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+![ITU logo: A globe with a lightning bolt and the letters ITU.](2dfa6ac3edfe874f68aa0cbccaa42322_img.jpg)
+
+ITU logo: A globe with a lightning bolt and the letters ITU.
+
+INTERNATIONAL TELECOMMUNICATION UNION
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.1000**
+
+(11/2001)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+Quality of service and performance
+
+---
+
+**Communications quality of service: A  
+framework and definitions**
+
+ITU-T Recommendation G.1000
+
+---
+
+# ITU-T G-SERIES RECOMMENDATIONS **TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS**
+
+|                                                                                                                                              |                      |
+|----------------------------------------------------------------------------------------------------------------------------------------------|----------------------|
+| INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS                                                                                             | G.100–G.199          |
+| GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS                                                                  | G.200–G.299          |
+| INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES                                                      | G.300–G.399          |
+| GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES | G.400–G.449          |
+| COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY                                                                                            | G.450–G.499          |
+| TESTING EQUIPMENTS                                                                                                                           | G.500–G.599          |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.600–G.699          |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.700–G.799          |
+| DIGITAL NETWORKS                                                                                                                             | G.800–G.899          |
+| DIGITAL SECTIONS AND DIGITAL LINE SYSTEM                                                                                                     | G.900–G.999          |
+| <b>QUALITY OF SERVICE AND PERFORMANCE</b>                                                                                                    | <b>G.1000–G.1999</b> |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.6000–G.6999        |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.7000–G.7999        |
+
+*For further details, please refer to the list of ITU-T Recommendations.*
+
+# **Communications quality of service: A framework and definitions**
+
+## **Summary**
+
+This Recommendation gives a framework and definitions for communications quality of service so as to establish a uniform approach to quality of service across the ITU-T and eliminate the confusion resulting from different frameworks and inconsistent definitions. Improved consistency regarding quality of service is needed throughout the communications industry, especially in IP-related areas within and outside the ITU.
+
+This Recommendation provides a practical "top-down" path from a general quality definition (ISO 8402) to a QoS definition (ITU-T Rec. E.800) to network performance (ITU-T Rec. I.350, and Y.1540) to a functional breakdown of all components of service quality (the QoS definition matrix of ETSI ETR 003). Additionally presented are four viewpoints of QoS that make the definitions and framework meaningful for everyone: users, vendors, network operators, service providers, etc.
+
+This overall approach is believed to be highly useful because it encourages a consistent and uniform mapping between the top-down, framework definitions of QoS and the bottom-up, operational measures on network elements. Advantages of using this approach is that it not only helps identify QoS-related problems, but it also facilitates quantifying the problem from multiple views; the customer's (e.g. surveys and subjective tests) and the service provider's (network measurements). This helps assure that resolving the problem in one domain (e.g. the provider) also resolves it in the other (e.g. the customer).
+
+### **Source**
+
+ITU-T Recommendation G.1000 was prepared by ITU-T Study Group 12 (2001-2004) and approved under the WTSA Resolution 1 procedure on 29 November 2001.
+
+## FOREWORD
+
+The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
+
+The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics.
+
+The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
+
+In some areas of information technology which fall within ITU-T's purview, the necessary standards are prepared on a collaborative basis with ISO and IEC.
+
+### NOTE
+
+In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+## INTELLECTUAL PROPERTY RIGHTS
+
+ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process.
+
+As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database.
+
+© ITU 2002
+
+All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.
+
+## CONTENTS
+
+|                                                                           | Page |
+|---------------------------------------------------------------------------|------|
+| 1 Scope.....                                                              | 1    |
+| 2 References.....                                                         | 1    |
+| 3 Definitions .....                                                       | 1    |
+| 4 Abbreviations and Acronyms .....                                        | 2    |
+| 5 Communications Quality of Service: A framework and approach.....        | 2    |
+| 5.1 An elaboration on definitions for Quality and Quality of Service..... | 2    |
+| 5.2 A framework for the quality of communications services.....           | 2    |
+| 5.3 Matrix for the determination of communications QoS criteria.....      | 3    |
+| 5.4 Relationship between QoS and Network performance.....                 | 4    |
+| 5.5 Four viewpoints of QoS.....                                           | 4    |
+| 5.5.1 Customer's requirements of QoS .....                                | 5    |
+| 5.5.2 QoS offered by the service provider.....                            | 5    |
+| 5.5.3 QoS achieved or delivered by the service provider.....              | 5    |
+| 5.5.4 QoS perceived by the customer .....                                 | 5    |
+| 5.6 Relationship between the four viewpoints of QoS .....                 | 6    |
+| 5.7 The Challenge of IP-related QoS.....                                  | 6    |
+| 6 Conclusion .....                                                        | 6    |
+| Appendix I – Bibliography .....                                           | 7    |
+
+
+
+# Communications quality of service: A framework and definitions
+
+## 1 Scope
+
+This Recommendation addresses the need for a consistent approach to QoS, with a goal of setting a well-defined and relevant (e.g. customer-affecting) approach that can be readily used to plan and deploy networks, and to monitor service quality. Improved consistency regarding QoS is needed throughout the communications industry, especially in IP-related areas.
+
+While there is an urgent need in the industry for more consistency and uniformity in QoS as it applies to IP-related networks and services, the QoS framework and approach presented here can be applied to all communications services, such as mobile, wireline, multimedia, etc.
+
+## 2 References
+
+The following ITU-T Recommendations and other references contain provisions, which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is published regularly.
+
+NOTE – The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
+
+- ETSI ETR 003 ed.2 (1994), *Network Aspects (NA); General Aspects of Quality of Service (QoS) and Network Performance (NP)*.
+- ISO 8402:1994, *Quality management and quality assurance – Vocabulary*.
+- ISO 9000:2000, *Quality management systems – Fundamentals and vocabulary*.
+- ITU-T Recommendation E.800 (1994), *Terms and definitions related to quality of service and network performance including dependability*.
+- ITU-T Recommendation G.1010 (2001), *End-user multimedia QoS categories*.
+- ITU-T Recommendation I.350 (1993), *General aspects of quality of service and network performance in digital networks, including ISDN*.
+- ITU-T Recommendation Y.1540 (1999), *Internet protocol data communication service – IP packet transfer and availability performance parameters*.
+
+# 3 Definitions
+
+This Recommendation defines the following terms:
+
+**3.1 quality:** the totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs (ISO 8402).
+
+**3.2 quality of service (QoS):** the collective effect of service performances, which determine the degree of satisfaction of a user of the service (ITU-T Rec. E.800).
+
+**3.3 QoS requirements of user/customer:** a statement of the level of quality *required* by the applications of customers/users of a service, which may be expressed non-technically.
+
+**3.4 QoS offered/planned by provider:** a statement of the level of quality *expected* to be offered to the customer by the service provider.
+
+**3.5 QoS delivered/achieved by provider:** a statement of the level of the *actual* quality achieved and delivered to the customer.
+
+**3.6 QoS perceived by user/customer:** is a statement expressing the level of quality that customers *believe* they have experienced.
+
+## **4 Abbreviations and Acronyms**
+
+This Recommendation uses the following abbreviations:
+
+ETSI European Telecommunications Standards Institute
+
+FITCE Federation of Telecommunications Engineers of the European Community (*Fédération des ingénieurs et experts des télécommunications de la Communauté Européenne*)
+
+IETF Internet Engineering Task Force
+
+IP Internet Protocol
+
+ISO International Organization for Standardization
+
+ITU-T International Telecommunication Union – Telecommunication Standardization Sector
+
+NI Network Interface
+
+NP Network Performance
+
+QoS Quality of Service
+
+SLA Service Level Agreement
+
+# **5 Communications Quality of Service: A framework and approach**
+
+## **5.1 An elaboration on definitions for Quality and Quality of Service**
+
+The term *quality of service* (QoS) is extensively used today, not just in the telecommunications world in which it has its roots, but increasingly regarding broadband, wireless and multimedia services that are IP-based. Networks and systems are gradually being designed in consideration of end-to-end performance required by user applications; however, the term QoS is usually not well-defined, is used loosely, or worst of all, misused.
+
+In looking for candidate definitions of quality and QoS, ISO 8402 provides a general definition of quality itself, whereas ITU-T Rec. E.800 provides a definition of QoS:
+
+As noted, a definition of general quality is provided in ISO 8402 as "*the totality of characteristics of an entity that bear on its ability to satisfy stated and implied needs.*" Similarly, ISO 9000 defines quality as the "*degree to which a set of inherent characteristics fulfills requirements.*" The ISO 8402 definition seems better from the user's view. In any event, QoS is clearly a subset of overall quality.
+
+As noted, ITU-T Rec. E.800 defines QoS as "*the collective effect of service performance which determine the degree of satisfaction of a user of the service.*"
+
+Most publications, including many standards, use the term QoS but either do not define it, or else point to one of these other few definitions. For example, in many industry standards, reports and specifications, QoS is not clearly defined, or else reference is made to ITU-T Rec. E.800.
+
+## **5.2 A framework for the quality of communications services**
+
+The framework described in ITU-T Rec. E.800 highlights some operational aspects of providing networks and services, but suffers from not being very application-oriented, and, in many areas, too
+
+vague to use. So while the definition of QoS in ITU-T Rec. E.800 is fairly widely used, the E.800 framework is not.
+
+Even if one adopts ISO 8402 as a definition of quality, and ITU-T Rec. E.800 as a definition of QoS, some way is still needed of relating the application-affecting functions of a service to the various criteria used to assess the quality with which these functions are performed. A telecommunications services quality framework [I.1] was adopted by the Federation of Telecommunications Engineers of the European Community (FITCE), with an extremely thorough assessment of how it could be used in practice. ETR 003 from ETSI provides a readily available source for this same framework and QoS approach, which is elaborated below.
+
+## 5.3 Matrix for the determination of communications QoS criteria
+
+Quality criteria of a telecommunications service may be derived from a matrix shown in Figure 1. Considerable thought has gone into its construction and has proven to be useful in identifying QoS criteria for customers before launching a new service (see ETSI ETR 003). The FITCE Study Commission found that, depending upon the granularity of QoS required for a service, as many as 43 QoS criteria may be deduced, and as few as 13 important ones for basic telephony service.
+
+|                                           |                                   | Service Quality Criteria |               |                   |                  |               |                 |                  |
+|-------------------------------------------|-----------------------------------|--------------------------|---------------|-------------------|------------------|---------------|-----------------|------------------|
+|                                           |                                   | SPEED<br>1               | ACCURACY<br>2 | AVAILABILITY<br>3 | RELIABILITY<br>4 | SECURITY<br>5 | SIMPLICITY<br>6 | FLEXIBILITY<br>7 |
+| <b>Service Function</b>                   |                                   |                          |               |                   |                  |               |                 |                  |
+| SERVICE<br>MANAGEMENT                     | Sales & Pre-Contract Activities 1 |                          |               |                   |                  |               |                 |                  |
+|                                           | Provision 2                       |                          |               |                   |                  |               |                 |                  |
+|                                           | Alteration 3                      |                          |               |                   |                  |               |                 |                  |
+|                                           | Service Support 4                 |                          |               |                   |                  |               |                 |                  |
+|                                           | Repair 5                          |                          |               |                   |                  |               |                 |                  |
+|                                           | Cessation 6                       |                          |               |                   |                  |               |                 |                  |
+| CONNECTION<br>QUALITY                     | Connection Establish. 7           |                          |               |                   |                  |               |                 |                  |
+|                                           | Information Transfer 8            |                          |               |                   |                  |               |                 |                  |
+|                                           | Connection Release 9              |                          |               |                   |                  |               |                 |                  |
+| Billing 10                                |                                   |                          |               |                   |                  |               |                 |                  |
+| Network/Service management by customer 11 |                                   |                          |               |                   |                  |               |                 |                  |
+
+**Figure 1/G.1000 – Matrix to facilitate identification of communications QoS criteria**
+
+This matrix may be used for any telecommunications service to determine the requisite QoS criteria. After determining the quality criteria, parameters can be defined and performance objectives set.
+
+## 5.4 Relationship between QoS and Network performance
+
+Network performance contributes towards QoS as experienced by the user/customer. Network performance may or may not be on an end to end basis. For example, access performance is usually separated from the core network performance in the operations of single IP network, while Internet performance often reflects the combined NP of several autonomous networks
+
+Other ITU-T Rec., such as I.350 and Y.1540, have gone on to develop far more detail for network performance and the NI-to-NI part of overall QoS, in addition to contrasting QoS and network performance. But ITU-T Rec. E.800 remains the most meaningful ITU-T definition from the user's perspective, even though, as mentioned, the framework of ITU-T Rec. E.800 is vague in areas and has thus led to a variety of interpretations. So while the framework of ITU-T Rec. E.800 needs to be more concrete and more application-oriented, its basic definition of QoS is good.
+
+## 5.5 Four viewpoints of QoS
+
+The QoS definition matrix of Figure 1 gives criteria for judging the quality of the communications functions that any service must support. However, even this definitional matrix can be viewed from different perspectives:
+
+- Customer's QoS requirements;
+- Service provider's offerings of QoS (or planned/targeted QoS);
+- QoS achieved or delivered;
+- Customer survey ratings of QoS.
+
+For any framework of QoS to be truly useful and practical enough to be used across the industry, it must be meaningful from these four viewpoints, which are illustrated in Figure 2 and defined thereafter. While Figure 2 shows the "top down" relationship of these viewpoints, it does not indicate how, for example, QoS actually gets implemented by the service provider. This requires many detailed methods done in a more "bottom up" operation that is not addressed in this Recommendation. (The point of this Recommendation is that a single QoS definition and framework can support all of the viewpoints of Figure 2.)
+
+![Diagram illustrating the four viewpoints of QoS, showing the relationship between Customer and Service Provider perspectives.](c2fc2621e8206d24427b56bcb2398fc0_img.jpg)
+
+The diagram is titled "Figure 2/G.1000 – The four viewpoints of QoS". It is divided into two vertical panels by a dashed line. The left panel is labeled "CUSTOMER" and the right panel is labeled "SERVICE PROVIDER".
+
+In the CUSTOMER panel, there are two boxes: "Customer's QoS Requirements" at the top and "QoS Perceived By Customer" at the bottom. A blue arrow points from the bottom box to the top box.
+
+In the SERVICE PROVIDER panel, there are two boxes: "QoS Offered By Provider" at the top and "QoS Achieved by Provider" at the bottom. A blue arrow points from the top box to the bottom box.
+
+Horizontal blue arrows connect the panels: one from "Customer's QoS Requirements" to "QoS Offered By Provider", and another from "QoS Achieved by Provider" to "QoS Perceived By Customer".
+
+A small text label "T1213040-02" is located at the bottom right of the diagram.
+
+Diagram illustrating the four viewpoints of QoS, showing the relationship between Customer and Service Provider perspectives.
+
+Figure 2/G.1000 – The four viewpoints of QoS
+
+### **5.5.1 Customer's requirements of QoS**
+
+QoS requirements by the customer state the level of quality required of a particular service, which may be expressed in non-technical language. The customer is not concerned with how a particular service is provided, or with any aspects of the network's internal design, but only with the resulting end-to-end service quality. From the customer's point of view, quality of service is expressed by parameters, which:
+
+- focus on user-perceived effects, rather than their causes within the network;
+- do not depend, in their definition, on assumptions about the internal design of the network;
+- take into account all aspects of the service from the customer's point of view;
+- may be assured to a user by the service providers, sometimes in contractual terms;
+- are described in network-independent terms and create a common language understandable by both the user and the service provider.
+
+Of special interest is ITU-T Rec. G.1010 on the QoS requirements of end users for all their applications. This Recommendation explicitly addresses the "customer requirement" viewpoint, which can be used by the service provider to plan the network service level.
+
+### **5.5.2 QoS offered by the service provider**
+
+QoS offered by the service provider is a statement of the level of quality expected to be offered to the customer by the service provider. The level of quality is expressed by values assigned to QoS parameters. The principal use of this form of QoS is for planning and for Service Level Agreements. Each service would have its own set of QoS parameters (as in the QoS Classes of ITU-T Rec. Y.1540 for IP service offers). The service provider may express the offered QoS in non-technical terms for the benefit of customers, and in technical terms for use within the business.
+
+QoS offered by the service provider can be used in planning documents, to specify measurement systems and also can be used to form the basis of the SLAs.
+
+For example, a service provider may state, for the benefit of customers, that the availability of basic telephony service is planned to be 99.95% in a year with not more than a 15-minute break at any one occasion, and not more than 3 breaks over the year.
+
+#### **5.5.3 QoS achieved or delivered by the service provider**
+
+QoS achieved by the service provider is a statement of the level of quality actually achieved and delivered to the customer. This is expressed by values assigned to parameters, which should be the same as specified for the offered QoS so that the two can be compared to determine what was actually achieved to assess the level of performance achieved. These performance figures are summarised for specified periods of time, e.g. for the previous month.
+
+For example, the service provider may state that the achieved availability for the previous quarter was 99.95% with five breaks of service of which one lasted 65 minutes. The QoS achieved or delivered is used by the industry, sometimes by regulators, for publication in the interests of customers.
+
+#### **5.5.4 QoS perceived by the customer**
+
+QoS perceived by the users or customers is a statement expressing the level of quality experienced they 'believe' they have experienced. The Perceived QoS is expressed, usually in terms of degrees of satisfaction and not in technical terms. Perceived QoS is assessed by customer surveys and from customer's own comments on levels of service. Perceived QoS can be used by the service provider to determine customer satisfaction of the service quality. For example, a customer may state that on an unacceptable number of occasions, there was difficulty in getting through the network to make a call and may give it a rating of 2 on a 5-point scale: 5 indicating excellent service. Ideally there would be 1:1 correspondence between delivered and perceived QoS.
+
+## 5.6 Relationship between the four viewpoints of QoS
+
+The customer's QoS requirements (such as those given in ITU-T Rec. G.1010) may be considered as the logical starting point. A set of customer's QoS requirements may be treated in isolation as far as its capture is concerned. This requirement is an input to the service provider for the determination of the QoS to be offered or planned. The service provider may not always be in a position to offer customers the level of QoS they require. Considerations such as cost of quality, strategic aspects of the service providers' business, benchmarking ("best in class") and other factors will influence the level of quality offered. The customer's requirements may also influence what monitoring systems are to be instituted for the determination of achieved QoS for the purpose of regular reports on achieved quality. The combination of relationships forms the basis of a practical and effective management of service quality; one measure of progress could be considered to be when the four views of QoS begin to converge for a given service.
+
+## 5.7 The Challenge of IP-related QoS
+
+There are many issues presented by the use of IP-based networks and services, such as the lack of proven, robust and scalable standard mechanisms for:
+
+- dynamic allocations of resources (like packet loss and delay) among network segments;
+- assuring that required end-to-end network performance objectives are in fact achieved;
+- seamless signaling of desired end-to-end QoS across both network and peer interfaces;
+- performance monitoring of IP-based networks and services that are consistent with methods used for network and service planning, and also meaningful to the user experience; and
+- rapid and complete restoration of IP layer connectivity following severe outages (or attacks) of heavily loaded networks.
+
+If such issues are to be addressed in a timely fashion, a consistent QoS approach is clearly helpful.
+
+## 6 Conclusion
+
+In summary, this Recommendation provides a practical "top-down" path from a general quality definition (ISO 8402) to a QoS definition (ITU-T Rec. E.800) to network performance (ITU-T Rec. I.350 and Y.1540) to a functional breakdown of all components of service quality (the QoS definition matrix of ETSI ETR 003). Additionally presented are four viewpoints of QoS that make the definitions and framework meaningful for everyone: users, vendors, network operators, service providers, etc.
+
+This overall approach is believed to be highly useful because it encourages a consistent and uniform mapping between the top-down, framework definitions of QoS and the bottom-up, operational measures on network elements. Advantages of using this approach are that it not only helps identify QoS-related problems, but it also facilitates quantifying the problem from multiple viewpoints: the customer's (e.g. surveys and subjective tests) and the service provider's (network measurements). This helps assure that resolving the problem in one domain (e.g. the provider) also resolves it in the other (e.g. the customer).
+
+## APPENDIX I
+
+### **Bibliography**
+
+- [I.1] RICHTERS (J.S.), DVORAK (C.A.): A Framework for Defining the Quality of Communications Services, *IEEE Communications Magazine*, Volume 26, Issue 10, pp. 17-23, October 1988.
+
+
+
+
+
+### SERIES OF ITU-T RECOMMENDATIONS
+
+|                 |                                                                                                                                |
+|-----------------|--------------------------------------------------------------------------------------------------------------------------------|
+| Series A        | Organization of the work of ITU-T                                                                                              |
+| Series B        | Means of expression: definitions, symbols, classification                                                                      |
+| Series C        | General telecommunication statistics                                                                                           |
+| Series D        | General tariff principles                                                                                                      |
+| Series E        | Overall network operation, telephone service, service operation and human factors                                              |
+| Series F        | Non-telephone telecommunication services                                                                                       |
+| <b>Series G</b> | <b>Transmission systems and media, digital systems and networks</b>                                                            |
+| Series H        | Audiovisual and multimedia systems                                                                                             |
+| Series I        | Integrated services digital network                                                                                            |
+| Series J        | Cable networks and transmission of television, sound programme and other multimedia signals                                    |
+| Series K        | Protection against interference                                                                                                |
+| Series L        | Construction, installation and protection of cables and other elements of outside plant                                        |
+| Series M        | TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits |
+| Series N        | Maintenance: international sound programme and television transmission circuits                                                |
+| Series O        | Specifications of measuring equipment                                                                                          |
+| Series P        | Telephone transmission quality, telephone installations, local line networks                                                   |
+| Series Q        | Switching and signalling                                                                                                       |
+| Series R        | Telegraph transmission                                                                                                         |
+| Series S        | Telegraph services terminal equipment                                                                                          |
+| Series T        | Terminals for telematic services                                                                                               |
+| Series U        | Telegraph switching                                                                                                            |
+| Series V        | Data communication over the telephone network                                                                                  |
+| Series X        | Data networks and open system communications                                                                                   |
+| Series Y        | Global information infrastructure and Internet protocol aspects                                                                |
+| Series Z        | Languages and general software aspects for telecommunication systems                                                           |
+
+\*21742\*

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+
+
+International Telecommunication Union
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.107.1**
+
+(06/2019)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+International telephone connections and circuits –  
+Transmission planning and the E-model
+
+---
+
+**Wideband E-model**
+
+Recommendation ITU-T G.107.1
+
+ITU-T
+
+![ITU logo](1d7527f4316cfe2d342b08d1653d1592_img.jpg)
+
+The logo of the International Telecommunication Union (ITU) features a stylized globe with a red lightning bolt striking across it. The letters "ITU" are prominently displayed in blue and white within the globe.
+
+ITU logo
+
+International  
+Telecommunication  
+Union
+
+# ITU-T G-SERIES RECOMMENDATIONS **TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS**
+
+|                                                                                                                                              |                    |
+|----------------------------------------------------------------------------------------------------------------------------------------------|--------------------|
+| INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS                                                                                             | G.100–G.199        |
+| <b>Transmission planning and the E-model</b>                                                                                                 | <b>G.100–G.109</b> |
+| General Recommendations on the transmission quality for an entire international telephone connection                                         | G.110–G.119        |
+| General characteristics of national systems forming part of international connections                                                        | G.120–G.129        |
+| General characteristics of the 4-wire chain formed by the international circuits and national extension circuits                             | G.130–G.139        |
+| General characteristics of the 4-wire chain of international circuits; international transit                                                 | G.140–G.149        |
+| General characteristics of international telephone circuits and national extension circuits                                                  | G.150–G.159        |
+| Apparatus associated with long-distance telephone circuits                                                                                   | G.160–G.169        |
+| Transmission plan aspects of special circuits and connections using the international telephone connection network                           | G.170–G.179        |
+| Protection and restoration of transmission systems                                                                                           | G.180–G.189        |
+| Software tools for transmission systems                                                                                                      | G.190–G.199        |
+| GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS                                                                  | G.200–G.299        |
+| INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES                                                      | G.300–G.399        |
+| GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES | G.400–G.449        |
+| COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY                                                                                            | G.450–G.499        |
+| TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS                                                                                       | G.600–G.699        |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.700–G.799        |
+| DIGITAL NETWORKS                                                                                                                             | G.800–G.899        |
+| DIGITAL SECTIONS AND DIGITAL LINE SYSTEM                                                                                                     | G.900–G.999        |
+| MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE – GENERIC AND USER-RELATED ASPECTS                                                             | G.1000–G.1999      |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.6000–G.6999      |
+| DATA OVER TRANSPORT – GENERIC ASPECTS                                                                                                        | G.7000–G.7999      |
+| PACKET OVER TRANSPORT ASPECTS                                                                                                                | G.8000–G.8999      |
+| ACCESS NETWORKS                                                                                                                              | G.9000–G.9999      |
+
+*For further details, please refer to the list of ITU-T Recommendations.*
+
+# Recommendation ITU-T G.107.1
+
+## Wideband E-model
+
+## Summary
+
+Recommendation ITU-T G.107.1 gives the algorithm for the wideband (WB) version of the E-model as the common ITU-T transmission rating model for planning speech services that provide WB speech transmission (50-7000 Hz). This computational model can be useful to transmission planners, to help ensure that users will be satisfied with end-to-end transmission performance. The primary output of the model is a scalar rating of transmission quality. A major feature of this model is the use of transmission impairment factors that reflect the effects of different types of degradations occurring on the entire transmission path, mouth-to-ear.
+
+## History
+
+| Edition | Recommendation | Approval   | Study Group | Unique ID*                                                                |
+|---------|----------------|------------|-------------|---------------------------------------------------------------------------|
+| 1.0     | ITU-T G.107.1  | 2011-12-14 | 12          | <a href="http://handle.itu.int/11.1002/1000/11453">11.1002/1000/11453</a> |
+| 2.0     | ITU-T G.107.1  | 2015-06-29 | 12          | <a href="http://handle.itu.int/11.1002/1000/12506">11.1002/1000/12506</a> |
+| 3.0     | ITU-T G.107.1  | 2019-06-29 | 12          | <a href="http://handle.itu.int/11.1002/1000/13925">11.1002/1000/13925</a> |
+
+## Keywords
+
+E-model, speech quality, transmission planning, wideband.
+
+---
+
+\* To access the Recommendation, type the URL <http://handle.itu.int/> in the address field of your web browser, followed by the Recommendation's unique ID. For example, <http://handle.itu.int/11.1002/1000/11830-en>.
+
+## FOREWORD
+
+The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
+
+The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics.
+
+The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
+
+In some areas of information technology which fall within ITU-T's purview, the necessary standards are prepared on a collaborative basis with ISO and IEC.
+
+## NOTE
+
+In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some other obligatory language such as "must" and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party.
+
+## INTELLECTUAL PROPERTY RIGHTS
+
+ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process.
+
+As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at <http://www.itu.int/ITU-T/ipr/>.
+
+© ITU 2019
+
+All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.
+
+## Table of Contents
+
+|                                                                          | Page |
+|--------------------------------------------------------------------------|------|
+| 1 Scope.....                                                             | 1    |
+| 2 References.....                                                        | 1    |
+| 3 Definitions .....                                                      | 2    |
+| 3.1 Terms defined elsewhere .....                                        | 2    |
+| 3.2 Terms defined in this Recommendation.....                            | 2    |
+| 4 Abbreviations and acronyms .....                                       | 2    |
+| 5 Conventions .....                                                      | 3    |
+| 6 WB E-model .....                                                       | 3    |
+| 6.1 Introduction .....                                                   | 3    |
+| 6.2 Transmission rating scale of the WB E-model.....                     | 3    |
+| 7 The structure and basic algorithms of the WB E-model.....              | 5    |
+| 7.1 Calculation of the transmission rating factor, R.....                | 6    |
+| 7.2 Basic signal-to-noise ratio, $R_o$ , WB .....                        | 6    |
+| 7.3 Simultaneous impairment factor, $I_s$ , WB .....                     | 6    |
+| 7.4 Delay impairment factor, $I_d$ , WB .....                            | 6    |
+| 7.5 Equipment impairment factor, $I_e$ , WB .....                        | 7    |
+| 7.6 Advantage factor, A.....                                             | 8    |
+| 7.7 Default values .....                                                 | 8    |
+| Annex A – MOS values derived from the transmission rating factor R ..... | 9    |
+| Bibliography.....                                                        | 10   |
+
+
+
+## Recommendation ITU-T G.107.1
+
+## Wideband E-model
+
+## 1 Scope
+
+This Recommendation describes the wideband (WB) version of a computational model, known as the E-model, that has proven useful as a transmission planning tool for assessing the combined effects of variations in several transmission parameters that affect conversational<sup>1</sup> quality. This computational model can be used, for example, by transmission planners to help ensure that users will be satisfied with end-to-end transmission performance whilst avoiding over-engineering of networks. It must be emphasized that the primary output from the model is the "rating factor"  $R$ , but this can be transformed to give estimates of customer opinion. Such estimates are only made for transmission planning purposes and not for actual customer opinion prediction (for which there is no agreed-upon model recommended by the ITU-T).
+
+This version is an adapted version of the NB (300-3400 Hz) E-model, typically referred to as "the E-model", which is described in [ITU-T G.107]. The WB version addresses scenarios which include WB (50-7000 Hz) transmission. It does not replace the narrowband (NB) E-model. Instead, it describes a separate WB-version of the model that uses, within limits, similar concepts and input parameters as the NB E-model. The current version captures the effects of talker echo, absolute delay, WB speech coding, and voice-over-IP packet-loss. Degradations which are covered but have not yet been studied in detail are listener echo. Degradations which are not yet covered are loudness loss, non-optimum sidetone levels, background noise at the sending and receiving side, circuit noise and quantizing distortions.
+
+For many parameter combinations of high importance to transmission planners, ITU-T G.107.1 can be used with confidence (e.g., loudness loss, coding distortions), but for some parameter combinations of high importance (e.g., the effects of delay in conjunction with other impairments), WB E-model predictions have been questioned and are currently under study.
+
+Regarding the interpretation of the WB E-model ratings, note that the current versions of [b-ITU-T G.108], [b-ITU-T G.108.1] and [b-ITU-T G.109] do not refer to the WB version described here, but only to the NB version of the E-model described in [ITU-T G.107].
+
+## 2 References
+
+The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
+
+[ITU-T G.107] Recommendation ITU-T G.107 (2015), *The E-model: a computational model for use in transmission planning*.  
+<<http://www.itu.int/rec/T-REC-G.107>>
+
+---
+
+<sup>1</sup> Conversational quality in this context refers to transmission characteristics, e.g., long transmission times, effects of talker echoes, etc. However, the E-model, as described in this Recommendation, is not intended to model transmission impairments during double talk situations.
+
+- [ITU-T G.113] Recommendation ITU-T G.113 (2007), *Transmission impairments due to speech processing*.  
+[<http://www.itu.int/rec/T-REC-G.113>](http://www.itu.int/rec/T-REC-G.113)
+- [ITU-T G.722] Recommendation ITU-T G.722 (2012), *7 kHz audio-coding within 64 kbit/s*.  
+[<http://www.itu.int/rec/T-REC-G.722>](http://www.itu.int/rec/T-REC-G.722)
+- [ITU-T O.41] Recommendation ITU-T O.41 (1994), *Psophometer for use on telephone-type circuits*.  
+[<http://www.itu.int/rec/T-REC-O.41>](http://www.itu.int/rec/T-REC-O.41)
+- [ITU-T P.800] Recommendation ITU-T P.800 (1996), *Methods for subjective determination of transmission quality*.  
+[<http://www.itu.int/rec/T-REC-P.800>](http://www.itu.int/rec/T-REC-P.800)
+- [ITU-T P.833] Recommendation ITU-T P.833 (2001), *Methodology for derivation of equipment impairment factors from subjective listening-only tests*.  
+[<http://www.itu.int/rec/T-REC-P.833>](http://www.itu.int/rec/T-REC-P.833)
+- [ITU-T P.833.1] Recommendation ITU-T P.833.1 (2009), *Methodology for the derivation of equipment impairment factors from subjective listening-only tests for wideband speech codecs*.  
+[<http://www.itu.int/rec/T-REC-P.833.1>](http://www.itu.int/rec/T-REC-P.833.1)
+- [ITU-T P.834] Recommendation ITU-T P.834 (2015), *Methodology for the derivation of equipment impairment factors from instrumental models*.  
+[<http://www.itu.int/rec/T-REC-P.834>](http://www.itu.int/rec/T-REC-P.834)
+- [ITU-T P.834.1] Recommendation ITU-T P.834.1 (2015), *Methodology for the derivation of equipment impairment factors from instrumental models for wideband speech codecs*.  
+[<http://www.itu.int/rec/T-REC-P.834.1>](http://www.itu.int/rec/T-REC-P.834.1)
+
+## 3 Definitions
+
+## 3.1 Terms defined elsewhere
+
+None.
+
+## 3.2 Terms defined in this Recommendation
+
+None.
+
+## 4 Abbreviations and acronyms
+
+This Recommendation uses the following abbreviations and acronyms:
+
+|      |                             |
+|------|-----------------------------|
+| LSTR | Listener Sidetone Rating    |
+| MOS  | Mean Opinion Score          |
+| NB   | Narrowband                  |
+| OLR  | Overall Loudness Rating     |
+| PCM  | Pulse Code Modulation       |
+| RLR  | Receive Loudness Rating     |
+| SLR  | Send Loudness Rating        |
+| STMR | Sidetone Masking Rating     |
+| TELR | Talker Echo Loudness Rating |
+| WB   | Wideband                    |
+
+## 5 Conventions
+
+None.
+
+# 6 WB E-model
+
+## 6.1 Introduction
+
+The complexity of modern networks requires that, for transmission planning, the many transmission parameters be not only considered individually, but also that their combined effects be taken into account. This can be done by "expert, informed guessing", but a more systematic approach is desirable, such as by using a computational model. The output from the model described here is a scalar transmission rating value,  $R$ , which varies directly with the overall conversational quality. [ITU-T G.113] gives guidance about specific impairments, including combined effects based upon a simplification of the model.
+
+## 6.2 Transmission rating scale of the WB E-model
+
+For the narrowband (NB) case described in [ITU-T G.107], the transmission rating scale ranges from  $R = 0$  (lowest possible quality) to  $R = 100$  (optimum quality). On this scale, a default NB transmission channel including logarithmic Pulse Code Modulation (PCM) coding and a noise floor (default parameter values according to Table 2 of [ITU-T G.107]) obtains a rating of  $R = 93.2$ . For a WB speech transmission channel, the quality is generally judged better than that for a NB channel. Thus, this scale range was extended in order to be also applicable to WB transmission scenarios. In the present WB version, the E-model is defined for a WB transmission channel of 50-7000 Hz, as it is defined in [ITU-T G.722].
+
+Unfortunately, it is not possible to obtain direct human judgements on the  $R$ -scale, as this scale has additivity properties which are not reflected by ordinary rating scales. Instead, for NB conditions, ITU-T recommends collecting judgements on a 5-point absolute category rating scale, see [ITU-T P.800]. The mean rating, averaged over all test participants and stimuli reflecting the same circuit condition, is then called a mean opinion score (MOS).
+
+It has been shown that MOS ratings differ between tests where only NB stimuli are presented and tests where both NB/WB or purely WB stimuli are presented, as the use of the scale is largely influenced by the stimulus set. On the other hand, there is also experimental evidence that judgements for WB samples collected in a purely WB context do not differ significantly from those collected in a mixed NB/WB context, see [b-Barriac] and [b-Takahashi]. In addition to the stimulus bandwidth, test results are influenced by the test participant group, the language, the participants' native country, etc. [b-Möller-01]. For a NB context, an average S-shaped relationship is defined between the  $R$ -scale (range [0;100]) and MOS ratings (range [1;4.5]) collected from "average" test participants in an "average" experimental setting, see Annex B and Appendix I of [ITU-T G.107].
+
+For a WB or a mixed NB/WB context, the  $R$ -scale was extended in a way which leaves the NB use of the scale unaffected, including the position of the reference connection (default parameter settings according to Table 2 of [ITU-T G.107]). Such an extension can be based on pairs of auditory tests in which the same (NB) test stimuli have been judged once in a purely NB and once in a mixed NB/WB context. The judgements on these common stimuli define a relationship between the use of the MOS-scale in a NB and in a mixed NB/WB context [b-Raake-01].
+
+Two pairs of tests have been carried out and will be considered in the following. Details on parts of the tests can be found in [b-Möller-02]. The MOS results from these tests have been transformed to the  $R$ -scale using the NB transformation rule given in Annex B of [ITU-T G.107]. The resulting  $R_{NB}$
+
+values (NB test) and  $R_{NB/WB}$  values (mixed NB/WB test) for the conditions which were common in each pair are displayed in Figure 1.
+
+![Figure 1: Comparison between R-values derived in a NB and in a mixed NB/WB context. The figure contains two scatter plots. The top plot shows R_NB on the y-axis (0 to 150) versus R_NB/WB on the x-axis (0 to 100). A diagonal line represents the relationship, ending at a point labeled R_max = 129. The bottom plot shows R_NB on the y-axis (0 to 140) versus R_NB/WB on the x-axis (0 to 100). A diagonal line represents the relationship, ending at a point labeled R_max = 129.5. Both plots show data points following the diagonal line, indicating a strong correlation between the two R-value scales.](4e0ade2f41b66d5602160da5cc978274_img.jpg)
+
+The figure consists of two vertically stacked scatter plots. The top plot has  $R_{NB}$  on the y-axis (0 to 150) and  $R_{NB/WB}$  on the x-axis (0 to 100). A solid diagonal line starts at (0,0) and ends at (100, 129), with an arrow pointing to the endpoint labeled  $R_{max} = 129$ . Data points are represented by open circles. The bottom plot has  $R_{NB}$  on the y-axis (0 to 140) and  $R_{NB/WB}$  on the x-axis (0 to 100). A solid diagonal line starts at (0,0) and ends at (100, 129.5), with an arrow pointing to the endpoint labeled  $R_{max} = 129.5$ . Data points are represented by open circles. Both plots show a strong positive correlation between the two R-value scales.
+
+Figure 1: Comparison between R-values derived in a NB and in a mixed NB/WB context. The figure contains two scatter plots. The top plot shows R\_NB on the y-axis (0 to 150) versus R\_NB/WB on the x-axis (0 to 100). A diagonal line represents the relationship, ending at a point labeled R\_max = 129. The bottom plot shows R\_NB on the y-axis (0 to 140) versus R\_NB/WB on the x-axis (0 to 100). A diagonal line represents the relationship, ending at a point labeled R\_max = 129.5. Both plots show data points following the diagonal line, indicating a strong correlation between the two R-value scales.
+
+**Figure 1 – Comparison between  $R$ -values derived in a NB and in a mixed NB/WB context**
+
+Due to the use of the NB relationship between MOS and  $R$  for deriving the  $R_{NB/WB}$  values, the maximum  $R_{NB/WB}$  value corresponding to MOS = 4.5 equals 100. The corresponding  $R_{NB}$  value of the panels in Figure 1 shows the amount by which the  $R$ -scale has to be extended in a NB/WB context. This maximum value is around  $R_{max} = 129$ . In other words, the NB transmission rating scale of the E-model has been extended by approximately 29% to reflect the quality improvement when migrating from NB to WB. This extended  $R$ -scale is a "universal"  $R$ -scale and it is applicable to both NB and WB transmission channels.
+
+The primary output of the WB E-model is the transmission rating  $R$ . However, the output can also give nominal estimates of user reactions, for instance in the form of MOS values, as described in Annex A.
+
+# 7 The structure and basic algorithms of the WB E-model
+
+The WB E-model is based on the equipment impairment factor method, following previous transmission rating models. The reference connection, as shown in Figure 2, is split into a send side
+
+and a receive side. The model estimates the conversational quality from mouth-to-ear as perceived by the user at the receive side, both as listener and talker.
+
+![Figure 2 – Reference connection of the WB E-model. This diagram illustrates the transmission path and associated parameters for the WB E-model. It shows a 'Send side' on the left and a 'Receive side' on the right, connected by a central 'Coding/decoding' block. Key parameters include: OLR (Overall Loudness Rating) as the sum of SLR (Send Loudness Rating) and RLR (Receive Loudness Rating); a '0 dBr point' reference; WEPL (Weighted Echo Pass Loss) and Round-trip delay Tr; STMR (Sidetone Masking Rating), LSTR (Listener Speech Transmission Ratio, defined as STMR + Dr), and TELR (Talker Echo Loudness Rating) on the receive side; and various delay and noise parameters like Mean one-way delay T, Absolute delay Ta, Circuit noise Nc, and Packet-loss probability Ppl.](5b4e774d63e0e0ed73801a9247755e5f_img.jpg)
+
+The diagram shows the reference connection for the WB E-model. On the left, the 'Send side' features a user icon with a 'Ds-factor' and 'Room noise Ps'. A signal path leads to a 'Coding/decoding' block. Above this block is a 'WEPL' and 'Round-trip delay Tr' loop. A '0 dBr point' is marked on the signal path. On the right, the 'Receive side' features another user icon with a 'Dr-factor' and 'Room noise Pr'. The signal path from the coding/decoding block to the receive side includes 'Circuit noise Nc referred to 0 dBr', 'Equipment impairment factor Ie', 'Packet-loss probability Ppl', and 'Packet-loss robustness factor Bpl'. Various delay and rating parameters are indicated with double-headed arrows: 'OLR' (Overall Loudness Rating) spanning both sides; 'SLR' (Send Loudness Rating) on the send side; 'RLR' (Receive Loudness Rating) on the receive side; 'Mean one-way delay T' between the send side and the receive side; 'Absolute delay Ta' between the send side and the receive side; and 'TELR' (Talker Echo Loudness Rating) on the receive side. Other parameters shown include 'Quantizing distortion qdu' and 'Advantage factor A'. The formula 'LSTR = STMR + Dr' is also shown.
+
+Figure 2 – Reference connection of the WB E-model. This diagram illustrates the transmission path and associated parameters for the WB E-model. It shows a 'Send side' on the left and a 'Receive side' on the right, connected by a central 'Coding/decoding' block. Key parameters include: OLR (Overall Loudness Rating) as the sum of SLR (Send Loudness Rating) and RLR (Receive Loudness Rating); a '0 dBr point' reference; WEPL (Weighted Echo Pass Loss) and Round-trip delay Tr; STMR (Sidetone Masking Rating), LSTR (Listener Speech Transmission Ratio, defined as STMR + Dr), and TELR (Talker Echo Loudness Rating) on the receive side; and various delay and noise parameters like Mean one-way delay T, Absolute delay Ta, Circuit noise Nc, and Packet-loss probability Ppl.
+
+Figure 2 – Reference connection of the WB E-model
+
+The transmission parameters used as an input to the computation model are shown in Figure 2. Values for room noise and for the  $D$ -factors are handled separately in the algorithm for the send side and receive side and may be of different amounts. The parameters send loudness rating (SLR), receive loudness rating (RLR) and circuit noise  $N_c$  are referred to a defined 0 dBr point. All other input parameters are either considered as values for the overall connection, such as overall loudness rating (OLR) (in any case, the sum of SLR and RLR), equipment impairment factors  $I_e$  and advantage factor  $A$ , or referred to only for the receive side, such as sidetone masking rating (STMR), LSTR, weighted echo pass loss (WEPL) (for calculation of listener echo) and talker echo loudness rating (TELR).
+
+There are three different parameters associated with transmission time. The absolute delay  $T_a$  represents the total one-way delay between the send side and receive side and is used to estimate the impairment due to excessive delay. The parameter mean one-way delay  $T$  represents the delay between the receive side (in talking state) and the point in a connection where a signal coupling occurs as a source of echo. The round-trip delay  $T_r$  only represents the delay in a 4-wire loop, where the "double reflected" signal will cause impairments due to listener echo.
+
+Not all parameters listed in Figure 2 are currently used in the WB E-model. However, they are displayed here in order to provide a full description of the transmission channel considering also degradations which might be included in an updated WB E-model at a later stage. The current version is based on [b-Raake-02] and on further extensions for talker echo and delay which have been presented in ITU's Telecommunication Sector (ITU-T).
+
+## 7.1 Calculation of the transmission rating factor, $R$
+
+For WB, the basic E-model formula (7-1 of [ITU-T G.107]) can be re-written as:
+
+$$R = R_{o, WB} - I_{s, WB} - I_{d, WB} - I_{e, eff, WB} + A \quad (7-1)$$
+
+$Ro, WB$  represents in principle the basic signal-to-noise ratio, including noise sources such as circuit noise and room noise. Factor  $Is, WB$  is a combination of all impairments which occur more or less simultaneously with the voice signal. Factor  $Id, WB$  represents the impairments caused by delay and the effective equipment impairment factor  $Ie, eff, WB$  represents impairments caused by low bit-rate codecs. It also includes impairment due to randomly distributed pack losses. The advantage factor  $A$  allows for compensation of impairment factors when the user benefits from other types of access. The term  $Id, WB$  is subdivided into further specific impairment values. The following clauses give the equations used in the WB E-model.
+
+## 7.2 Basic signal-to-noise ratio, $Ro, WB$
+
+The basic signal-to-noise ratio  $Ro, WB$  is defined by:
+
+$$Ro, WB = 129 \quad (7-2)$$
+
+The effects of circuit noise  $Nc$ , room noise at send side  $Ps$  and room noise at receive side  $Pr$  are items for further study.
+
+## 7.3 Simultaneous impairment factor, $Is, WB$
+
+The factor  $Is, WB$  is the sum of all impairments which may occur more or less simultaneously with the voice transmission. This aspect has not been analysed for the WB case so far, thus it is set to:
+
+$$Is, WB = 0 \quad (7-3)$$
+
+## 7.4 Delay impairment factor, $Id, WB$
+
+$Id, WB$ , the impairment factor representing all impairments due to delay of voice signals, is further divided into the three factors  $Idte, WB$ ,  $Idle, WB$  and  $Idd$ :
+
+$$Id, WB = Idte, WB + Idle, WB + Idd \quad (7-4)$$
+
+The factor  $Idte, WB$  gives an estimate for the impairments due to talker echo:
+
+$$Idte, WB = \left[ \frac{Roe - Re, WB}{2} + \sqrt{\frac{(Roe - Re, WB)^2}{4} + 100} - 1 \right] (1 - e^{-T}) \quad (7-5)$$
+
+where:
+
+$$Roe = -1.5(No, WB - RLR) \quad (7-6)$$
+
+$$Re, WB = 80 + 3(TERV, WB - 14) \quad (7-7)$$
+
+$$TERV, WB = TELR + K - 40 \log \frac{1 + \frac{T}{10}}{1 + \frac{T}{150}} + 6e^{-0.3T^2} \quad (7-8)$$
+
+For  $T < 100$  ms:
+
+$$K = 0.08 \cdot T + 10 \quad (7-9)$$
+
+For  $T \geq 100$  ms:
+
+$$K = 18 \quad (7-10)$$
+
+For the WB case, we currently do not assume any mutual influence between talker echo and sidetone.
+
+The factor  $Idle, WB$  represents impairments due to listener echo. This impairment has not been specifically studied for the WB case, but it is assumed that the degradations will be similar to those of the NB case, so the formulae are congruent with the ones of the NB E-model:
+
+$$Idle, WB = \frac{Ro, WB - Rle}{2} + \sqrt{\frac{(Ro, WB - Rle)^2}{4} + 169} \quad (7-11)$$
+
+where:
+
+$$Rle = 10.5(WEPL + 7)(Tr + 1)^{-0.25} \quad (7-12)$$
+
+The factor  $Idd$  represents the impairment caused by an absolute delay  $Ta$  that is too long, which occurs even with perfect echo cancelling.
+
+For  $Ta \leq 100$  ms:
+
+$$Idd = 0$$
+
+For  $Ta > 100$  ms:
+
+$$Idd = 25 \cdot 1.29 \left\{ (1 + X^6)^{\frac{1}{6}} - 3 \left( 1 + \left[ \frac{X}{3} \right]^6 \right)^{\frac{1}{6}} + 2 \right\} \quad (7-13)$$
+
+with:
+
+$$X = \frac{\log\left(\frac{Ta}{100}\right)}{\log 2} \quad (7-14)$$
+
+## 7.5 Equipment impairment factor, $Ie, WB$
+
+The values for the equipment impairment factor  $Ie, WB$  of elements using low bit-rate codecs are not related to other input parameters. They depend on subjective MOS test results as well as on network experience. Refer to Appendix IV of [ITU-T G.113] for the currently recommended values of  $Ie, WB$ .
+
+In case of packet-loss, the packet-loss dependent effective equipment impairment factor  $Ie, eff, WB$  is derived using the codec-specific value for the equipment impairment factor at zero packet-loss  $Ie, WB$  and the packet-loss robustness factor  $Bpl$ . With the packet-loss probability  $Ppl$ ,  $Ie, eff, WB$  is calculated using the equation:
+
+$$Ie, eff, WB = Ie, WB + (95 - Ie, WB) \cdot \frac{Ppl}{Ppl + Bpl} \quad (7-15)$$
+
+As can be seen from Equation 7-15, the effective equipment impairment factor in case of  $Ppl = 0$  (no packet-loss) is equal to the  $Ie, WB$  value defined in Appendix IV of [ITU-T G.113].
+
+Corresponding values for  $Bpl, wb$  can be found in Appendix IV of [ITU-T G.113].
+
+One should derive  $Ie, eff, WB$  by using the  $Ie, wb$  and  $Bpl$  values if they are provided in [ITU-T G.113]. If, for practical reasons, it is difficult to observe the packet-loss rate ( $Ppl$ ), one can use the approach found in [ITU-T P.834.1] to directly derive  $Ie, eff, WB$ .
+
+If  $Ie, wb$  is derived directly by using the instrumental method recommended in [ITU-T P.834.1], it already reflects the effect of packet-loss introduced in the preparation of speech materials under test. Therefore, one should not use equation (7-15). Rather, one should use the  $Ie, wb$  value derived by [ITU-T P.834.1] in  $Ie, eff, WB$  in equation (7-1).
+
+## 7.6 Advantage factor, $A$
+
+Background information on the advantage factor  $A$  can be found in Appendix II of [ITU-T G.113]. As this effect has not yet been studied for the WB case, it is recommended to set:
+
+$$A = 0 \quad (7-16)$$
+
+## 7.7 Default values
+
+For all input parameters used in the algorithm of the E-model, the default values are listed in Table 1. It is strongly recommended to use these default values for all parameters which are not varied during planning calculation.
+
+**Table 1 – Default values and permitted ranges for the parameters**
+
+| Parameter                               | Abbr.  | Unit  | Default value | Permitted range | Remark       |
+|-----------------------------------------|--------|-------|---------------|-----------------|--------------|
+| Send loudness rating                    | SLR    | dB    | +8            | -               | (Note 3)     |
+| Receive loudness rating                 | RLR    | dB    | +2            | -               | (Note 3)     |
+| Sidetone masking rating                 | STMR   | dB    | 15            | 10 ... 20       | (Note 2)     |
+| Listener sidetone rating                | LSTR   | dB    | 18            | 13 ... 23       | (Note 2)     |
+| D-Value of telephone, send side         | Ds     | –     | 3             | -               | (Note 3)     |
+| D-Value of telephone, receive side      | Dr     | –     | 3             | -               | (Note 3)     |
+| Talker echo loudness rating             | TELR   | dB    | 65            | 5 ... 65        |              |
+| Weighted echo path loss                 | WEPL   | dB    | 110           | 5 ... 110       |              |
+| Mean one-way delay of the echo path     | T      | ms    | 0             | 0 ... 500       |              |
+| Round-trip delay in a 4-wire loop       | Tr     | ms    | 0             | 0 ... 1000      |              |
+| Absolute delay in echo-free connections | Ta     | ms    | 0             | 0 ... 500       |              |
+| Equipment impairment factor             | Ie, WB | –     | 0             | 0 ... 56        | (Note 4)     |
+| Packet-loss robustness factor           | Bpl    | –     | 4.3           | 4.3 ... 7.3     | (Notes 3, 4) |
+| Random packet-loss probability          | Ppl    | %     | 0             | 0 ... 20        | (Notes 3, 4) |
+| Circuit noise referred to 0 dBr-point   | Nc     | dBm0p | –70           | -               | (Note 3)     |
+| Noise floor at the receive side         | Nfor   | dBmp  | –96           | –               | (Note 3)     |
+| Room noise at the send side             | Ps     | dB(A) | 35            | -               | (Note 3)     |
+| Room noise at the receive side          | Pr     | dB(A) | 35            | -               | (Note 3)     |
+| Advantage factor                        | A      | –     | 0             | 0 ... 20        |              |
+
+NOTE 1 – Total values between microphone or receiver and 0 dBr-point.  
+NOTE 2 – Fixed relation: LSTR = STMR + D.  
+NOTE 3 – Currently under study.  
+NOTE 4 – If Ppl > 0%, then the Bpl must match the codec, packet size, and PLC assumed.
+
+## Annex A
+
+### MOS values derived from the transmission rating factor $R$
+
+(This annex forms an integral part of this Recommendation.)
+
+The transmission rating factor  $R$  can be in the range from 0 to 129, where  $R = 0$  represents an extremely bad quality and  $R = 129$  represents a very high quality in the WB case. An estimated mean opinion score ( $\text{MOS}_{\text{CQEW}}$ ) for the conversational situation on the scale 1-5 can be obtained from the  $R$ -factor by using the equations:
+
+$$R_x = \frac{R}{1.29} \quad (\text{A-1})$$
+
+$$\text{For } R_x < 0: \quad \text{MOS}_{\text{CQEW}} = 1$$
+
+$$\text{For } 0 < R_x < 100: \quad \text{MOS}_{\text{CQEW}} = 1 + 0.035 R_x + R_x(R_x - 60)(100 - R_x)7 \cdot 10^{-6} \quad (\text{A-2})$$
+
+$$\text{For } R_x > 100: \quad \text{MOS}_{\text{CQEW}} = 4.5$$
+
+## Bibliography
+
+- [b-ITU-T G.108] Recommendation ITU-T G.108 (1999), *Application of the E-model: A planning guide*.
+- [b-ITU-T G.108.1] Recommendation ITU-T G.108.1 (2000), *Guidance for assessing conversational speech transmission quality effects not covered by the E-model*.
+- [b-ITU-T G.109] Recommendation ITU-T G.109 (1999), *Definition of categories for speech transmission quality*.
+- [b-Barriac] Barriac, V., Le Saout, J.-Y., and Lockwood C. (2004), *Discussion on Unified Methodologies for the Comparison of Voice Quality of Narrowband and Wideband Scenarios*, ETSI Workshop on Wideband Speech Quality in Terminals and Networks: Assessment and Prediction, pp. 75-79.
+- [b-Möller-01] Möller, S. (2000), *Assessment and Prediction of Speech Quality in Telecommunications*, Springer.
+- [b-Möller-02] Möller, S., Raake, A., Kitawaki, N., Takahashi, A., Wältermann, M. (2006), *Impairment Factor Framework for Wideband Speech Codecs*, IEEE Trans. Audio, Speech and Language Processing 14(6), pp. 1969-1976.
+- [b-Raake-01] Raake, A. (2006), *Speech Quality of VoIP – Assessment and Prediction*, Chichester, UK, Wiley.
+- [b-Raake-02] Raake, A., Möller, S., Wältermann, M., Côté, N., Ramirez, J.-P. (2010), *Parameter-based Prediction of Speech Quality in Listening Context – Towards a WB E-Model*, in: Second International Workshop on Quality of Multimedia Experience (QoMEX'10), June 21-23, Trondheim, 182-187.
+- [b-Takahashi] Takahashi, A., Kurashima, A., and Yoshino, H. (2005), *Subjective Quality Index for Compatibly Evaluating Narrowband and Wideband Speech*, Prague, Czech Republic, MESAQIN.
+
+
+
+## SERIES OF ITU-T RECOMMENDATIONS
+
+|                 |                                                                                                                                                           |
+|-----------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------|
+| Series A        | Organization of the work of ITU-T                                                                                                                         |
+| Series D        | Tariff and accounting principles and international telecommunication/ICT economic and policy issues                                                       |
+| Series E        | Overall network operation, telephone service, service operation and human factors                                                                         |
+| Series F        | Non-telephone telecommunication services                                                                                                                  |
+| <b>Series G</b> | <b>Transmission systems and media, digital systems and networks</b>                                                                                       |
+| Series H        | Audiovisual and multimedia systems                                                                                                                        |
+| Series I        | Integrated services digital network                                                                                                                       |
+| Series J        | Cable networks and transmission of television, sound programme and other multimedia signals                                                               |
+| Series K        | Protection against interference                                                                                                                           |
+| Series L        | Environment and ICTs, climate change, e-waste, energy efficiency; construction, installation and protection of cables and other elements of outside plant |
+| Series M        | Telecommunication management, including TMN and network maintenance                                                                                       |
+| Series N        | Maintenance: international sound programme and television transmission circuits                                                                           |
+| Series O        | Specifications of measuring equipment                                                                                                                     |
+| Series P        | Telephone transmission quality, telephone installations, local line networks                                                                              |
+| Series Q        | Switching and signalling, and associated measurements and tests                                                                                           |
+| Series R        | Telegraph transmission                                                                                                                                    |
+| Series S        | Telegraph services terminal equipment                                                                                                                     |
+| Series T        | Terminals for telematic services                                                                                                                          |
+| Series U        | Telegraph switching                                                                                                                                       |
+| Series V        | Data communication over the telephone network                                                                                                             |
+| Series X        | Data networks, open system communications and security                                                                                                    |
+| Series Y        | Global information infrastructure, Internet protocol aspects, next-generation networks, Internet of Things and smart cities                               |
+| Series Z        | Languages and general software aspects for telecommunication systems                                                                                      |

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+
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.1091**
+
+(10/2014)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+Multimedia Quality of Service and performance – Generic  
+and user-related aspects
+
+# --- **Quality of Experience requirements for telepresence services**
+
+Recommendation ITU-T G.1091
+
+## ITU-T G-SERIES RECOMMENDATIONS **TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS**
+
+|                                                                                                                                              |                      |
+|----------------------------------------------------------------------------------------------------------------------------------------------|----------------------|
+| INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS                                                                                             | G.100–G.199          |
+| GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS                                                                  | G.200–G.299          |
+| INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES                                                      | G.300–G.399          |
+| GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES | G.400–G.449          |
+| COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY                                                                                            | G.450–G.499          |
+| TRANSMISSION MEDIA AND OPTICAL SYSTEMS CHARACTERISTICS                                                                                       | G.600–G.699          |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.700–G.799          |
+| DIGITAL NETWORKS                                                                                                                             | G.800–G.899          |
+| DIGITAL SECTIONS AND DIGITAL LINE SYSTEM                                                                                                     | G.900–G.999          |
+| <b>MULTIMEDIA QUALITY OF SERVICE AND PERFORMANCE – GENERIC AND USER-RELATED ASPECTS</b>                                                      | <b>G.1000–G.1999</b> |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.6000–G.6999        |
+| DATA OVER TRANSPORT – GENERIC ASPECTS                                                                                                        | G.7000–G.7999        |
+| PACKET OVER TRANSPORT ASPECTS                                                                                                                | G.8000–G.8999        |
+| ACCESS NETWORKS                                                                                                                              | G.9000–G.9999        |
+
+*For further details, please refer to the list of ITU-T Recommendations.*
+
+# Recommendation ITU-T G.1091
+
+# Quality of Experience requirements for telepresence services
+
+## Summary
+
+Telepresence is an interactive audio-visual communications experience between remote locations, where users enjoy a strong sense of realism and presence between participants by optimizing a variety of attributes such as audio and video quality, eye contact, gaze awareness, body language, spatial audio, coordinated environments and natural image size. In this regard, delivering a high-level of quality of experience (QoE) is an important factor. Recommendation ITU-T G.1091 provides guidelines for achieving high QoE in telepresence services. First, a high-level description of a telepresence system is described, with an overview of important use cases. Then, specific audio and visual parameters, and other characteristics that are important from a QoE perspective are defined. Where possible, specific values for the parameters necessary to achieve high QoE are recommended, along with references to associated test methodologies.
+
+## History
+
+| Edition | Recommendation | Approval   | Study Group | Unique ID*                                                                |
+|---------|----------------|------------|-------------|---------------------------------------------------------------------------|
+| 1.0     | ITU-T G.1091   | 2014-10-29 | 12          | <a href="http://handle.itu.int/11.1002/1000/12310">11.1002/1000/12310</a> |
+
+## Keywords
+
+QoE, telepresence.
+
+---
+
+\* To access the Recommendation, type the URL <http://handle.itu.int/> in the address field of your web browser, followed by the Recommendation's unique ID. For example, <http://handle.itu.int/11.1002/1000/11830-en>.
+
+## FOREWORD
+
+The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications, information and communication technologies (ICTs). The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
+
+The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics.
+
+The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
+
+In some areas of information technology which fall within ITU-T's purview, the necessary standards are prepared on a collaborative basis with ISO and IEC.
+
+## NOTE
+
+In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure, e.g., interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some other obligatory language such as "must" and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party.
+
+## INTELLECTUAL PROPERTY RIGHTS
+
+ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process.
+
+As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementers are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database at <http://www.itu.int/ITU-T/ipr/>.
+
+© ITU 2015
+
+All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.
+
+## Table of Contents
+
+###### Page
+
+|      |                                                               |   |
+|------|---------------------------------------------------------------|---|
+| 1    | Scope.....                                                    | 1 |
+| 2    | References.....                                               | 1 |
+| 3    | Definitions .....                                             | 1 |
+| 3.1  | Terms defined elsewhere .....                                 | 1 |
+| 3.2  | Terms defined in this Recommendation.....                     | 2 |
+| 4    | Abbreviations and acronyms .....                              | 2 |
+| 5    | Conventions .....                                             | 2 |
+| 5.1  | Requirement terminology .....                                 | 2 |
+| 5.2  | Icons used .....                                              | 2 |
+| 6    | High-level QoE considerations for telepresence services ..... | 3 |
+| 6.1  | Telepresence system description .....                         | 4 |
+| 6.2  | Use cases .....                                               | 4 |
+| 7    | Audio characteristics .....                                   | 5 |
+| 7.1  | Send and receive loudness ratings .....                       | 5 |
+| 7.2  | Send and receive side frequency responses.....                | 5 |
+| 7.3  | Send and receive side distortion .....                        | 6 |
+| 7.4  | Send and receive side noise .....                             | 6 |
+| 7.5  | Directionality of audio (spatial characteristics).....        | 6 |
+| 7.6  | Acoustic echo/double talk .....                               | 6 |
+| 7.7  | Intelligibility of audio.....                                 | 6 |
+| 8    | Video characteristics.....                                    | 6 |
+| 8.1  | Actual size .....                                             | 6 |
+| 8.2  | Resolution (definition).....                                  | 6 |
+| 8.3  | Image quality .....                                           | 6 |
+| 8.4  | Eye gaze.....                                                 | 7 |
+| 8.5  | Spatial considerations for video .....                        | 7 |
+| 9    | Display characteristics .....                                 | 7 |
+| 9.1  | Screen size .....                                             | 7 |
+| 9.2  | Aspect ratios .....                                           | 7 |
+| 9.3  | Continuity across borders .....                               | 7 |
+| 9.4  | Colour space (4:2:2, 4:2:0, 4:4:4).....                       | 7 |
+| 9.5  | Colour fidelity .....                                         | 7 |
+| 9.6  | Mirrored image.....                                           | 7 |
+| 10   | Audiovisual characteristics .....                             | 7 |
+| 10.1 | Absolute delay of audio and video streams .....               | 8 |
+| 10.2 | Lip sync .....                                                | 8 |
+| 11   | Room characteristics.....                                     | 8 |
+
+|                                           | Page |
+|-------------------------------------------|------|
+| 11.1 Background noise .....               | 8    |
+| 11.2 Reverberation .....                  | 8    |
+| 11.3 Lighting .....                       | 8    |
+| 11.4 Sound uniformity .....               | 8    |
+| 11.5 Impact of seating arrangements ..... | 8    |
+| 11.6 Temperature .....                    | 8    |
+| 11.7 Humidity .....                       | 8    |
+| 11.8 Audibility in local room .....       | 8    |
+| 12 Other characteristics .....            | 9    |
+| 12.1 Ease of use .....                    | 9    |
+| Bibliography .....                        | 10   |
+
+# Recommendation ITU-T G.1091
+
+# Quality of Experience requirements for telepresence services
+
+# 1 Scope
+
+This Recommendation provides guidelines for achieving high quality of experience (QoE) in telepresence services. First, a high-level description of a telepresence system is described, with an overview of important use cases. Then, specific audio and visual parameters, and other characteristics that are important from a QoE perspective are defined. Where possible, specific values for the parameters necessary to achieve high QoE are recommended, along with references to associated test methodologies. The list of parameters and requirements is not to be considered as an exhaustive list, and it is expected that this Recommendation will be updated as more experience is gained with telepresence systems.
+
+# 2 References
+
+The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
+
+- [ITU-T F.734] Recommendation ITU-T F.734 (2014), *Definitions, requirements and use cases for telepresence systems*.
+- [ITU-T G.131] Recommendation ITU-T G.131 (2003), *Talker echo and its control*.
+- [ITU-T H.420] Recommendation ITU-T H.420 (2014), *Telepresence system architecture*.
+- [ITU-T P.300] Recommendation ITU-T P.300 (2001), *Transmission performance of group audio terminals (GATs)*.
+
+# 3 Definitions
+
+## 3.1 Terms defined elsewhere
+
+This Recommendation uses the following terms defined elsewhere:
+
+- 3.1.1 actual size** [ITU-T F.734]: A rendered figure in a display is the same size as if the person is in the room.
+- 3.1.2 gaze awareness** [ITU-T F.734]: Awareness of gaze direction of persons by watching their eyes, head and body position. There is awareness of both direct gaze and averted gaze. Direct gaze is the perception of persons looking at each other directly and simultaneously. Averted gaze is the perception that other persons are looking at someone/something else in the environment.
+- 3.1.3 gaze direction** [ITU-T F.734]: The vector positioned along the visual axis, pointing from the fovea of the looker through the centre of the pupil to the gazed-at spot.
+- 3.1.4 eye contact** [ITU-T F.734]: Technically equivalent to a mutual direct-gaze between two persons communicating.
+- 3.1.5 render** [b-IETF RFC 7262]: The process of generating a representation from a media, such as displayed motion video or sound emitted from loudspeakers.
+
+**3.1.6 telepresence** [ITU-T F.734]: An interactive audio-visual communications experience between remote locations, where the users enjoy a strong sense of realism and presence between participants by optimizing a variety of attributes such as audio and video quality, eye contact, gaze awareness, body language, spatial audio, coordinated environments and natural image size.
+
+**3.1.7 telepresence system** [ITU-T H.420]: A set of functions, devices and network elements which are able to capture, deliver, manage and render multiple high quality interactive audio and video signals in a telepresence conference. An appropriate number of devices (e.g., cameras, screens, loudspeakers, microphones, codecs, multipoint control units, PCs) and environmental characteristics are used to establish telepresence.
+
+## **3.2 Terms defined in this Recommendation**
+
+None.
+
+# **4 Abbreviations and acronyms**
+
+This Recommendation uses the following abbreviations and acronyms:
+
+|      |                              |
+|------|------------------------------|
+| CRC  | Call and Resource Controller |
+| GW   | Gateway                      |
+| IP   | Internet Protocol            |
+| MCU  | Multipoint Control Unit      |
+| PC   | Personal Computer            |
+| PLMN | Public Land Mobile Network   |
+| QoE  | Quality of Experience        |
+
+# **5 Conventions**
+
+## **5.1 Requirement terminology**
+
+In this Recommendation the following conventions are used:
+
+- "Shall" indicates a mandatory requirement.
+- "Should" indicates a suggested but optional course of action.
+- "May" indicates an optional course of action rather than a recommendation that something take place.
+
+## **5.2 Icons used**
+
+Figure 1 shows the meaning of icons used in this Recommendation.
+
+![A grid of 28 icons representing various telepresence and network components. The icons are arranged in 7 rows and 4 columns. Row 1: Single-screen telepresence endpoint (one screen with two people), Two-screen telepresence endpoint (two screens, one with two people, one with a bar chart), Three-screen telepresence endpoint (three screens, each with two people), Document camera (a physical camera). Row 2: Gate keeper (a box with two people and a shield), ITU-T H.320 endpoint (a small box labeled 'ITU-T H.320'), Legacy ITU-T H.323 endpoint (a box with a screen), Loudspeaker (a circular speaker). Row 3: Main camera (three camera icons), Management system (a laptop showing a network diagram), Main display (three screens, each with two people), Microphone (a vintage microphone). Row 4: Mobile phone (a mobile phone), Multipoint control unit (a box with three people and arrows), Personal computer (a laptop), Phone (a desktop telephone). Row 5: Presentation display (a monitor labeled 'HDTV'), Router (a circular icon with four arrows), SIP endpoint (a box labeled 'SIP'), Switch (a box with five horizontal lines). Row 6: Telepresence control unit (a box with people and a 'DBS' icon), Telepresence media codec (a box with a vertical bar and arrows), Telepresence media input/output control unit (a box with four arrows in a circle), Telepresence media processor (a box with three horizontal lines and arrows). Row 7: Touch control panel (a hand touching a screen), Video/voice gateway (a circular icon with 'X' and 'V' and arrows). The text 'G.1091(14)_F01' is in the bottom right corner.](fd955384881fd240be5518d3050588d9_img.jpg)
+
+A grid of 28 icons representing various telepresence and network components. The icons are arranged in 7 rows and 4 columns. Row 1: Single-screen telepresence endpoint (one screen with two people), Two-screen telepresence endpoint (two screens, one with two people, one with a bar chart), Three-screen telepresence endpoint (three screens, each with two people), Document camera (a physical camera). Row 2: Gate keeper (a box with two people and a shield), ITU-T H.320 endpoint (a small box labeled 'ITU-T H.320'), Legacy ITU-T H.323 endpoint (a box with a screen), Loudspeaker (a circular speaker). Row 3: Main camera (three camera icons), Management system (a laptop showing a network diagram), Main display (three screens, each with two people), Microphone (a vintage microphone). Row 4: Mobile phone (a mobile phone), Multipoint control unit (a box with three people and arrows), Personal computer (a laptop), Phone (a desktop telephone). Row 5: Presentation display (a monitor labeled 'HDTV'), Router (a circular icon with four arrows), SIP endpoint (a box labeled 'SIP'), Switch (a box with five horizontal lines). Row 6: Telepresence control unit (a box with people and a 'DBS' icon), Telepresence media codec (a box with a vertical bar and arrows), Telepresence media input/output control unit (a box with four arrows in a circle), Telepresence media processor (a box with three horizontal lines and arrows). Row 7: Touch control panel (a hand touching a screen), Video/voice gateway (a circular icon with 'X' and 'V' and arrows). The text 'G.1091(14)\_F01' is in the bottom right corner.
+
+**Figure 1 – Icons used in this Recommendation**
+
+# 6 High-level QoE considerations for telepresence services
+
+This clause provides a high-level description of telepresence systems. Some scenarios and use cases are also provided to provide a context for assessing the important factors from the point of view of QoE.
+
+## 6.1 Telepresence system description
+
+The term "telepresence systems" is used to refer to conferencing systems that provide high definition and high quality audio/video, enabling a "being-there" experience. A telepresence system includes endpoints (both telepresence and legacy), multipoint control unit (MCU), call and resource controller (CRC), gateway (GW), management system, etc., as shown in Figure 2. [ITU-T H.420], "Telepresence System Architecture", provides a detailed description of a telepresence system.
+
+![Figure 2: High-level telepresence architecture network model. This diagram shows a central cloud labeled 'IP network' connected to various devices. On the left, a 'PLMN' cloud connects to a mobile phone and a desktop phone. A central router connects to a 'SIP' server and an 'ITU-T H.320' gateway. The 'IP network' cloud is also connected to several telepresence endpoints, represented by icons of people and screens. Some endpoints are connected directly to the IP network, while others are connected through intermediate routers or gateways. The diagram illustrates a complex network architecture for telepresence systems.](cfda9df1319e04207eb28bcefd1dab7b_img.jpg)
+
+Figure 2: High-level telepresence architecture network model. This diagram shows a central cloud labeled 'IP network' connected to various devices. On the left, a 'PLMN' cloud connects to a mobile phone and a desktop phone. A central router connects to a 'SIP' server and an 'ITU-T H.320' gateway. The 'IP network' cloud is also connected to several telepresence endpoints, represented by icons of people and screens. Some endpoints are connected directly to the IP network, while others are connected through intermediate routers or gateways. The diagram illustrates a complex network architecture for telepresence systems.
+
+**Figure 2 – High-level telepresence architecture network model**
+
+## 6.2 Use cases
+
+Clause 6 of [ITU-T F.734], "Definitions, requirements, and use cases for telepresence systems", presents a set of use cases describing typical scenarios. The most common and basic use cases are introduced and illustrated in detail, which are described from the viewpoint of the users.
+
+### 6.2.1 Use case 1: Symmetric point-to-point meeting
+
+In this use case, each of the two sites has an identical number of screens, with cameras having fixed fields of view, and one camera for each screen. The number of screens or cameras is not relevant as long as both sites have the same number. The sound type is the same at each end. Figure 3 illustrates the use case.
+
+![Figure 3: Use case 1: Symmetric point-to-point meeting. This diagram shows two identical 'Two-screen telepresence endpoint' units connected via a central cloud labeled 'IP network'. Each endpoint consists of two screens: one showing two people and the other showing a bar chart. Above each screen is a camera. The endpoints are connected to the IP network cloud, which in turn connects to the other endpoint, illustrating a symmetric point-to-point meeting setup.](54fabc351eda5228d2fa28cd9ba07971_img.jpg)
+
+Figure 3: Use case 1: Symmetric point-to-point meeting. This diagram shows two identical 'Two-screen telepresence endpoint' units connected via a central cloud labeled 'IP network'. Each endpoint consists of two screens: one showing two people and the other showing a bar chart. Above each screen is a camera. The endpoints are connected to the IP network cloud, which in turn connects to the other endpoint, illustrating a symmetric point-to-point meeting setup.
+
+**Figure 3 – Use case 1: Symmetric point-to-point meeting**
+
+For further information, see clause 3.1 of [b-IETF RFC 7205].
+
+### 6.2.2 Use case 2: Multipoint meeting
+
+In this use case of a multipoint telepresence conference, there are more than two sites participating. Figure 4 illustrates the use case. The number of screens or cameras can be different or the same. Each site can have different audio/video characteristics.
+
+For further information, see clause 3.3 of [b-IETF RFC 7205].
+
+![Diagram illustrating Use case 2: Multipoint meeting. A central cloud labeled 'IP network' is connected to three telepresence endpoints. The top-left endpoint is a 'Two-screen telepresence endpoint' with two screens. The top-right endpoint is a 'Three-screen telepresence endpoint' with three screens. The bottom endpoint is also a 'Three-screen telepresence endpoint' with three screens. Each screen displays a person icon, and each endpoint has three camera icons above its screens. The diagram is labeled G.1091(14)_F04 in the bottom right corner.](5a4e62bead259c258d069fd3663ea670_img.jpg)
+
+Diagram illustrating Use case 2: Multipoint meeting. A central cloud labeled 'IP network' is connected to three telepresence endpoints. The top-left endpoint is a 'Two-screen telepresence endpoint' with two screens. The top-right endpoint is a 'Three-screen telepresence endpoint' with three screens. The bottom endpoint is also a 'Three-screen telepresence endpoint' with three screens. Each screen displays a person icon, and each endpoint has three camera icons above its screens. The diagram is labeled G.1091(14)\_F04 in the bottom right corner.
+
+Figure 4 – Use case 2: Multipoint meeting
+
+# 7 Audio characteristics
+
+This clause describes the audio characteristics related to QoE of telepresence services in more detail.
+
+## 7.1 Send and receive loudness ratings
+
+Telepresence systems shall send audio to other telepresence systems with a SendLoudnessRating of  $(+13 - F_s)$  dB, measured in accordance with [ITU-T P.300].
+
+In order to take into account the difference between the reference test positioning and the actual microphone-talker operating distance ( $d_s$ ), for which the terminal is adjusted, the following correction factor  $F_s$  is defined:
+
+$$F_s[\text{dB}] = 20 \text{ Log} \left( \frac{d_s}{0.5} \right) \quad (d_s \text{ in metres})$$
+
+Telepresence systems shall receive audio from other telepresence systems with a nominal Receive Loudness Rating of  $(+5 - F_r)$  dB, measured in accordance with [ITU-T P.300].
+
+In order to take into account the difference between the reference test positioning and the actual loudspeaker-listener operating distance ( $d_r$ ) for which the terminal is adjusted, the following correction factor  $F_r$  is defined:
+
+$$F_r[\text{dB}] = 20 \text{ Log} \left( \frac{d_r}{0.5} \right) \quad (d_r \text{ in metres})$$
+
+## 7.2 Send and receive side frequency responses
+
+Telepresence systems shall provide audio with send and receive side frequency responses from 100 Hz to 20 kHz.
+
+## **7.3 Send and receive side distortion**
+
+Telepresence systems shall provide audio with send and receive side distortion less than 3%. Note, different specifications exist for send and receive.
+
+For send side distortion the requirement is 2% (1% is a desired objective).
+
+For received side distortion the requirement is 3%.
+
+## **7.4 Send and receive side noise**
+
+Send side noise should be less than –63 dBm0p. Receive side noise should be less than 40 dBA (under maximum volume).
+
+### **7.5 Directionality of audio (spatial characteristics)**
+
+In telepresence systems, all the seats shall sense directions (e.g., left, middle, right for three screen telepresence endpoint) for rendering.
+
+The person who is talking shall be identified in correct spatial geometry.
+
+## **7.6 Acoustic echo/double talk**
+
+Single talk: The weighted terminal coupling loss (TCLw) should be greater than or equal to 45 dB.
+
+Double talk: Talker echo loss rating (TELR, as defined in [ITU-T G.131]) double talk should be greater than 27 dB.
+
+## **7.7 Intelligibility of audio**
+
+Telepresence systems shall provide audio with intelligibility greater than 94%.
+
+# **8 Video characteristics**
+
+This clause describes the video characteristics related to QoE of telepresence services in more detail.
+
+## **8.1 Actual size**
+
+Telepresence systems shall provide an actual-sized video display as far as possible:
+
+The difference in size between the original and displayed video should be limited within 10% of the original.
+
+## **8.2 Resolution (definition)**
+
+Telepresence systems shall provide video with resolution higher than 720 P.
+
+Telepresence systems shall provide a harmonic view among image displays of different resolutions.
+
+(One example could be that multiple video captures of different resolutions are to be displayed in one room.)
+
+## **8.3 Image quality**
+
+Telepresence systems shall provide images without noticeable geometric distortion.
+
+The overall geometric distortion should be less than 10%.
+
+Telepresence systems shall provide images without noticeable noise.
+
+## **8.4 Eye gaze**
+
+The remote participants' video images should be rendered within a tolerable angle in both horizontal and vertical axes from the gaze direction between the two communicating participants. The tolerable angles should be 3-5 degrees horizontal and up to 12 degrees upward and 8 degrees downward.
+
+## **8.5 Spatial considerations for video**
+
+Telepresence systems shall keep the spatial relationship of video as of physical relationship of participants.
+
+Sync of videos in different screens should also be considered.
+
+# **9 Display characteristics**
+
+This clause describes the display characteristics related to QoE of telepresence services in more detail. In contrast to the video characteristics, displays characteristics focus on physical arrangement for screens, consistency across multiple screens, etc.
+
+## **9.1 Screen size**
+
+Ideally, all screens of a telepresence room should be of the same size. However, if different sizes are used in the same room, they should display images for different purposes.
+
+## **9.2 Aspect ratios**
+
+Telepresence systems shall provide a harmonized view among screens of different aspect ratios.
+
+The displayed image size should not differ by more than 5% from the original image size even when different aspect ratios are used.
+
+## **9.3 Continuity across borders**
+
+Telepresence systems shall provide a continuous view across frame borders of adjacent screens.
+
+This is related to clause 9.1 and also illustrated in Figure 10 in clause 7 of [ITU-T F.734].
+
+The width of the discontinuous part should be less than 3 cm.
+
+### **9.4 Colour space (4:2:2, 4:2:0, 4:4:4)**
+
+Telepresence systems shall provide a harmonized view among displays of different colour spaces.
+
+## **9.5 Colour fidelity**
+
+Telepresence systems shall ensure unnoticeable change in displayed colour, especially for skin colour and white balancing.
+
+## **9.6 Mirrored image**
+
+Telepresence systems shall not provide mirrored images of participants by default.
+
+When the local images are displayed on the local screens, it is recommended to provide mirrored images for the participants.
+
+# **10 Audiovisual characteristics**
+
+This clause describes the audiovisual characteristics related to QoE of telepresence services in more detail.
+
+## **10.1 Absolute delay of audio and video streams**
+
+- For telepresence systems: the end-to-end video delay shall be less than or equal to 320 ms.
+- End-to-end audio delay shall be less than or equal to 280 ms.
+
+## **10.2 Lip sync**
+
+Audio streams should be less than 40 ms ahead of video streams, or less than 60 ms behind video streams.
+
+# **11 Room characteristics**
+
+### **11.1 Background noise**
+
+Telepresence systems shall enjoy a comfortable environment with background noise less than 40 dBA.
+
+## **11.2 Reverberation**
+
+Early reflecting sound should be between 1 kHz and 8 kHz and 15 ms after direct sound should attenuate 10 dB against direct sound.
+
+## **11.3 Lighting**
+
+Telepresence systems shall provide a suitable lighting mechanism in the room, which includes but is not limited to, gamut, colour temperature, colour rendering index, type of illumination, etc.
+
+Telepresence systems should provide a colour temperature of 6'500 K, a luminance of 250 Lux.
+
+## **11.4 Sound uniformity**
+
+Telepresence systems shall provide a uniform sound effect (to within 3 dB) for users in the same room.
+
+## **11.5 Impact of seating arrangements**
+
+Please note that not all people share the same QoE in the room.
+
+The maximum QoE difference should be less than 20%.
+
+### **11.6 Temperature**
+
+The room temperature should be set between 10~25 C, a suggested temperature is 18 C.
+
+## **11.7 Humidity**
+
+The relative humidity should be between 40% and 60%.
+
+## **11.8 Audibility in local room**
+
+Audibility for a single word should be greater than or equal to 90%.
+
+# **12 Other characteristics**
+
+## **12.1 Ease of use**
+
+The user interface for frequently used functions shall be optimized. The number of pushes or touches should be no more than five.
+
+A tutorial should be provided.
+
+The set-up of a telepresence call shall not exceed seven seconds.
+
+## Bibliography
+
+- [b-IETF RFC 7205] IETF RFC 7205 (2014), *Use Cases for Telepresence Multistreams.*
+- [b-IETF RFC 7262] IETF RFC 7262 (2014), *Requirements for Telepresence Multistreams.*
+
+
+
+## SERIES OF ITU-T RECOMMENDATIONS
+
+|                 |                                                                                             |
+|-----------------|---------------------------------------------------------------------------------------------|
+| Series A        | Organization of the work of ITU-T                                                           |
+| Series D        | General tariff principles                                                                   |
+| Series E        | Overall network operation, telephone service, service operation and human factors           |
+| Series F        | Non-telephone telecommunication services                                                    |
+| <b>Series G</b> | <b>Transmission systems and media, digital systems and networks</b>                         |
+| Series H        | Audiovisual and multimedia systems                                                          |
+| Series I        | Integrated services digital network                                                         |
+| Series J        | Cable networks and transmission of television, sound programme and other multimedia signals |
+| Series K        | Protection against interference                                                             |
+| Series L        | Construction, installation and protection of cables and other elements of outside plant     |
+| Series M        | Telecommunication management, including TMN and network maintenance                         |
+| Series N        | Maintenance: international sound programme and television transmission circuits             |
+| Series O        | Specifications of measuring equipment                                                       |
+| Series P        | Terminals and subjective and objective assessment methods                                   |
+| Series Q        | Switching and signalling                                                                    |
+| Series R        | Telegraph transmission                                                                      |
+| Series S        | Telegraph services terminal equipment                                                       |
+| Series T        | Terminals for telematic services                                                            |
+| Series U        | Telegraph switching                                                                         |
+| Series V        | Data communication over the telephone network                                               |
+| Series X        | Data networks, open system communications and security                                      |
+| Series Y        | Global information infrastructure, Internet protocol aspects and next-generation networks   |
+| Series Z        | Languages and general software aspects for telecommunication systems                        |

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+
+
+![ITU logo](2dfa6ac3edfe874f68aa0cbccaa42322_img.jpg)
+
+The logo of the International Telecommunication Union (ITU) features the letters 'ITU' in a bold, sans-serif font, superimposed on a stylized globe with intersecting lines.
+
+ITU logo
+
+INTERNATIONAL TELECOMMUNICATION UNION
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.114**
+
+(05/2003)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+International telephone connections and circuits – General  
+Recommendations on the transmission quality for an  
+entire international telephone connection
+
+---
+
+**One-way transmission time**
+
+ITU-T Recommendation G.114
+
+---
+
+# ITU-T G-SERIES RECOMMENDATIONS **TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS**
+
+|                                                                                                                                              |                    |
+|----------------------------------------------------------------------------------------------------------------------------------------------|--------------------|
+| INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS                                                                                             | G.100–G.199        |
+| General definitions                                                                                                                          | G.100–G.109        |
+| <b>General Recommendations on the transmission quality for an entire international telephone connection</b>                                  | <b>G.110–G.119</b> |
+| General characteristics of national systems forming part of international connections                                                        | G.120–G.129        |
+| General characteristics of the 4-wire chain formed by the international circuits and national extension circuits                             | G.130–G.139        |
+| General characteristics of the 4-wire chain of international circuits; international transit                                                 | G.140–G.149        |
+| General characteristics of international telephone circuits and national extension circuits                                                  | G.150–G.159        |
+| Apparatus associated with long-distance telephone circuits                                                                                   | G.160–G.169        |
+| Transmission plan aspects of special circuits and connections using the international telephone connection network                           | G.170–G.179        |
+| Protection and restoration of transmission systems                                                                                           | G.180–G.189        |
+| Software tools for transmission systems                                                                                                      | G.190–G.199        |
+| GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS                                                                  | G.200–G.299        |
+| INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES                                                      | G.300–G.399        |
+| GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES | G.400–G.449        |
+| COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY                                                                                            | G.450–G.499        |
+| TESTING EQUIPMENTS                                                                                                                           | G.500–G.599        |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.600–G.699        |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.700–G.799        |
+| DIGITAL NETWORKS                                                                                                                             | G.800–G.899        |
+| DIGITAL SECTIONS AND DIGITAL LINE SYSTEM                                                                                                     | G.900–G.999        |
+| QUALITY OF SERVICE AND PERFORMANCE – GENERIC AND USER-RELATED ASPECTS                                                                        | G.1000–G.1999      |
+| TRANSMISSION MEDIA CHARACTERISTICS                                                                                                           | G.6000–G.6999      |
+| DIGITAL TERMINAL EQUIPMENTS                                                                                                                  | G.7000–G.7999      |
+| DIGITAL NETWORKS                                                                                                                             | G.8000–G.8999      |
+
+*For further details, please refer to the list of ITU-T Recommendations.*
+
+## **ITU-T Recommendation G.114**
+
+# **One-way transmission time**
+
+## **Summary**
+
+This Recommendation provides guidance on the effect of end-to-end one-way delay (sometimes termed latency), and an upper bound one-way network delay.
+
+While it is recommended that a one-way delay of 400 ms should not be exceeded for general network planning, it is important to appreciate that highly interactive tasks (e.g., many voice calls, interactive data applications, video conferencing) can be affected by much lower delays.
+
+The effects of delays below 500 ms on conversational speech are estimated using a curve derived from the E-model (ITU-T Rec. G.107).
+
+This version constitutes a major revision of this Recommendation in order to align with other ITU-T Recommendations of the G.100 series.
+
+###### **Source**
+
+ITU-T Recommendation G.114 was approved by ITU-T Study Group 12 (2001-2004) under the ITU-T Recommendation A.8 procedure on 6 May 2003. It includes the modifications introduced by ITU-T Rec. G.114 (2003) Appendix II approved on 30 September 2003.
+
+## FOREWORD
+
+The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of telecommunications. The ITU Telecommunication Standardization Sector (ITU-T) is a permanent organ of ITU. ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
+
+The World Telecommunication Standardization Assembly (WTSA), which meets every four years, establishes the topics for study by the ITU-T study groups which, in turn, produce Recommendations on these topics.
+
+The approval of ITU-T Recommendations is covered by the procedure laid down in WTSA Resolution 1.
+
+In some areas of information technology which fall within ITU-T's purview, the necessary standards are prepared on a collaborative basis with ISO and IEC.
+
+## NOTE
+
+In this Recommendation, the expression "Administration" is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+Compliance with this Recommendation is voluntary. However, the Recommendation may contain certain mandatory provisions (to ensure e.g. interoperability or applicability) and compliance with the Recommendation is achieved when all of these mandatory provisions are met. The words "shall" or some other obligatory language such as "must" and the negative equivalents are used to express requirements. The use of such words does not suggest that compliance with the Recommendation is required of any party.
+
+## INTELLECTUAL PROPERTY RIGHTS
+
+ITU draws attention to the possibility that the practice or implementation of this Recommendation may involve the use of a claimed Intellectual Property Right. ITU takes no position concerning the evidence, validity or applicability of claimed Intellectual Property Rights, whether asserted by ITU members or others outside of the Recommendation development process.
+
+As of the date of approval of this Recommendation, ITU had not received notice of intellectual property, protected by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database.
+
+© ITU 2003
+
+All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU.
+
+## CONTENTS
+
+|                                                                                 | Page |
+|---------------------------------------------------------------------------------|------|
+| 1 Introduction .....                                                            | 1    |
+| 2 References.....                                                               | 1    |
+| 3 Applicability to speech transmission quality – Use of the E-model.....        | 2    |
+| 4 Recommendations for one-way transmission time.....                            | 2    |
+| 5 Estimating end-to-end delay based on assemblies of transmission elements..... | 3    |
+| Annex A – End-to-end delay estimation.....                                      | 4    |
+| A.1 Planning values for the delay of transmission elements.....                 | 4    |
+| A.2 Codec delay .....                                                           | 5    |
+| A.3 Delay due to IP delay variation buffer .....                                | 7    |
+| Appendix I – Delay introduced by coder-related processing.....                  | 8    |
+| Appendix II – Guidance on one-way delay for voice over IP .....                 | 12   |
+| II.1 Introduction .....                                                         | 12   |
+| II.2 Achieving satisfactory delay .....                                         | 12   |
+
+
+
+# One-way transmission time
+
+# 1 Introduction
+
+This Recommendation provides guidance on the effect of end-to-end one-way delay (sometimes termed latency), and an upper bound on one-way network delay. The effect of delay on speech transmission quality can be estimated by the use of a curve derived from the Transmission Rating Model of ITU-T Rec. G.107 [3], which is the recommended ITU-T method for end-to-end speech transmission planning. ITU-T Rec. G.108 [4] gives detailed examples on how to use the model to assess the transmission performance of connections involving various impairments, including one-way delay; and ITU-T Rec. G.109 [5] maps transmission rating predictions of the model into categories of speech transmission quality. Thus, while ITU-T Rec. G.114 provides useful information regarding one-way delay as a parameter by itself, ITU-T Rec. G.107 [3] (and its ITU-T Rec. G.108 [4] and ITU-T Rec. G.109 [5] companions) should be used to assess the effects of delay in conjunction with other impairments (e.g., distortions due to speech processing).
+
+Highly interactive tasks (e.g., some speech, video conferencing and interactive data applications) may be affected by delays below 100 ms, as per test result documented in Annex B of previous versions of ITU-T Rec. G.114. For this reason, previous versions of this Recommendation noted that if delays were kept below 150 ms, then most *applications* would not be significantly affected. Additionally, an upper limit of 400 ms for *network* planning purposes was always a part of ITU-T Rec. G.114. However, this parallel treatment of network delays on one hand, with application ("mouth-to-ear") level delays on the other hand, led to confusion in how ITU-T Rec. G.114 should be applied.
+
+Fortunately, with the development and approval of the E-model (ITU-T Rec. G.107 [3]), which is based on subjective tests of delay (among other parameters), there now exists an agreed way of estimating the effects of delay on mouth-to-ear speech transmission quality.
+
+Accordingly, simple and straightforward guidance can now be provided for the effects of delay on speech transmission and is given in the present Recommendation.
+
+The lack of similar tools for non-speech applications is a subject for further study, so this Recommendation can only provide general planning guidance.
+
+# 2 References
+
+The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. All Recommendations and other references are subject to revision; users of this Recommendation are therefore encouraged to investigate the possibility of applying the most recent edition of the Recommendations and other references listed below. A list of the currently valid ITU-T Recommendations is regularly published. The reference to a document within this Recommendation does not give it, as a stand-alone document, the status of a Recommendation.
+
+- [1] ITU-T Recommendation G.100 (2001), *Definitions used in Recommendations on general characteristics of international telephone connections and circuits*.
+- [2] ITU-T Recommendation G.101 (1996), *The transmission plan*.
+- [3] ITU-T Recommendation G.107 (2003), *The E-Model, a computational model for use in transmission planning*.
+- [4] ITU-T Recommendation G.108 (1999), *Application of the E-model: A planning guide*.
+
+- [5] ITU-T Recommendation G.109 (1999), *Definition of categories of speech transmission quality*.
+- [6] ITU-T Recommendation G.131 (1996), *Control of talker echo*.
+- [7] ITU-T Recommendation G.168 (2002), *Digital network echo cancellers*.
+- [8] ITU-T Recommendation G.763 (1998), *Digital circuit multiplication equipment using G.726 ADPCM and digital speech interpolation*.
+- [9] ITU-T Recommendation G.764 (1990), *Voice packetization – Packetized voice protocols*.
+- [10] ITU-T Recommendation G.766 (1996), *Facsimile demodulation/remodulation for digital circuit multiplication equipment*.
+- [11] ITU-T Recommendation G.767 (1998), *Digital circuit multiplication equipment using 16 kbit/s LD-CELP, digital speech interpolation and facsimile demodulation/remodulation*.
+- [12] ITU-T Recommendation Q.551 (2002), *Transmission characteristics of digital exchanges*.
+- [13] ITU-T Recommendation Y.1541 (2002), *Network performance objectives for IP-based services*.
+
+# 3 Applicability to speech transmission quality – Use of the E-model
+
+This Recommendation provides end-to-end limits for mean one-way delay, independent of other transmission impairments. The need to consider the combined effects of all impairments on speech transmission quality is addressed by the Transmission Rating Model of ITU-T Rec. G.107 [3], which is the recommended ITU-T method for end-to-end speech transmission planning. ITU-T Rec. G.108 [4] gives detailed examples on how to use the model to assess the transmission performance of connections involving various impairments, including delay; and ITU-T Rec. G.109 [5] maps transmission rating predictions of the model into categories of speech transmission quality. Thus, while this Recommendation provides useful information regarding mean one-way delay as a parameter by itself, ITU-T Rec. G.107 [3] (and its ITU-T Rec. G.108 [4] and ITU-T Rec. G.109 [5] companions) should be used to assess the effects of delay in conjunction with other impairments (e.g., distortions due to speech processing).
+
+# 4 Recommendations for one-way transmission time
+
+Regardless of the type of application, it is recommended to not exceed a one-way delay of 400 ms for general network planning (i.e., UNI to UNI, as illustrated, for example, in ITU-T Rec. Y.1541 [13]), a value that allows flexibility in deploying global networks, without making an excessive number of user experiences unacceptable.
+
+However, it is desirable to keep the delays seen by user applications as low as possible. The E-model should be used to estimate the effect of one-way delay (including all delay sources, i.e., "mouth-to-ear") on speech transmission quality for conversational speech as shown below. For non-speech applications such as interactive data or video, there are no agreed-upon assessment tools like the E-model, so the effects of delay on such applications must be carefully monitored. Although a few applications may be slightly affected by end-to-end (i.e., "mouth-to-ear" in the case of speech) delays of less than 150 ms, if delays can be kept below this figure, most applications, both speech and non-speech, will experience essentially transparent interactivity.
+
+While delays above 400 ms are unacceptable for general network planning purposes, it is recognized that in some exceptional cases this limit will be exceeded. An example of such an exception is an unavoidable double satellite hop for a hard-to-reach location, the impact of which can be estimated by use of the advantage factor in the E-model.
+
+Regarding the use of the E-model for speech applications, the effect of delay can be seen in the following graph of Transmission Rating,  $R$ , versus delay. Also shown are the speech quality categories of ITU-T Rec. G.109 [5], which translate the  $R$  values to levels of user acceptance.
+
+![Figure 1/G.114: Determination of the effects of absolute delay by the E-model. The graph plots the E-model rating R (y-axis, 50 to 100) against Mouth-to-ear-delay/ms (x-axis, 0 to 500). The curve starts at R ≈ 94 for 0 ms delay and decreases as delay increases. The graph is divided into five horizontal color-coded regions representing user satisfaction levels: 'Users very satisfied' (green, R ≈ 94-100), 'Users satisfied' (light green, R ≈ 90-94), 'Some users dissatisfied' (yellow, R ≈ 80-90), 'Many users dissatisfied' (pink, R ≈ 70-80), and 'Nearly all users dissatisfied' (red, R ≈ 50-70). The curve crosses the boundaries between these regions at approximately 150 ms, 250 ms, 350 ms, and 450 ms. Beyond 500 ms, the curve continues as a dashed line. The label 'G.114_F01' is in the bottom right corner.](b93cbfb52e37619e688175a6aad9edd9_img.jpg)
+
+Figure 1/G.114: Determination of the effects of absolute delay by the E-model. The graph plots the E-model rating R (y-axis, 50 to 100) against Mouth-to-ear-delay/ms (x-axis, 0 to 500). The curve starts at R ≈ 94 for 0 ms delay and decreases as delay increases. The graph is divided into five horizontal color-coded regions representing user satisfaction levels: 'Users very satisfied' (green, R ≈ 94-100), 'Users satisfied' (light green, R ≈ 90-94), 'Some users dissatisfied' (yellow, R ≈ 80-90), 'Many users dissatisfied' (pink, R ≈ 70-80), and 'Nearly all users dissatisfied' (red, R ≈ 50-70). The curve crosses the boundaries between these regions at approximately 150 ms, 250 ms, 350 ms, and 450 ms. Beyond 500 ms, the curve continues as a dashed line. The label 'G.114\_F01' is in the bottom right corner.
+
+**Figure 1/G.114 – Determination of the effects of absolute delay by the E-model**
+
+NOTE 1 – The curve in Figure 1 is based on the effect of pure delay only, i.e., in the complete absence of any echo. This is calculated by setting the G.107 E-model parameter  $T_a$  equal to the total value of one-way delay from mouth-to-ear, with all other E-model input parameter values set to their default values. The effect of echo, as would be incurred due to imperfect echo control, will result in lower speech quality for a given value of one-way delay.
+
+NOTE 2 – The calculation also assumes an Equipment Impairment Factor ( $I_e$ ) of zero. Non-zero values, as would be incurred due to speech coding/processing, will result in lower speech quality for a given value of one-way delay.
+
+NOTE 3 – For one-way delay values exceeding 500 ms, the graph is continued as a dashed line to indicate that these results are not fully validated, but is the best estimate of what should be expected, and, therefore, provides useful guidance.
+
+# 5 Estimating end-to-end delay based on assemblies of transmission elements
+
+The nominal delay values and general planning rules given in Annex A, and the coder-related delays of Appendix I, may be used to estimate the total end-to-end transmission time.
+
+## Annex A
+
+### End-to-end delay estimation
+
+### A.1 Planning values for the delay of transmission elements
+
+**Table A.1/G.114 – Planning values for the delay of transmission elements**
+
+| Transmission or processing system                                                                                 | Contribution to one-way transmission time | Remarks                                                               |
+|-------------------------------------------------------------------------------------------------------------------|-------------------------------------------|-----------------------------------------------------------------------|
+| Terrestrial coaxial cable or radio-relay system: FDM and digital transmission                                     | 4 µs/km                                   | Allows for delay in repeaters and regenerators                        |
+| Optical fibre cable system, digital transmission                                                                  | 5 µs/km (Note 1)                          |                                                                       |
+| Submarine coaxial cable system                                                                                    | 6 µs/km                                   |                                                                       |
+| Submarine optical fibre system:<br>– transmit terminal<br>– receive terminal                                      | 13 ms<br>10 ms                            | Worst case                                                            |
+| Satellite system:<br>– 400 km altitude<br>– 14 000 km altitude<br>– 36 000 km altitude                            | 12 ms<br>110 ms<br>260 ms                 |                                                                       |
+| FDM channel modulator or demodulator                                                                              | 0.75 ms (Note 2)                          |                                                                       |
+| PLMS (Public Land Mobile System)<br>– objective 40 ms                                                             | 80-110 ms                                 |                                                                       |
+| H.260-series video coders and decoders                                                                            | Further study (Note 3)                    |                                                                       |
+| DCME (ITU-T Rec. G.763 [8]) per pair: for speech, VBD, and non-remodulated fax                                    | 30 ms                                     | Half the sum of transmission times in both directions of transmission |
+| DCME (ITU-T Rec. G.767 [11]) per pair: for speech, VBD, and non-remodulated fax                                   | 30 ms                                     |                                                                       |
+| DCME (ITU-T Rec. G.766 [10] in conjunction with ITU-T Recs G.763 [8] or G.767 [11]) per pair: for remodulated fax | 200 ms                                    |                                                                       |
+| PCME (ITU-T Rec. G.764 [9]) per pair:<br>– with speech and non-remodulated VBD<br>– with remodulated VBD          | 35 ms<br>70 ms                            |                                                                       |
+| Transmultiplexer                                                                                                  | 1.5 ms (Note 4)                           | Half the sum of transmission times in both directions of transmission |
+| Digital transit exchange, digital-digital                                                                         | 0.45 ms (Note 5)                          |                                                                       |
+| Digital local exchange, analogue-analogue                                                                         | 1.5 ms (Note 5)                           |                                                                       |
+| Digital local exchange, analogue subscriber line-digital junction                                                 | 0.975 ms (Note 5)                         |                                                                       |
+| Digital local exchange, digital subscriber line-digital junction                                                  | 0.825 ms (Note 5)                         |                                                                       |
+| Echo cancellers                                                                                                   | 0.5 ms (Note 6)                           |                                                                       |
+| ATM (CBR using AAL 1)                                                                                             | 6.0 ms (Note 7)                           |                                                                       |
+
+**Table A.1/G.114 – Planning values for the delay of transmission elements**
+
+NOTE 1 – This value is provisional and is under study.
+
+NOTE 2 – These values allow for group-delay distortion around frequencies of peak speech energy and for delay of intermediate higher order multiplex and through-connecting equipment.
+
+NOTE 3 – Further study required. Delay for these devices is usually non-constant, and the range varies by implementation. Current implementations are of the order of several hundred milliseconds, and considerable delay is added to audio channels to achieve lip-synchronization. Manufacturers are encouraged to reduce their contribution to transmission time, in accordance with this Recommendation.
+
+NOTE 4 – For satellite digital communications where the transmultiplexer is located at the earth station, this value may be increased to 3.3 ms.
+
+NOTE 5 – These are mean values: depending on traffic loading, higher values can be encountered, e.g., 0.75 ms (1.950 ms, 1.350 ms, or 1.250 ms) with 0.95 probability of not exceeding. (For details see ITU-T Rec. Q.551 [12].)
+
+NOTE 6 – This is averaged for both directions of transmission.
+
+NOTE 7 – This is the cell formation delay of 64 kbit/s stream when it completely fills the cell (one voice channel per VC). In practical applications, additional delay will result, e.g., from cell loss detection and buffering. Other delays may be applicable to other AALs and cell mapping arrangements, and are for further study.
+
+### **A.2 Codec delay**
+
+Modern speech codecs operate on collections of speech samples known as frames. Each block of input speech samples is processed into a compressed frame. The coded speech frame is not generated until all speech samples in the input block have been collected by the encoder. Thus, there is a delay of one frame before processing can begin. In addition, many coders also look into the succeeding frame to improve compression efficiency. The length of this advance look is known as the look-ahead time of the coder. The time required to process an input frame is assumed to be the same as the frame length since efficient use of processor resources will be accomplished when an encoder/decoder pair (or multiple encoder/decoder pairs operating in parallel on multiple input streams) fully uses the available processing power (evenly distributed in the time domain). Thus, the delay through an encoder/decoder pair is normally assumed to be:
+
+$$2 \times \text{frame size} + \text{look-ahead}$$
+
+#### **A.2.1 Delay in wirebound environment**
+
+If the output facility is running at the same rate as the speech codec (e.g., an 8 kbit/s facility for ITU-T Rec. G.729), then an additional frame of delay is incurred when clocking the compressed frame to the facility. Thus, the maximum delay attributable to codec-related processing in conventional wirebound systems (i.e., the PSTN) is:
+
+$$3 \times \text{frame size} + \text{look-ahead}$$
+
+#### **A.2.2 Delay in mobile and wireless environment**
+
+If the output facility is a mobile network or a cordless facility, then the frame output by the encoder will function similar to the operation in wirebound environment but an additional delay is incurred for attaching the compressed frame to the airpath (assumed again that the mobile facility is running at the same rate as the speech codec). Thus, the maximum delay attributable to codec-related processing in mobile and wireless systems is:
+
+$$3 \times \text{frame size} + \text{look-ahead} + \text{air interface framing}$$
+
+#### **A.2.3 Delay in IP environment (one frame per packet)**
+
+If the output facility is an IP network, then the frame output by the encoder will instantaneously be dropped into an IP packet. The additional delay required for IP packet assembly and presentation to the underlying link layer will depend on the link layer. When the link layer is a LAN (e.g., Ethernet), this additional time will usually be quite small. Thus, the minimum delay attributable to codec-related processing in IP-based systems is:
+
+$$2 \times \text{frame size} + \text{look-ahead}$$
+
+When the link layer is one with lower clock rate (e.g., Modem connection) or one with high traffic load (e.g., congested LAN), the additional delay will increase substantially. In order to clock compressed frames at least with the same rate to the facility as the speech samples are collected at the input of the encoder, the additional delay should not exceed one frame size. Thus, the maximum delay attributable to codec-related processing in IP-based systems operating in real-time is:
+
+$$3 \times \text{frame size} + \text{look-ahead}$$
+
+#### **A.2.4 Delay in IP environment (multiple frames per packet)**
+
+If multiple voice frames are grouped together into a single IP packet, further delay is added to the speech signal. This delay will be at least the duration of one extra voice frame at the encoder for each additional voice frame added to the IP packet. Thus, the minimum delay attributable to codec-related processing in IP-based systems with multiple frames per packet is:
+
+$$(N + 1) \times \text{frame size} + \text{look-ahead}$$
+
+where N is the number of frames in each packet.
+
+When the link layer is one with lower clock rate (e.g., Modem connection) or one with high traffic load (e.g., congested LAN), additional delay will be incurred in delivering the packet to the facility. In order to clock compressed frames at least with the same rate to the facility as the speech samples are collected at the input of the encoder, the additional delay should, in case of multiple frames per packet, not exceed the length of the frames contained in one packet. It should be noted that clocking out a packet to the IP facility cannot start before all speech frames for this packet are available. Thus, the maximum delay attributable to codec-related processing in IP-based systems operating in real-time with multiple frames per packet is:
+
+$$(2N + 1) \times \text{frame size} + \text{look-ahead}$$
+
+where N is the number of frames in each packet.
+
+The Figure A.1 provides an example for  $N = 2$ :
+
+![Figure A.1/G.114 – Example: Composition of total codec-related delay in an IP Environment for N = 2. The diagram shows a sequence of events on a timeline, with horizontal arrows indicating durations. The events are: 1. collection of speech samples for frame #1; 2. look ahead; 3. processing of speech samples for frame #1; 4. collection of speech samples for frame #2; 5. look ahead; 6. processing of speech samples for frame #2; 7. clocking out a packet with two speech frames to the IP facility; 8. minimum delay to be attributed to the codec; 9. maximum delay to be attributed to the codec. The label G.114_FA.1 is at the bottom right.](4e4be0bd8b235167902f2c03e41da651_img.jpg)
+
+collection of speech samples for frame #1
+
+look ahead
+
+processing of speech samples for frame #1
+
+collection of speech samples for frame #2
+
+look ahead
+
+processing of speech samples for frame #2
+
+clocking out a packet with two speech frames to the IP facility
+
+minimum delay to be attributed to the codec
+
+maximum delay to be attributed to the codec
+
+G.114\_FA.1
+
+Figure A.1/G.114 – Example: Composition of total codec-related delay in an IP Environment for N = 2. The diagram shows a sequence of events on a timeline, with horizontal arrows indicating durations. The events are: 1. collection of speech samples for frame #1; 2. look ahead; 3. processing of speech samples for frame #1; 4. collection of speech samples for frame #2; 5. look ahead; 6. processing of speech samples for frame #2; 7. clocking out a packet with two speech frames to the IP facility; 8. minimum delay to be attributed to the codec; 9. maximum delay to be attributed to the codec. The label G.114\_FA.1 is at the bottom right.
+
+**Figure A.1/G.114 – Example: Composition of total codec-related delay in an IP Environment for  $N = 2$**
+
+### A.3 Delay due to IP delay variation buffer
+
+Packetized transmission systems exhibit variable delay (jitter) in packet delivery time. This is caused by the fact that different packets carrying speech samples of the same telephone conversation may encounter varying queue lengths, or different routes through the network. Details of this effect depend strongly on the specific mechanisms for transport, queuing or prioritization, which may be implemented in such a system. Nevertheless, delay variation must be removed prior to replaying the speech to the user, otherwise significant degradation will be noticeable.
+
+This is typically achieved by collecting packets in a de-jitter buffer at the receive side. This buffer rearranges the timely order of the packets, and is sized to take into account a certain range of network delay variation, effectively delaying all packets to correspond to the delay of the packet with the longest tolerable transit time. If the delivery time of a packet exceeds the length of the receive buffer, then this packet "arrives too late" with respect to its intended play-out time, and will be discarded. Consequently, the speech carried in this packet is lost for the decoding process. This "packet loss" impairs speech transmission quality (see ITU-T Rec. G.113).
+
+The contribution of the de-jitter buffer to one-way delay is based on the average time packets spend in the buffer, which is less than the peak buffer size. Depending on the specific type of implementation, as well as on the proper adjustment of the de-jitter buffer, this may be as low as one half of the peak buffer size (assuming symmetrically distributed delays). Packets that encounter the minimum transfer delay will wait the maximum time in the de-jitter buffer before being played out as a synchronous stream, while the reverse is true for packets with the maximum accommodated transfer delay (these packets spend the minimum time in the de-jitter buffer). For PLANNING PURPOSES, IT IS RECOMMENDED to assume that, a de-jitter buffer adds one half of its peak delay to the mean network delay.
+
+*Example* (taken from Appendix III/Y.1541 [13]):
+
+A jitter buffer designed to compensate for 50 ms packet delay variation range will introduce 25 ms additional delay, on average.
+
+Further Guidance on the effects of packet delay, as caused by de-jitter buffer, is provided by ITU-T Rec. Y.1541 [13].
+
+It should be noted that, with the use of dynamic de-jitter buffer implementations, the delay of the speech replayed to the user will be subjected to infrequent transitional delay variations when the de-jitter buffer resizes.
+
+## Appendix I
+
+## Delay introduced by coder-related processing
+
+**Table I.1/G.114 – Delay values for coders in wirebound applications**
+
+| Coder type | Rate (kbit/s) | Frame size (ms) | Look-ahead (ms) | Mean one-way delay introduced by coder-related processing (ms) | Reference                 |
+|------------|---------------|-----------------|-----------------|----------------------------------------------------------------|---------------------------|
+| PCM        | 64            | 0.125           | 0               | 0.375                                                          | G.711, G.712              |
+| ADPCM      | 40            | 0.125           | 0               | 0.375                                                          | G.726, G.727              |
+| ADPCM      | 32            | 0.125           | 0               | 0.375                                                          | G.721(1988), G.726, G.727 |
+| ADPCM      | 24            | 0.125           | 0               | 0.375                                                          | G.726, G.727              |
+| ADPCM      | 16            | 0.125           | 0               | 0.375                                                          | G.726, G.727              |
+| LD-CELP    | 16            | 0.625           | 0               | 1.875                                                          | G.728                     |
+| LD-CELP    | 12.8          | 0.625           | 0               | 1.875                                                          | G.728                     |
+| CS-ACELP   | 8             | 10              | 5               | 35                                                             | G.729                     |
+| VSELP      | 7.95          | 20              | 0               | 60                                                             | IS-54-B, TTA              |
+| ACELP      | 7.4           | 20              | 5               | 65                                                             | IS-641, TTA               |
+| QCELP      | 8             | 20              | 0               | 60                                                             | IS-96-A                   |
+| RCELP      | 8             | 20              | 10              | 70                                                             | IS-127                    |
+| VSELP      | 6.7           | 20              | 5               | 65                                                             | Japanese PDC              |
+| RPE-LTP    | 13            | 20              | 0               | 60                                                             | GSM 06.10, Full-rate      |
+| VSELP      | 5.6           | 20              | 0               | 60                                                             | GSM 06.20, Half-rate      |
+| ACELP      | 12.2          | 20              | 0               | 60                                                             | GSM 06.60, Enhanced FR    |
+| ACELP      | 5.3           | 30              | 7.5             | 97.5                                                           | G.723.1                   |
+| MP-MLQ     | 6.3           | 30              | 7.5             | 97.5                                                           | G.723.1                   |
+
+NOTE 1 – The PCM coder converts from analogue to digital and vice-versa while all other coders refer to the PCM domain; for PCM in the analogue domain, additional delay is incurred (0.375 ms).
+
+NOTE 2 – For wirebound applications, the mean one-way delay introduced by codec-related processing =  $3 \times \text{frame size} + \text{look-ahead}$  (see A.2.1).
+
+**Table I.2/G.114 – Delay values for coders in mobile or cordless applications**
+
+| Coder type | Rate (kbit/s) | Frame size (ms) | Look-ahead (ms) | Air interface framing (ms) | Mean one-way delay introduced by coder-related processing (ms) | Reference                       |
+|------------|---------------|-----------------|-----------------|----------------------------|----------------------------------------------------------------|---------------------------------|
+| PCM        | 64            | 0.125           | 0               | (See Note 3)               |                                                                | G.711, G.712                    |
+| ADPCM      | 40            | 0.125           | 0               | (See Note 3)               |                                                                | G.726, G.727                    |
+| ADPCM      | 32            | 0.125           | 0               | 13.625                     | 14                                                             | G.721(1988), G.726, G.727, DECT |
+| ADPCM      | 24            | 0.125           | 0               | (See Note 3)               |                                                                | G.726, G.727                    |
+| ADPCM      | 16            | 0.125           | 0               | (See Note 3)               |                                                                | G.726, G.727                    |
+| LD-CELP    | 16            | 0.625           | 0               | (See Note 3)               |                                                                | G.728                           |
+| LD-CELP    | 12.8          | 0.625           | 0               | (See Note 3)               |                                                                | G.728                           |
+| CS-ACELP   | 8             | 10              | 5               | (See Note 3)               |                                                                | G.729                           |
+| VSELP      | 7.95          | 20              | 0               |                            |                                                                | IS-54-B, TIA                    |
+| ACELP      | 7.4           | 20              | 5               |                            |                                                                | IS-641, TIA                     |
+| QCELP      | 8             | 20              | 0               |                            |                                                                | IS-96-A                         |
+| RCELP      | 8             | 20              | 10              |                            |                                                                | IS-127                          |
+| VSELP      | 6.7           | 20              | 5               |                            |                                                                | Japanese PDC                    |
+| RPE-LTP    | 13            | 20              | 0               | 35                         | 95                                                             | GSM 06.10, Full-rate            |
+| VSELP      | 5.6           | 20              | 0               | 35                         | 95                                                             | GSM 06.20, Half-rate            |
+| ACELP      | 12.2          | 20              | 0               | 35                         | 95                                                             | GSM 06.60, Enhanced FR          |
+| ACELP      | 5.3           | 30              | 7.5             | (See Note 3)               |                                                                | G.723.1                         |
+| MP-MLQ     | 6.3           | 30              | 7.5             | (See Note 3)               |                                                                | G.723.1                         |
+
+NOTE 1 – The PCM coder converts from analogue to digital and vice-versa while all other coders refer to the PCM domain; for PCM in the analogue domain, additional delay is incurred (0.375 ms).
+
+NOTE 2 – For mobile or cordless applications the mean one-way delay introduced by codec-related processing =  $3 \times \text{frame size} + \text{look-ahead} + \text{air interface framing}$  (see A.2.2)
+
+NOTE 3 – For the marked types of coders, Study Group 12 is not aware of any mobile or cordless application.
+
+**Table I.3/G.114 – Delay values for coders in IP-based applications  
+(one frame per packet)**
+
+| Coder type | Rate (kbit/s) | Frame size (ms) | Look-ahead (ms) | Mean one-way delay introduced by coder-related processing (ms) (see Note 2) |         | Reference                 |
+|------------|---------------|-----------------|-----------------|-----------------------------------------------------------------------------|---------|---------------------------|
+|            |               |                 |                 | Minimum                                                                     | Maximum |                           |
+| PCM        | 64            | 0.125           | 0               | 0.25                                                                        | 0.375   | G.711, G.712              |
+| ADPCM      | 40            | 0.125           | 0               | 0.25                                                                        | 0.375   | G.726, G.727              |
+| ADPCM      | 32            | 0.125           | 0               | 0.25                                                                        | 0.375   | G.721(1988), G.726, G.727 |
+| ADPCM      | 24            | 0.125           | 0               | 0.25                                                                        | 0.375   | G.726, G.727              |
+| ADPCM      | 16            | 0.125           | 0               | 0.25                                                                        | 0.375   | G.726, G.727              |
+| LD-CELP    | 16            | 0.625           | 0               | 1.25                                                                        | 1.875   | G.728                     |
+| LD-CELP    | 12.8          | 0.625           | 0               | 1.25                                                                        | 1.875   | G.728                     |
+| CS-ACELP   | 8             | 10              | 5               | 25                                                                          | 35      | G.729                     |
+| VSELP      | 7.95          | 20              | 0               | 40                                                                          | 60      | IS-54-B, TIA              |
+| ACELP      | 7.4           | 20              | 5               | 45                                                                          | 65      | IS-641, TIA               |
+| QCELP      | 8             | 20              | 0               | 40                                                                          | 60      | IS-96-A                   |
+| RCELP      | 8             | 20              | 10              | 50                                                                          | 70      | IS-127                    |
+| VSELP      | 6.7           | 20              | 5               | 45                                                                          | 65      | Japanese PDC              |
+| RPE-LTP    | 13            | 20              | 0               | 40                                                                          | 60      | GSM 06.10, Full-rate      |
+| VSELP      | 5.6           | 20              | 0               | 40                                                                          | 60      | GSM 06.20, Half-rate      |
+| ACELP      | 12.2          | 20              | 0               | 40                                                                          | 60      | GSM 06.60, Enhanced FR    |
+| ACELP      | 5.3           | 30              | 7.5             | 67.5                                                                        | 97.5    | G.723.1                   |
+| MP-MLQ     | 6.3           | 30              | 7.5             | 67.5                                                                        | 97.5    | G.723.1                   |
+
+NOTE 1 – The PCM codec converts from analogue to digital and vice-versa while all other coders refer to the PCM domain; for PCM in the analogue domain, additional delay is incurred (0.375 ms).
+
+NOTE 2 – For IP-related applications, the mean one-way delay introduced by codec-related processing:  
+ $= 2 \times \text{frame size} + \text{look-ahead}$  (minimum, see A.2.3)  
+ $= 3 \times \text{frame size} + \text{look-ahead}$  (maximum, see A.2.3).
+
+**Table I.4/G.114 – Delay values for coders in IP-based applications  
+(multiple frames per packet)**
+
+| Coder type | Rate (kbit/s) | Frame size (ms) | Look-ahead (ms) | Mean one-way delay introduced by coder-related processing (ms) (see Note 2) |                            | Reference                 |
+|------------|---------------|-----------------|-----------------|-----------------------------------------------------------------------------|----------------------------|---------------------------|
+|            |               |                 |                 | Minimum                                                                     | Maximum                    |                           |
+| PCM        | 64            | 0.125           | 0               | $(N + 1) \times 0.125$                                                      | $(2N + 1) \times 0.125$    | G.711, G.712              |
+| ADPCM      | 40            | 0.125           | 0               | $(N + 1) \times 0.125$                                                      | $(2N + 1) \times 0.125$    | G.726, G.727              |
+| ADPCM      | 32            | 0.125           | 0               | $(N + 1) \times 0.125$                                                      | $(2N + 1) \times 0.125$    | G.721(1988), G.726, G.727 |
+| ADPCM      | 24            | 0.125           | 0               | $(N + 1) \times 0.125$                                                      | $(2N + 1) \times 0.125$    | G.726, G.727              |
+| ADPCM      | 16            | 0.125           | 0               | $(N + 1) \times 0.125$                                                      | $(2N + 1) \times 0.125$    | G.726, G.727              |
+| LD-CELP    | 16            | 0.625           | 0               | $(N + 1) \times 0.625$                                                      | $(2N + 1) \times 0.625$    | G.728                     |
+| LD-CELP    | 12.8          | 0.625           | 0               | $(N + 1) \times 0.625$                                                      | $(2N + 1) \times 0.625$    | G.728                     |
+| CS-ACELP   | 8             | 10              | 5               | $(N + 1) \times 10 + 5$                                                     | $(2N + 1) \times 10 + 5$   | G.729                     |
+| VSELP      | 7.95          | 20              | 0               | $(N + 1) \times 20$                                                         | $(2N + 1) \times 20$       | IS-54-B, TIA              |
+| ACELP      | 7.4           | 20              | 5               | $(N + 1) \times 20 + 5$                                                     | $(2N + 1) \times 20 + 5$   | IS-641, TIA               |
+| QCELP      | 8             | 20              | 0               | $(N + 1) \times 20$                                                         | $(2N + 1) \times 20$       | IS-96-A                   |
+| RCELP      | 8             | 20              | 10              | $(N + 1) \times 20 + 10$                                                    | $(2N + 1) \times 20 + 10$  | IS-127                    |
+| VSELP      | 6,7           | 20              | 5               | $(N + 1) \times 20 + 5$                                                     | $(2N + 1) \times 20 + 5$   | Japanese PDC              |
+| RPE-LTP    | 13            | 20              | 0               | $(N + 1) \times 20$                                                         | $(2N + 1) \times 20$       | GSM 06.10, Full-rate      |
+| VSELP      | 5.6           | 20              | 0               | $(N + 1) \times 20$                                                         | $(2N+1) \times 20$         | GSM 06.20, Half-rate      |
+| ACELP      | 12.2          | 20              | 0               | $(N + 1) \times 20$                                                         | $(2N + 1) \times 20$       | GSM 06.60, Enhanced FR    |
+| ACELP      | 5.3           | 30              | 7.5             | $(N + 1) \times 30 + 7.5$                                                   | $(2N + 1) \times 30 + 7.5$ | G.723.1                   |
+| MP-MLQ     | 6.3           | 30              | 7.5             | $(N + 1) \times 30 + 7.5$                                                   | $(2N + 1) \times 30 + 7.5$ | G.723.1                   |
+
+NOTE 1 – The PCM codec converts from analogue to digital and vice-versa while all other coders refer to the PCM domain; for PCM in the analogue domain, additional delay is incurred (0.375 ms).
+
+NOTE 2 – For IP-related applications with multiple frames per packet, the mean one-way delay introduced by codec-related processing can be calculated as follows:
+
+$= (N + 1) \times \text{frame size} + \text{look-ahead}$  (minimum, see A.2.4)
+
+$= (2N + 1) \times \text{frame size} + \text{look-ahead}$  (maximum, see A.2.4).
+
+NOTE 3 – N = number of frames per packet.
+
+## Appendix II
+
+## Guidance on one-way delay for voice over IP
+
+### II.1 Introduction
+
+This appendix gives additional guidance on the application of ITU-T Rec. G.114. The main purpose is to provide practical information for end-to-end VoIP network planning. Also, this appendix provides a linkage to the IP network delay objectives in ITU-T Rec. Y.1541.
+
+### II.2 Achieving satisfactory delay
+
+For many *intra-regional* (e.g., within Africa, Europe, North America) routes in the range of 5000 km or less, users of VoIP connections are likely to experience mouth-to-ear delays <150 ms. Appendix III/Y.1541 illustrates this calculation using reference terminals with a total of 50 ms mean delay (10 ms packets). The calculation shows that the 100 ms objective of Y.1541 Class 0 can be met with a well-engineered access network (with a T1 or E1 rate or larger as Y.1541 requires) and with as many as 12 network routers. Appendix X/Y.1541 shows that similar speech quality can be maintained with reference terminals contributing a total delay of a less stringent 80 ms (using 20 ms packets and robust packet loss concealment).
+
+For *inter-regional* routes covered terrestrially, even those traversing the 27 500 km of the ITU's traditional worst-case Hypothetical Reference Connection, a VoIP mouth-to-ear path is likely to see a delay of just over 300 ms. This assumes terminals contributing a total of 80 ms delay (20 ms packets), a well-engineered access network and supporting IP network paths encountering 20 or fewer network routers (as per Appendix III/Y.1541). Of course, it is extremely unlikely that the worst case of 27 500 km will be encountered by many calls. For the much more frequent inter-regional calls of, for example, 10 000 km or less, the corresponding delays would be approximately 225 ms; certainly not as low (or as desirable) as 150 ms, but still quite satisfactory for the vast majority of users.
+
+Whilst delays in the mid-200 ms range may not be a serious problem for long inter-regional calls, where users expect calls to be somewhat different from regional calls, it is critical that network planners do not allow local and regional calls to encounter such delays because user expectations are that such calls be completely delay-transparent.
+
+Whilst it is recognized that using VoIP technologies will increase the delays well above that of non-packetized TDM transmission, this analysis demonstrates that the widespread use of end-end VoIP need not cause problematic delays if appropriate care and planning is exercised.
+
+
+
+## SERIES OF ITU-T RECOMMENDATIONS
+
+|                 |                                                                                                                                |
+|-----------------|--------------------------------------------------------------------------------------------------------------------------------|
+| Series A        | Organization of the work of ITU-T                                                                                              |
+| Series B        | Means of expression: definitions, symbols, classification                                                                      |
+| Series C        | General telecommunication statistics                                                                                           |
+| Series D        | General tariff principles                                                                                                      |
+| Series E        | Overall network operation, telephone service, service operation and human factors                                              |
+| Series F        | Non-telephone telecommunication services                                                                                       |
+| <b>Series G</b> | <b>Transmission systems and media, digital systems and networks</b>                                                            |
+| Series H        | Audiovisual and multimedia systems                                                                                             |
+| Series I        | Integrated services digital network                                                                                            |
+| Series J        | Cable networks and transmission of television, sound programme and other multimedia signals                                    |
+| Series K        | Protection against interference                                                                                                |
+| Series L        | Construction, installation and protection of cables and other elements of outside plant                                        |
+| Series M        | TMN and network maintenance: international transmission systems, telephone circuits, telegraphy, facsimile and leased circuits |
+| Series N        | Maintenance: international sound programme and television transmission circuits                                                |
+| Series O        | Specifications of measuring equipment                                                                                          |
+| Series P        | Telephone transmission quality, telephone installations, local line networks                                                   |
+| Series Q        | Switching and signalling                                                                                                       |
+| Series R        | Telegraph transmission                                                                                                         |
+| Series S        | Telegraph services terminal equipment                                                                                          |
+| Series T        | Terminals for telematic services                                                                                               |
+| Series U        | Telegraph switching                                                                                                            |
+| Series V        | Data communication over the telephone network                                                                                  |
+| Series X        | Data networks and open system communications                                                                                   |
+| Series Y        | Global information infrastructure and Internet protocol aspects                                                                |
+| Series Z        | Languages and general software aspects for telecommunication systems                                                           |

marked/G/T-REC-G.115-199602-I_PDF-E/raw.md

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+
+
+![ITU logo: A globe with the letters ITU and a lightning bolt.](2dfa6ac3edfe874f68aa0cbccaa42322_img.jpg)
+
+ITU logo: A globe with the letters ITU and a lightning bolt.
+
+INTERNATIONAL TELECOMMUNICATION UNION
+
+**ITU-T**
+
+**G.115**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+(02/96)
+
+**TRANSMISSION SYSTEMS AND MEDIA**
+
+**GENERAL CHARACTERISTICS OF INTERNATIONAL  
+TELEPHONE CONNECTIONS AND INTERNATIONAL  
+TELEPHONE CIRCUITS**
+
+---
+
+**MEAN ACTIVE SPEECH LEVEL FOR  
+ANNOUNCEMENTS AND SPEECH  
+SYNTHESIS SYSTEMS**
+
+**ITU-T Recommendation G.115**
+
+(Previously "CCITT Recommendation")
+
+---
+
+# FOREWORD
+
+The ITU-T (Telecommunication Standardization Sector) is a permanent organ of the International Telecommunication Union (ITU). The ITU-T is responsible for studying technical, operating and tariff questions and issuing Recommendations on them with a view to standardizing telecommunications on a worldwide basis.
+
+The World Telecommunication Standardization Conference (WTSC), which meets every four years, establishes the topics for study by the ITU-T Study Groups which, in their turn, produce Recommendations on these topics.
+
+The approval of Recommendations by the Members of the ITU-T is covered by the procedure laid down in WTSC Resolution No. 1 (Helsinki, March 1-12, 1993).
+
+ITU-T Recommendation G.115 was prepared by ITU-T Study Group 12 (1993-1996) and was approved under the WTSC Resolution No. 1 procedure on the 6th of February 1996.
+
+---
+
+# NOTE
+
+In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+# CONTENTS
+
+|                           | <i>Page</i> |
+|---------------------------|-------------|
+| 1 Introduction .....      | 1           |
+| 2 Recommended level ..... | 1           |
+| 3 Measuring method.....   | 1           |
+| 4 Reference point .....   | 2           |
+| References .....          | 2           |
+
+# **SUMMARY**
+
+This Recommendation contains a recommendation for the mean active speech level to be used for setting the levels of announcement machines and speech synthesis systems in the telephone network. A level of  $-19$  dBm0 is recommended, which presumes that the SLR of the telephones in the network is 8 dB, the long-term target value (referred to a 0 dBr point). If the present SLR in a national network is different, the recommended level can be adjusted, using a formula supplied, to a level consistent with that of live speech in the network. The recommended level is defined at the 0 dBr point, and to measure the level a speech voltmeter conforming to the requirements of Method B speech level meters in Recommendation P.56 should be used.
+
+# MEAN ACTIVE SPEECH LEVEL FOR ANNOUNCEMENTS AND SPEECH SYNTHESIS SYSTEMS
+
+(Geneva, 1996)
+
+# 1 Introduction
+
+With the deployment of automatic announcement systems and of speech synthesis systems in the telephone network, it is necessary to specify the proper level of the generated speech. The actual speech levels of live speech in the network depend on the send and receive efficiencies of the telephone sets. Recommendation P.310 specifies long-term objectives for the SLR and RLR of digital telephone sets. Different networks have different values of SLR and RLR in their telephones. Thus, the speech levels vary from one network to another. Information on actual speech levels in different networks and for different types of calls, e.g. national and international, is found in [1].
+
+A recommendation on speech levels must take these differences into account. In this Recommendation, a recommended level based on the long-term SLR and RLR objectives is provided. As an option, a formula for adjusting the level to that of a particular network is also provided.
+
+## 2 Recommended level
+
+It is recommended that the mean active speech level generated in announcement machines in the telephone network be set to a nominal output level of  $-19 \text{ dBm0}$ .
+
+This value presumes that the target value for SLR is 8 dB, referred to a 0 dBr point. Together with the target value for RLR of 2 dB, the recommended value of  $-19 \text{ dBm0}$  will produce an average listening level at the subscriber's end that corresponds to the preferred listening level, as defined in Recommendation P.10.
+
+The recommended speech level also conforms with the existing levels in national networks, if the present SLR values are around 8 dB. If the national transmission plan for the present mixed analogue-digital network states a nominal  $\text{SLR}_n$  (referred to a 0 dBr point) that differs significantly from 8 dB, an interim value for the level from announcement machines and synthesis systems which conforms better with the present levels in the network may be achieved by a correction for the difference.
+
+The correction,  $C = 8 - \text{SLR}_n \text{ dB}$ , is added to the recommended value of  $-19 \text{ dBm0}$ , in order to produce a higher or lower level, depending on the sign of the correction. By this correction, the interim level becomes:
+
+$L_i = -19 + C \text{ dBm0}$ , which is equivalent to:
+
+$L_i = -11 - \text{SLR}_n \text{ dBm0}$ .
+
+NOTE – For synthetic speech, it should be checked that the level balance between the individual words in an announcement, in particular for digits, is the same as for real speech.
+
+## 3 Measuring method
+
+The active speech level shall be measured in accordance with P.56 Method B. The necessary precision can be obtained for a duration of speech of at least 5 seconds. For longer announcements than 20 seconds, it is recommended that at least 20 seconds of active speech be measured.
+
+Instead of a speech signal, an artificial voice signal according to Recommendation P.50 may be used for setting the level.
+
+## 4 Reference point
+
+The reference point for the recommended speech level is 0 dBr as defined in Recommendation G.121.
+
+# References
+
+- [1] ITU-T COM 12-R 9-E (1994), *Report of WP 3/12 meeting* (Geneva, 23-24 February 1994), page 15, Table 1.
+- [2] ITU-T COM 12-50-E (1995), *Basis for Recommendation G.115 on speech levels*, (Rapporteur Q.17/12).
+- [3] ITU-T Recommendation P.310 (1996), *Transmission characteristics for telephone band (300-3400 Hz) digital telephones*.
+- [4] ITU-T Recommendation P.10 (1993), *Vocabulary of terms on telephone transmission quality and telephone sets*.
+- [5] ITU-T Recommendation P.56 (1993), *Objective measurement of active speech level*.
+- [6] ITU-T Recommendation P.50 (1993), *Artificial voices*.
+- [7] ITU-T Recommendation G.121 (1993), *Loudness Ratings (LRs) of national systems*.