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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.105**
+
+**TRANSMISSION SYSTEMS AND MEDIA**
+
+**GENERAL CHARACTERISTICS OF INTERNATIONAL  
+TELEPHONE CONNECTIONS AND INTERNATIONAL  
+TELEPHONE CIRCUITS**
+
+---
+
+**HYPOTHETICAL REFERENCE CONNECTION  
+FOR CROSSTALK STUDIES**
+
+**ITU-T Recommendation G.105**
+
+(Extract from the *Blue Book*)
+
+---
+
+# NOTES
+
+1 ITU-T Recommendation G.105 was published in Fascicle III.1 of the *Blue Book*. This file is an extract from the *Blue Book*. While the presentation and layout of the text might be slightly different from the *Blue Book* version, the contents of the file are identical to the *Blue Book* version and copyright conditions remain unchanged (see below).
+
+2 In this Recommendation, the expression “Administration” is used for conciseness to indicate both a telecommunication administration and a recognized operating agency.
+
+# **HYPOTHETICAL REFERENCE CONNECTION FOR CROSSTALK STUDIES**
+
+*(Geneva, 1980)*
+
+## **1 Purpose**
+
+This Recommendation gives guidance concerning the application of Recommendation P.16 [1] in the general switched telephone network and recommends the structure and parameters of a hypothetical reference connection specifically designed for crosstalk studies.
+
+## **2 General remarks**
+
+### **2.1 *Accuracy of fundamental data***
+
+2.1.1 There is always some degree of uncertainty in applying to real telephone conversation the results of tests in which subjects were asked to listen attentively to see if they were able to detect the presence of intelligible crosstalk. Furthermore, this type of test cannot be expected to indicate reliably the extent to which a subscriber's confidence in the privacy of his own conversation is undermined by overhearing another conversation. Hence in general the aim should be to reduce the risk of potentially intelligible crosstalk as much as possible.
+
+2.1.2 In applying the calculation method given in Recommendation P.16 [1], errors can occur if the distributions of crosstalk attenuations and loudness ratings are skew, rather than normal, or are truncated by test acceptance procedures. This arises because we are generally seeking low probabilities of encountering intelligible crosstalk which are highly dependent on the tails of distributions being accurately defined. One way of avoiding this difficulty is to apply Monte-Carlo methods as described, for example, in the CCITT manual cited in [2], taking care to make enough iterations to secure the necessary accuracy.
+
+2.1.3 Considerable care must be taken to obtain representative values of the loss and noise in crosstalk paths being studied. In particular, errors arising from small changes in mean values can easily result in the calculated probability of overhearing being in error by a factor of 10 or more (see, for example, [3]).
+
+### **2.2 *Effect of line and room noise***
+
+2.2.1 The masking effect of line noise is another aspect which is important and raises some difficulties. On the one hand if, for the purpose of establishing crosstalk limits, the level of line noise is assumed to be negligible, unrealistic demands may be placed on the crosstalk attenuation required to be introduced by items of plant. On the other hand, if it is assumed that circuits and exchanges in service introduce noise power levels comparable with their design objectives, e.g. the well known 4 pW0p/km, the incidence of overhearing may be unacceptably high, particularly when the network is lightly loaded so that noise power levels can be expected to be at their lowest.
+
+As in many transmission studies, a compromise has to be made somewhere between these extremes. In some cases, it may be necessary to rely on measurements of noise power levels on established plant during light and busy traffic periods. However, it must not be overlooked that limits devised now must, if possible, take the future into account. It is a wise principle that the successful performance of equipment in one part of the network should not be dependent upon adventitious imperfections of other parts of the network, particularly if such imperfections are likely to be eliminated or reduced in the future, e.g. by new designs of local exchange or by the extensive use of digital long-distance transmission systems.
+
+2.2.2 Unlike line noise the effect of room noise can be reduced by a determined listener. Hence Recommendation P.16 [1] recommends that negligible room noise be assumed when deriving a design objective for equipment.
+
+### 2.3 Probabilities and distributions involved
+
+2.3.1 When constructing the distribution of crosstalk attenuation introduced by equipment and cables, it is appropriate to consider only the worst (acceptable) values. For example, in a 10-pair cable only the worst disturber for each pair should be taken into account, i.e. 10 values. This distribution should not be diluted by the other 80 better values. In the busy period the worst potential disturber of a particular pair can be relied upon to be activated.
+
+2.3.2 In respect of intelligible crosstalk between local calls established in the same local exchange network, the probability of a potentially disturbing subscriber making a call at the same time as the disturbed subscriber can be significantly low certainly in the case of residential subscribers, although this is probably not the case for business subscribers and PBXs. Information concerning this topic and showing how to calculate the probabilities concerned will be found in [4].
+
+2.3.3 Multiple entries into a telephone connection of intelligible crosstalk signals all at significant levels and all derived from one source is so unlikely an event that it may be ignored for the purposes of deriving design limits. Hence the crosstalk mechanism of interest is assumed to be the dominant one when deriving limits, and all other sources are deemed to be negligible, and may thus attract the whole of the allowance.
+
+However, when a network performance objective for crosstalk has to be divided among the exchanges and circuits making up the connection, it may be necessary to give some consideration to the number of potential crosstalk paths from different sources. For example, crosstalk limits may be assigned to complete paths through an exchange and to complete junction or trunk circuits. Thus, on simple other-exchange connections (ignoring, for the moment, crosstalk arising within local cables) there are three dominant sources of crosstalk, and if, for example, the aim were to be not greater than 1 in 100 for such connections, the probability of overhearing from each source should be reduced to 1 in 300 (assuming equal probabilities and no correlation between the sources).
+
+Figures 1/G.105 and 2/G.105 illustrate some crosstalk paths of significance.
+
+## 3 Hypothetical reference connections for crosstalk
+
+Figure 3/G.105 illustrates the essential elements of two hypothetical reference connections appropriate to crosstalk studies in respect of telephone circuits and exchanges. It will be observed that the connections are much simpler than the corresponding ones in Recommendation G.103 used for studying noise and loss. It would be inappropriate to study the risk of potentially intelligible crosstalk between a pair of 12-circuit connections of near maximum length and noise, in order to arrive at, for example, a limit for channel equipment crosstalk, because the majority use of the channel equipment bought and installed to the specification is in much simpler, quieter, and more numerous connections.
+
+![Diagram illustrating far-end and near-end crosstalk paths between two telephone connections. The top path (Far-end paths) shows a 'Far-end disturber' on the left connected to a 'Listener's correspondent (assumed to be silent)' on the right. The bottom path (Near-end paths) shows a 'Disturbed listener' on the right connected to a 'Near-end disturber' on the right. Both paths pass through a 'Subscriber line', 'Local exchange', 'Repeater station', and 'Transmission system' (in that order from left to right). The diagram is labeled CCITT - 23 160.](6324b252294c0f5d4e34dad4a1202075_img.jpg)
+
+The diagram illustrates two hypothetical reference connections for crosstalk studies. The top connection, labeled 'Far-end paths', consists of a 'Far-end disturber' on the left, a 'Subscriber line', a 'Local exchange', a 'Repeater station', and a 'Transmission system'. A 'Listener's correspondent (assumed to be silent)' is connected to the right end of this path. The bottom connection, labeled 'Near-end paths', consists of a 'Transmission system', a 'Repeater station', a 'Local exchange', and a 'Subscriber line'. A 'Disturbed listener' is connected to the right end of this path, and a 'Near-end disturber' is connected to the right end of the bottom path. Arrows indicate the flow of signals and crosstalk between the two paths.
+
+Diagram illustrating far-end and near-end crosstalk paths between two telephone connections. The top path (Far-end paths) shows a 'Far-end disturber' on the left connected to a 'Listener's correspondent (assumed to be silent)' on the right. The bottom path (Near-end paths) shows a 'Disturbed listener' on the right connected to a 'Near-end disturber' on the right. Both paths pass through a 'Subscriber line', 'Local exchange', 'Repeater station', and 'Transmission system' (in that order from left to right). The diagram is labeled CCITT - 23 160.
+
+*Note* - Individual crosstalk limits for "repeater stations" (e.g. multiplexing equipment) and "transmission systems" are not the subject of this Recommendation which only deals with subscriber lines, exchanges, and interexchange circuits. In particular, limits recommended for circuits would be apportioned by the competent CCI Study Group(s).
+
+FIGURE 1/G.105
+
+Some far-end and near-end crosstalk paths of significance when considering potentially intelligible between telephone connections
+
+![Diagram showing two hypothetical crosstalk reference paths in a local exchange network. The top path shows a 'Disturber' and 'Disturbed' line originating from the 'Same distribution point', passing through two exchange switching stages (marked with asterisks), with crosstalk components labeled Xc and Xe. The bottom path shows a 'Disturber' and 'Disturbed' line originating from 'Different distribution points', also passing through two exchange switching stages, with crosstalk components labeled Xe and Xc. A note at the bottom states: '* All the subscriber lines are here shown equipped with additional amplification, but this is not always the case in practice.' The diagram is labeled CCITT - 23 170.](f9a14fbfecbd7d059226cc93677d721b_img.jpg)
+
+CCITT - 23 170
+
+\* All the subscriber lines are here shown equipped with additional amplification, but this is not always the case in practice.
+
+Diagram showing two hypothetical crosstalk reference paths in a local exchange network. The top path shows a 'Disturber' and 'Disturbed' line originating from the 'Same distribution point', passing through two exchange switching stages (marked with asterisks), with crosstalk components labeled Xc and Xe. The bottom path shows a 'Disturber' and 'Disturbed' line originating from 'Different distribution points', also passing through two exchange switching stages, with crosstalk components labeled Xe and Xc. A note at the bottom states: '\* All the subscriber lines are here shown equipped with additional amplification, but this is not always the case in practice.' The diagram is labeled CCITT - 23 170.
+
+*Note 1* - On own-exchange calls, overhearing between customers served by different distribution points may be assumed to be due only to exchange crosstalk or to crosstalk arising within local cables (near-end or far-end) on the far side of the exchange switching equipment. For other-exchange calls, the crosstalk paths are assumed to occur within the exchange and between junction or trunk circuits.
+
+*Note 2* - In the case of overhearing between customers served by the same distribution point, it should also be assumed that crosstalk can arise within the local cable (near-end crosstalk) or other permanently connected equipment. The particular customers who are unfavourably located in this respect will depend to a great extent on the type of local telephone circuits in use. When current-regulated telephones are used, customers on limiting length local lines are most at risk because the sensitivities of the telephone instrument are highest on these lines.
+
+*Note 3* - The effect of additional exchange amplification sometimes associated with long lines must be included where appropriate.
+
+FIGURE 2/G.105  
+Some hypothetical crosstalk reference paths for studying crosstalk in the local exchange network
+
+![Diagram (a) Far-end crosstalk paths. A schematic showing a disturbing talker on the left connected via a 4 dB (SLR) path to a network. The network consists of multiple nodes labeled with exchange noise (N_e) and circuit noise (N_c). Signal paths are interrupted by switches (X) with associated losses (X_e for exchange, X_c for circuit). Specific loss values like 1 dB, 0.5 dB, 4 dB, and 3 dB are marked at various points. The path leads to a disturbed listener on the right with a -4.5 dB (RLR) level. Dashed lines indicate remainders of connections. Negligible room noise is noted at the listener end.](547f726730e589392f239257a833ede3_img.jpg)
+
+Diagram (a) Far-end crosstalk paths. A schematic showing a disturbing talker on the left connected via a 4 dB (SLR) path to a network. The network consists of multiple nodes labeled with exchange noise (N\_e) and circuit noise (N\_c). Signal paths are interrupted by switches (X) with associated losses (X\_e for exchange, X\_c for circuit). Specific loss values like 1 dB, 0.5 dB, 4 dB, and 3 dB are marked at various points. The path leads to a disturbed listener on the right with a -4.5 dB (RLR) level. Dashed lines indicate remainders of connections. Negligible room noise is noted at the listener end.
+
+a) *Far-end crosstalk paths*
+
+![Diagram (b) Near-end crosstalk paths. A schematic showing a disturbing talker and a disturbed listener both on the right side. The signal from the talker (4 dB SLR) travels left into the network, encounters various nodes (N_e, N_c) and losses (X_e, X_c, 1 dB, 0.5 dB, 4 dB, 3 dB), and then crosstalks back to the listener (-4.5 dB RLR). Dashed lines represent connection remainders. Negligible room noise is noted at the listener. The code CCITT 42140 is at the bottom right.](a7d78d22e465dea388b31d0739f9d0cd_img.jpg)
+
+Diagram (b) Near-end crosstalk paths. A schematic showing a disturbing talker and a disturbed listener both on the right side. The signal from the talker (4 dB SLR) travels left into the network, encounters various nodes (N\_e, N\_c) and losses (X\_e, X\_c, 1 dB, 0.5 dB, 4 dB, 3 dB), and then crosstalks back to the listener (-4.5 dB RLR). Dashed lines represent connection remainders. Negligible room noise is noted at the listener. The code CCITT 42140 is at the bottom right.
+
+b) *Near-end crosstalk paths*
+
+*Note 1* - The disturbed connection is taken to be a very simple one, the disturbed listener being connected to a local exchange co-sited with the trunk exchange (e.g. the first ISC or the national primary centre).
+
+*Note 2* - Suitable values for the various circuit and exchange noise powers are
+
+|                           |                         |                                 |
+|---------------------------|-------------------------|---------------------------------|
+| Circuit noise ( $N_c$ ):  | subscribers local line: | 100 pWp                         |
+|                           | 4-wire circuit:         | 500 pW0p                        |
+|                           | (Satellite circuit:     | 10 000 pW0p)                    |
+| Exchange noise ( $N_e$ ): | local exchange:         | 50 pWp or pW0p (as appropriate) |
+|                           | 4-wire exchange:        | 100 pW0p                        |
+
+*Note 3* - In accordance with the convention adopted in Recommendation G.103, the send switching level at all exchanges is shown as 0 dB. In practice, other values of relative level are encountered and must be taken into account in the study.
+
+*Note 4* - Only one crosstalk mechanism is assumed to be dominant at any one time.
+
+FIGURE 3/G.105  
+**Hypothetical reference connections for crosstalk  
+between switched telephone connections**
+
+## References
+
+- [1] CCITT Recommendation *Subjective effects of direct crosstalk; Thresholds of audibility and intelligibility*, Vol. V, Rec. P.16.
+- [2] CCITT Manual *Transmission planning of switched telephone networks*, ITU, Geneva, 1976.
+- [3] *Social Crosstalk in the Local Area Network*, Electrical Communication (ITT), Vol. 49, No. 4, pp. 406-417, 1974.
+- [4] LAPSA (P. M.): Calculation of multidisturber crosstalk probabilities, *Bell System Technical Journal*, Vol. 55, No. 7, September 1976.

marked/G/T-REC-G.1072-202001-I_PDF-E/raw.md

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+
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.1072**
+
+(01/2020)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+Multimedia Quality of Service and performance – Generic  
+and user-related aspects
+
+---
+
+**Opinion model predicting gaming quality of  
+experience for cloud gaming services**
+
+Recommendation ITU-T G.1072
+
+# 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.1072
+
+# Opinion model predicting gaming quality of experience for cloud gaming services
+
+## Summary
+
+Recommendation ITU-T G.1072 presents an opinion model that predicts the overall gaming quality of experience (QoE) of non-expert gamers for cloud gaming services. The model uses an impairment factor approach in which the impact of typical Internet protocol (IP) network parameters and video encoding parameters on the video and input quality is estimated. The knowledge summarized in Recommendations ITU-T G.1032 and ITU-T P.809 serves as a basis for the development of the model. The model is a network planning tool which can be used by stakeholders to manage resource allocation and to configure IP-network transmission settings such as the selection of encoding framerates, resolutions and bitrates, under the assumption that the network is prone to packet loss and latency. Depending on whether the respective stakeholder has a priori knowledge of the type of game being offered through the cloud gaming service, either a default mode, which assumes the game to be highly sensitive towards delays and frame losses as well as having high encoding complexity, or an extended mode, which uses an adjusted model coefficient to increase the prediction accuracy, can be used.
+
+## History
+
+| Edition | Recommendation | Approval   | Study Group | Unique ID*                                                                |
+|---------|----------------|------------|-------------|---------------------------------------------------------------------------|
+| 1.0     | ITU-T G.1072   | 2020-01-13 | 12          | <a href="http://handle.itu.int/11.1002/1000/14151">11.1002/1000/14151</a> |
+
+## Keywords
+
+Cloud gaming, game, mean opinion score (MOS), modelling, quality of experience (QoE).
+
+---
+
+\* 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 2020
+
+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.....                                            | 3    |
+| 4 Abbreviations and acronyms .....                                                       | 3    |
+| 5 Conventions .....                                                                      | 3    |
+| 6 Areas of application.....                                                              | 4    |
+| 6.1 Application range for the model.....                                                 | 4    |
+| 6.2 Modes of operation.....                                                              | 5    |
+| 7 Gaming QoE prediction.....                                                             | 5    |
+| 7.1 Model structure.....                                                                 | 5    |
+| 7.2 Core model .....                                                                     | 6    |
+| 8 Calculation of quality impairment estimations.....                                     | 7    |
+| 8.1 Spatial video quality modeling.....                                                  | 7    |
+| 8.2 Temporal video quality impairment factor (I_TVQ) due to frame rate reduction(s)..... | 8    |
+| 8.3 Input quality modeling .....                                                         | 9    |
+| 9 Performance of the model.....                                                          | 9    |
+| Annex A – Description of the modes used for the model considering content classes .....  | 11   |
+| Bibliography.....                                                                        | 12   |
+
+
+
+# Opinion model predicting gaming quality of experience for cloud gaming services
+
+# 1 Scope
+
+This Recommendation describes a model that delivers predicted mean opinion scores (MOS) on a 5-point absolute category rating (ACR) scale, see [ITU-T P.800.1], [ITU-T P.910], based on the impact of impairments introduced by typical Internet protocol (IP) networks on the quality experienced by players using a cloud gaming system. This Recommendation targets cloud gaming services that perform video streaming over real-time transport protocol (RTP) (over user datagram protocol (UDP)) and which select various video encoding parameters to adapt to the network throughput, packet loss, and end-to-end delay.
+
+The focus of the described model is to predict gaming quality of experience (QoE) by considering relevant factors that are identified and discussed in [ITU-T G.1032]. The impairment factors are derived based on network and encoding parameters. By analysing the suitability of a variety of quality features for the prediction of the overall gaming QoE, an impairment model inspired by the E-model [b-ITU-T G.107] was developed.
+
+The model is a network planning tool which can be used by various stakeholders for purposes such as resource allocation and configuration of IP-network transmission settings such as the selection of resolution and bitrates, under the assumption that the network is prone to packet loss and latency.
+
+The model offers two different modes: a default mode where no information about the game type is considered, and an extended mode, for which various impairment factors based on a content classification with respect to the encoding complexity as well as the delay and frame loss sensitivity of a game are considered. Depending on whether the respective stakeholder (cloud gaming service provider or the network planner) has a priori knowledge of the type of game being offered through the cloud gaming service, the appropriate mode can be used. More information on the modes is given in clause 6.2 and Annex A.
+
+Virtual reality games requiring 3D rendering devices, mobile input, and output devices, as well as input devices other than keyboard and mouse are not within the scope of this model. Nevertheless, the model might apply to such systems as well. This is an item for further study. The model is also not designed to predict the influence of the game design or the motivation of users to play them. The subjective ratings collected to develop the model are primarily derived from non-expert gamers and hence, the model predictions might not be accurate for highly experienced gamers due to their different expectations and sensitivity towards the various degradations. Furthermore, the influence of social factors are not considered in this model. While the focus of the described model is on cloud gaming, some parts may also be relevant for online gaming (where the game is primarily executed on the client) or passive gaming video streaming (where only the video content is streamed to passive viewers of the game) applications. With respect to the technologies considered, the model addresses cloud gaming services using graphics processing unit (GPU) hardware accelerator engines for video compression, and H.264 [ITU-T H.264] as the video compression standard.
+
+# 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.1032] Recommendation ITU-T G.1032 (2017), *Influence factors on gaming quality of experience*.
+- [ITU-T G.1071] Recommendation ITU-T G.1071 (2016), *Opinion model for network planning of video and audio streaming applications*.
+- [ITU-T H.264] Recommendation ITU-T H.264 (2019), *Advanced video coding for generic audiovisual services*.
+- [ITU-T P.800.1] Recommendation ITU-T P.800.1 (2016), *Mean opinion score (MOS) terminology*.
+- [ITU-T P.809] Recommendation ITU-T P.809 (2018), *Subjective evaluation methods for gaming quality*.
+- [ITU-T P.910] Recommendation ITU-T P.910 (2008), *Subjective video quality assessment methods for multimedia applications*.
+- [ITU-T P.1201.2] Recommendation ITU-T P.1201.2 (2012), *Parametric non-intrusive assessment of audiovisual media streaming quality – Higher resolution application area*.
+- [ITU-T P.1401] Recommendation ITU-T P.1401 (2020), *Methods, metrics, and procedures for statistical evaluation, qualification, and comparison of objective quality prediction models*.
+
+# 3 Definitions
+
+## 3.1 Terms defined elsewhere
+
+This Recommendation uses the following terms defined elsewhere:
+
+**3.1.1 cloud gaming** [ITU-T G.1032]: Cloud gaming is characterized by game content delivered from a server to a client as a video stream with game controls sent from the client to the server. The execution of the game logic, rendering of the virtual scene, and video encoding is performed at the server, while the client is responsible for video decoding and capturing of client input.
+
+**3.1.2 game** [b-Juul]: A game is a rule-based system with a variable and quantifiable outcome, where different outcomes are assigned different values, the player exerts effort in order to influence the outcome, the player feels emotionally attached to the outcome, and the consequences of the activity are optional and negotiable.
+
+**3.1.3 model, model algorithm** [b-ITU-T P.1203]: An algorithm with the purpose of estimating the subjective (perceived) quality of a media sequence.
+
+**3.1.4 slicing artefacts** [ITU-T G.1071]: Artefacts that are introduced when packet losses are concealed through the use of a packet loss concealment (PLC) scheme to repair erroneous frames.
+
+**3.1.5 freezing artefacts** [ITU-T G.1071]: Artefacts that are introduced when the packet loss concealment (PLC) scheme of the receiver replaces the erroneous frames (either due to packet loss or error propagation) with the previous error-free frame until a decoded picture without errors have been received. Since the erroneous frames are not displayed, this type of artefact is also referred to as freezing with skipping.
+
+## 3.2 Terms defined in this Recommendation
+
+This Recommendation defines the following terms:
+
+**3.2.1 input quality:** Describes the playability of a game scenario in terms of the components responsiveness, immediate feedback, and controllability.
+
+**3.2.2 spatial video quality:** Spatial component of video quality composed of the dimensions unclearness and fragmentation as presented in [b-Schiffner], which can be influenced by artefacts such as blockiness and blur.
+
+**3.2.3 temporal video quality:** Temporal component of video quality represented by the dimensions discontinuity as presented in [b-Schiffner], which can be influenced by artefacts such as freezing and jerkiness.
+
+**3.2.4 average frames per second:** The average number of successfully transmitted frames per second during a video stream.
+
+# 4 Abbreviations and acronyms
+
+This Recommendation uses the following abbreviations and acronyms:
+
+|         |                                        |
+|---------|----------------------------------------|
+| 3D      | Three-dimensional space                |
+| ACR     | Absolute Category Rating               |
+| Avg_FPS | Average Frames Per Second              |
+| FEC     | Forward Error Correction               |
+| FR_enc  | Encoding Framerate                     |
+| GoP     | Group of Pictures                      |
+| GPU     | Graphics Processing Unit               |
+| HEVC    | High Efficiency Video Coding           |
+| IP      | Internet Protocol                      |
+| IPQ     | Input Quality                          |
+| llhq    | Low Latency, High Quality              |
+| MOS     | Mean Opinion Score                     |
+| PLC     | Packet Loss Concealment                |
+| PLCC    | Pearson Linear Correlation Coefficient |
+| QoE     | Quality of Experience                  |
+| RMSE    | Root-Mean-Square Error                 |
+| RTP     | Real-time Transport Protocol           |
+| SVQ     | Spatial Video Quality                  |
+| TVQ     | Temporal Video Quality                 |
+| UDP     | User Datagram Protocol                 |
+
+# 5 Conventions
+
+Within the scope of this Recommendation, the person interacting with a game is referred to as the player, whereas the software, which is used in a cloud gaming set-up to display a remotely rendered game video stream, is referred to as the client. Furthermore, when referring to a delay, the round-
+
+trip time for transmitting a video and command stream is considered, excluding additional delays such as system tick rate, rendering delay, encoding/decoding time, refresh rate of the display, and any other processing between client and servers.
+
+# 6 Areas of application
+
+## 6.1 Application range for the model
+
+The described model was developed based on a dataset of subjective assessment studies using the interactive and passive viewing-and-listening paradigms proposed in [ITU-T P.809]. The passive viewing-and-listening paradigm was used to cover a broader range of video encoding parameters and games. It must be noted that packet loss in the conducted interactive tests resulted in freezing artefacts, while in the passive viewing-and-listening tests, slicing artefacts occurred. The factors and application range summarized in Table 1 are considered.
+
+**Table 1 – Factors and application ranges of the model**
+
+| Application information                   | Interactive                      | Passive viewing-and-listening    |
+|-------------------------------------------|----------------------------------|----------------------------------|
+|                                           | Value range, unit                |                                  |
+| Sequence duration                         | 90 seconds                       | 30 seconds                       |
+| Screen size                               | 24"                              | 24"                              |
+| Input devices                             | Mouse and keyboard               | –                                |
+| Packetization                             | RTSP (over RTP/UDP/IP)           | RTSP (over RTP/UDP/IP)           |
+| Video codec                               | H.264 using NVENC                | H.264 using NVENC                |
+| Resolution                                | 1080p                            | 720p, 1080p                      |
+| Coded video bitrate (mbps)                | 1-50                             | 0.3-50                           |
+| Frame rate (fps)                          | 10, 20, 30, 60                   | 20, 30, 60                       |
+| Group of pictures ( <i>Note 1</i> )       | Infinite                         | Infinite and 0.5, 1, 2 seconds   |
+| Pre-set                                   | llhq (low latency, high quality) | llhq (low latency, high quality) |
+| Encoding mode                             | CBR                              | CBR                              |
+| Video compression                         | Standard H.264, Main 4.0         | Standard H.264, Main 4.0         |
+| Audio codec                               | AC3                              | –                                |
+| Coded audio bitrate (kbps)                | 192 (stereo)                     | –                                |
+| Audio sample rate (Hz)                    | 48,000                           | –                                |
+| Packet loss degradation ( <i>Note 2</i> ) | uniform loss (0-5%)              | uniform loss (0-2%)              |
+| Delay range                               | 0-400 ms                         | –                                |
+
+NOTE 1 – The low latency, high quality (llhq) preset does not use B frames and it uses an infinite GoP length by default. In case of a corrupted frame, no spatial artefacts will be visible but instead the FEC will lead to a replacement of the corrupted frame resulting in jerkiness of the video (freezing artefacts). However, for conducted passive tests, we also considered different GoP sizes, especially when considering different packet loss scenarios.
+
+NOTE 2 – The assumption of a uniform probability of packet loss is a hypothetical one which does not reflect the situation in real-life networks, where losses would typically occur in bursts. The modelling of bursty packet loss is an item for further study.
+
+The following factors were not considered during model design:
+
+- Audio/video sync distortions
+- Different levels of audio quality
+
+- Packet loss and delay distribution
+- Video codecs for which the model is not validated (MPEG2, HEVC, VP9, AV1, etc.)
+- Transcoding solutions
+- The effects of noise and color correctness in a video
+- Different ranges of parameters than the ones tested, e.g. delay of 800 ms, display size lower/higher than 24".
+
+## 6.2 Modes of operation
+
+The impact of network and encoding distortions on various quality features perceived by a player depends strongly on the sensitivity of a game towards these degradations. In order to reach a higher accuracy of the model, two modes of operation are defined depending on whether a network planner or service provider can make any assumption on the type of game that is targeted or not. If the network planner or cloud gaming provider has knowledge of the type of the targeted game with respect to its encoding complexity and sensitivity towards delay and frame losses, the quality of experience (QoE) can be predicted more accurately by considering the content classes. If the network planner cannot make any assumption on the game type, the highest content class will be assumed, which is referred to as the default mode in the rest of this document. Therefore, in the default mode, the model will result in a pessimistic quality prediction for games that are not of high complexity and sensitivity. Considerations on how to quantify game characteristics to derive the classes considered for the model are given in Annex A.
+
+# 7 Gaming QoE prediction
+
+## 7.1 Model structure
+
+The model structure, as illustrated in Figure 1, is composed of two main modules, namely input quality (*IPQ*) and video quality (*VQ*).
+
+The model considers two types of input parameters: network and encoding parameters. For the network parameters, delay and packet loss (*PL*) are used. For the encoding parameters the video resolution (*Res*), encoding framerate (*FR\_enc*), and the bitrate (*Br*) used for the video stream are used.
+
+Furthermore, five different estimations of quality impairments expressed on the R-scale, namely *I\_VQ\_cod*, *I\_VQ\_trans*, *I\_TVQ*, *I\_IPQ\_frames*, and *I\_IPQ\_delay*, are calculated based on the previously mentioned input parameters. Their calculation is described in the following subsections.
+
+To predict the overall gaming QoE (*MOS\_QoE*), the estimated quality impairments are weighted with the coefficients *a*, *b*, *c*, *d*, and *e*, which depend on the game type. Next, their sum is subtracted from a reference value, *R\_max*, resulting in *R\_QoE*. Finally, the predicted *MOS\_QoE* is calculated using a conversion to the MOS-scale.
+
+![Figure 1 – Model structure. A block diagram showing the flow from input parameters to the final MOS_QoE output. Inputs are grouped into Encoding parameters (Resolution, Bitrate, Framerate), Network parameters (Packet loss, Delay), and Game classification (Encoding complexity, Frame loss sensitivity, Delay sensitivity). These feed into three impairment calculation blocks: 'Based on SVQ' (I_VQ_cod, I_VQ_trans), 'Based on TVQ' (I_TVQ), and 'Based on IPQ' (I_IPQ_frames, I_IPQ_delay). A central 'Avg_FPS' block also receives inputs and feeds into the TVQ and IPQ blocks. The outputs of the impairment blocks are weighted (a, b, c, d, e) and summed to produce R_QoE. R_QoE is then subtracted from R_max and mapped to MOS_QoE.](d4af765160d04ecef538e5066006dc77_img.jpg)
+
+G.1072(20)\_F01
+
+Figure 1 – Model structure. A block diagram showing the flow from input parameters to the final MOS\_QoE output. Inputs are grouped into Encoding parameters (Resolution, Bitrate, Framerate), Network parameters (Packet loss, Delay), and Game classification (Encoding complexity, Frame loss sensitivity, Delay sensitivity). These feed into three impairment calculation blocks: 'Based on SVQ' (I\_VQ\_cod, I\_VQ\_trans), 'Based on TVQ' (I\_TVQ), and 'Based on IPQ' (I\_IPQ\_frames, I\_IPQ\_delay). A central 'Avg\_FPS' block also receives inputs and feeds into the TVQ and IPQ blocks. The outputs of the impairment blocks are weighted (a, b, c, d, e) and summed to produce R\_QoE. R\_QoE is then subtracted from R\_max and mapped to MOS\_QoE.
+
+**Figure 1 – Model structure**
+
+## 7.2 Core model
+
+The core model predicting gaming QoE is defined as:
+
+$$R_{QoE} = R_{max} - a \cdot I_{VQ_{cod}} - b \cdot I_{VQ_{trans}} - c \cdot I_{TVQ} - d \cdot I_{IPQ_{frames}} - e \cdot I_{IPQ_{delay}} \quad (1)$$
+
+$$MOS_{QoE} = \text{MOS\_from\_R} ( R_{QoE} ) \quad (2)$$
+
+where,
+
+$R_{QoE}$  is the overall estimated gaming QoE expressed on the R-scale, where 0 is the worst quality and 100 the best quality;
+
+$MOS_{QoE}$  is the overall estimated gaming QoE expressed on the MOS-scale, where 1 is the worst quality and 5 is the best quality;
+
+$R_{max}$  is the reference value indicating the best possible gaming QoE (= 100) on the R-scale;
+
+$I_{VQ_{cod}}$  is the estimated spatial video quality impairment for video compression artefacts on the R-scale;
+
+$I_{VQ_{trans}}$  is the estimated spatial video quality impairment for video transmission errors on the R-scale;
+
+$I_{TVQ}$  is the estimated temporal video quality impairment for frame rate reductions on the R-scale;
+
+$I_{IPQ_{frames}}$  is the estimated input quality impairment for frame rate reductions on the R-scale; and
+
+$I_{IPQ_{delay}}$  is the estimated input quality impairment for network delay degradations on the R-scale;
+
+with the constant coefficients  $a$ ,  $b$ ,  $c$ ,  $d$ , and  $e$  values summarized in Table 2.
+
+**Table 2 – Weighting factors of the final model**
+
+| Coefficient | <i>a</i> | <i>b</i> | <i>c</i> | <i>d</i> | <i>e</i> |
+|-------------|----------|----------|----------|----------|----------|
+| Value       | 0.788    | 0.896    | 0.227    | 0.625    | 0.848    |
+
+*MOS\_max*, the MOS value corresponding to *R\_max*, is 4.64, whereas the lowest possible MOS value, *MOS\_min*, is 1.3. The following transformation from [ITU-T P.1201.2] is used to derive *MOS\_QoE*:
+
+```
+function MOS_QoE = MOS_from_R(R_QoE)
+
+set MOS_MAX = 4.64;
+set MOS_MIN = 1.3;
+
+if (R_QoE > 0 & R_QoE < 100),
+    MOS_QoE = (1+(MOS_MAX-MOS_MIN)/100×R_QoE+R_QoE×(R_QoE-60)×(100-R_QoE)×7.0E-6);
+elseif (R_QoE >= 100),
+    MOS_QoE = MOS_MAX;
+else
+    MOS_QoE = MOS_MIN;
+end
+```
+
+# 8 Calculation of quality impairment estimations
+
+This clause describes the five estimations of quality impairments.
+
+## 8.1 Spatial video quality modeling
+
+The spatial video quality is modelled based on [ITU-T G.1071]. Since different encoding settings, as well as datasets, are used here, the model proposed in [ITU-T G.1071] is retrained.
+
+### 8.1.1 Video quality impairment factor (*I\_VQ\_cod*) due to video compression artefacts
+
+For the calculation of the impairment factor *I\_VQ\_Cod*, the parameters bitrate (*BR*), resolution (*Res*), and encoding framerate (*FR\_enc*) are used, where the number of pixels per video frame (*NumPixelPerFrame*) is calculated as the product of height and width of the video, e.g., 1920 × 1080 for 1080p resolution). It must be noted that the *ContentComplexity* here is not the same as the proposed encoding complexity classification, which is used for the model coefficients for each class separately. Based on models proposed in [ITU-T G.1071] and [ITU-T P.1201.2], *I\_VQ\_Cod* is defined as:
+
+$$I_{VQ_{cod}} = a_{1V} \cdot \exp(a_{2V} \cdot BitPerPixel) + a_{3V} \cdot ContentComplexity + a_{4V} \quad (3)$$
+
+where *a<sub>1V</sub>*, *a<sub>2V</sub>*, *a<sub>3V</sub>* and *a<sub>4V</sub>* are the constant coefficients and
+
+$$ContentComplexity = a_{31} \cdot \exp(a_{32} \cdot BitPerPixel) + a_{33} \quad (4)$$
+
+$$BitPerPixel = \frac{Bitrate \cdot 10^6}{NumPixelPerFrame \cdot Framerate} \quad (5)$$
+
+where, *a<sub>31</sub>*, *a<sub>32</sub>* and *a<sub>33</sub>* are constant coefficients.
+
+### 8.1.2 Video quality impairment factor (*I\_VQ\_trans*) due to video transmission errors
+
+For the calculation of the impairment factor *I\_VQ\_Trans*, the parameters packet loss (*PL*) and results from *I\_VQ\_Cod* are used, following the equations below which are based on the models proposed in [ITU-T G.1071] and [ITU-T P.1201.2].
+
+$$I_{VQ_{trans}} = c_{1V} \cdot \log(c_{2V} \cdot LossMagnitudeE + 1) \quad (6)$$
+
+$$LossMagnitudeE = q_1 \cdot \exp(q_2 \cdot LossMagnitudeNP) - q_1 \quad (7)$$
+
+$$LossMagnitudeNP = \frac{(c_{21} - I_{codn}) \cdot PL}{c_{23} \cdot I_{codn} + PL} \quad (8)$$
+
+$$I_{codn} = \begin{cases} I_{VQcod}, & \text{if } I_{VQcod} \leq 65 \\ 65, & \text{else} \end{cases} \quad (9)$$
+
+where,  $c_{1V}$ ,  $c_{2V}$ ,  $q_1$ ,  $q_2$ ,  $c_{21}$ , and  $c_{23}$  are the coefficients derived based on the training dataset. The list of coefficients for each class of encoding complexity is given in Table 3. It must be noted that packet loss (PL) in Eqn. 8 should be zero if the packet loss concealment (PLC) scheme is causing freezing artefacts.
+
+**Table 3 – Coefficients of retrained G.1071 impairments for each encoding complexity in which classes 1, 2 and 3 represent low, medium and high complexity classes respectively**
+
+| Coefficient | Class 1   | Class 2    | Class 3 (default mode) |
+|-------------|-----------|------------|------------------------|
+| $a_{1V}$    | 52.5052   | 37.9882    | 47.7463                |
+| $a_{2V}$    | -28.017   | -13.7208   | -12.07                 |
+| $a_{3V}$    | -2.68405  | 8.57837    | 9.05168                |
+| $a_{4V}$    | 5.46648   | 3.26581    | 3.41919                |
+| $a_{31}$    | 12.4214   | 6.83276    | 7.62306                |
+| $a_{32}$    | -28.0192  | -127.997   | -167.838               |
+| $a_{33}$    | 0.215799  | 0.479595   | 0.0760333              |
+| $c_{1V}$    | 19.7092   | 0.612879   | 1.57176                |
+| $c_{2V}$    | 3358.31   | 0.00139396 | 3.68596                |
+| $c_{21}$    | 28.3699   | 56.2893    | 74.0571                |
+| $c_{23}$    | 0.0234973 | 0.0047567  | 0.00406                |
+| $q_1$       | 0.0016474 | 0.0581327  | 2.58892e-08            |
+| $q_2$       | 0.0895914 | 2.38014    | 0.868407               |
+
+## 8.2 Temporal video quality impairment factor ( $I\_TVQ$ ) due to frame rate reduction(s)
+
+For the impairment factor  $I\_TVQ$ , the parameters  $Avg\_FPS$  and  $FR\_enc$  are used. The following equation for  $I\_TVQ$  is obtained.
+
+$$I_{TVQ} = d_1 + d_2 \cdot FR_{enc}^2 + d_3 \cdot FR_{enc} + d_4 \cdot \text{Log}(\text{FrameLossRate}) \quad (10)$$
+
+where,
+
+$$\text{FrameLossRate} = \frac{FR_{enc} - Avg\_FPS}{FR_{enc}} \cdot 100 \quad (11)$$
+
+where, if  $Delay < 16$  ms,  $Avg\_FPS = FR\_enc$ , else:
+
+$$Avg\_FPS = FR_{enc} \cdot \exp(-(d_5 + d_6 \cdot FR_{enc} + d_7 \cdot \text{Bitrate} \cdot FR_{enc}) \cdot (d_8 \cdot Delay - d_9) \cdot PL) \quad (12)$$
+
+where,  $d_5 = 0.08526$ ,  $d_6 = 0.00073$ ,  $d_7 = 1.425\text{e-}07$ ,  $d_8 = 0.09656$ ,  $d_9 = 1.5$ . It must be noted that packet loss (PL) in Eqn. 12 should be zero if the packet loss concealment (PLC) scheme is
+
+causing slicing artefacts. The constant coefficients  $d_1, d_2, d_3, \text{ and } d_4$  with respective values for each frame loss sensitivity class mentioned in Table 4.
+
+**Table 4 – Coefficients of I\_TVQ for each frame loss sensitivity class**
+
+| Coefficient | Low sensitive | High sensitive (default mode) |
+|-------------|---------------|-------------------------------|
+| $d_1$       | 29.13         | 47.03                         |
+| $d_2$       | 0.01344       | 0.01747                       |
+| $d_3$       | −1.283        | −1.823                        |
+| $d_4$       | 6.724         | 10.7                          |
+
+## 8.3 Input quality modeling
+
+### 8.3.1 Input quality impairment factor (I\_IPQ\_frames) due to frame rate reduction(s)
+
+For the calculation of the impairment factor  $I\_IPQ\_frames$ , the parameters encoding framerate ( $FR\_enc$ ) and the average frames per second ( $Avg\_FPS$ ), see Eqn. 12, are used. The coefficients used are given for each frame loss sensitivity class in Table 5. The impairment is modeled as:
+
+$$I_{IPQFrames} = e_1 + e_2 \cdot FrameRate^2 + e_3 \cdot FrameRate + e_4 \cdot Log(FrameLossRate) \quad (13)$$
+
+**Table 5 – Coefficients of I\_IPQ\_frames for each frame loss sensitivity class**
+
+| Coefficient | Low sensitive | High sensitive (default mode) |
+|-------------|---------------|-------------------------------|
+| $e_1$       | 23.43         | 54.71                         |
+| $e_2$       | 0.008574      | 0.02589                       |
+| $e_3$       | −0.9253       | −2.485                        |
+| $e_4$       | 5.855         | 9.306                         |
+
+### 8.3.2 Input quality impairment factor (I\_IPQ\_delay) due to network delay degradations
+
+For the calculation of the impairment factor  $I\_IPQ\_delay$ , the parameter network delay ( $Delay$ ) is used. The impairment is modeled as:
+
+$$I_{IPQDelay} = \frac{f_1}{1 + \exp(f_2 - f_3 \cdot Delay)} + f_4 \quad (14)$$
+
+The coefficients obtained for each delay sensitivity class are summarized in Table 6.
+
+**Table 6 – Coefficients of I\_IPQ\_delay for each delay sensitivity class**
+
+| Coefficient | Low sensitive | High sensitive (default mode) |
+|-------------|---------------|-------------------------------|
+| $f_1$       | 47.97         | 90                            |
+| $f_2$       | 2.097         | 1.191                         |
+| $f_3$       | 0.01073       | 0.009775                      |
+| $f_4$       | −4.567        | −18.73                        |
+
+# 9 Performance of the model
+
+On the evaluation dataset, unknown to the model, the performance of the model is reported in Table 7 in terms of root-mean-square error (RMSE) and Pearson linear correlation coefficient
+
+(PLCC), see [ITU-T P.1401]. The prediction of the gaming QoE in comparison with the subjective ratings is shown as a scatter plot in Figure 2 for the default mode, and for the extended mode. It must be noted that due to the same range of parameters used in the training and test set, the result could be optimistic.
+
+**Table 7 – Final prediction performance of the model on the test set**
+
+|      | Considering classification<br>(extended mode) |           | Without considering classification<br>(default mode) |           |
+|------|-----------------------------------------------|-----------|------------------------------------------------------|-----------|
+|      | R-scale                                       | MOS-scale | R-scale                                              | MOS-scale |
+| RMSE | 8.03                                          | 0.33      | 12.19                                                | 0.47      |
+| PLCC | 0.89                                          | 0.90      | 0.80                                                 | 0.82      |
+
+![Figure 2: Scatter plots of predicted vs. assessed MOS ratings for gaming QoE. (a) Default mode: Predicted mean opinion score of gaming QoE (Y-axis) vs. Assessed mean opinion score of gaming QoE (X-axis). The data points are more scattered. (b) Extended mode: Predicted mean opinion score of gaming QoE (Y-axis) vs. Assessed mean opinion score of gaming QoE (X-axis). The data points are more tightly clustered, indicating better prediction accuracy. Both plots have axes ranging from 1 to 5. A small label 'G.1072(20)_F02' is present in the bottom right of plot (b).](cbc4516eb885829fe8c9dabc0946dcbe_img.jpg)
+
+Figure 2: Scatter plots of predicted vs. assessed MOS ratings for gaming QoE. (a) Default mode: Predicted mean opinion score of gaming QoE (Y-axis) vs. Assessed mean opinion score of gaming QoE (X-axis). The data points are more scattered. (b) Extended mode: Predicted mean opinion score of gaming QoE (Y-axis) vs. Assessed mean opinion score of gaming QoE (X-axis). The data points are more tightly clustered, indicating better prediction accuracy. Both plots have axes ranging from 1 to 5. A small label 'G.1072(20)\_F02' is present in the bottom right of plot (b).
+
+**Figure 2 – Scatter plot of the predicted and assessed MOS ratings on the test dataset without using content classification (default mode) on the left, and using the content classification (extended mode) on the right**
+
+It can be observed that the extended mode due to content information, results in a higher accuracy than the default mode where no content information is used. Nevertheless, the model results in a good performance for both modes.
+
+# Annex A
+
+## Description of the modes used for the model considering content classes
+
+(This annex forms an integral part of this Recommendation.)
+
+The model described in this Recommendation has two modes, a default mode and an extended mode, depending on the game type. The game type is determined by encoding complexity, and sensitivity towards delay and frame losses.
+
+Encoding complexity is determined, amongst others, by:
+
+- Movements of a virtual camera
+- Texture details
+- Frequency of movements of game objects
+
+Sensitivity towards delay is determined, amongst others, by:
+
+- Continuous (e.g., mouse) or discrete type of input (e.g., keyboard)
+- Number of possible input directions in the virtual scene
+- Minimum number of required actions
+- Available time interval for a player to perform a desired interaction
+- Predictability of game events
+
+Sensitivity towards frame losses is determined, amongst others, by:
+
+- Movements of a virtual camera
+- Frequency of game object movements
+- Pace of the interaction with the game
+
+As long as no information on these characteristics is available to the user of the model, the default mode should be used. In case that it is known that the model shall be applied to games of low or medium encoding complexity, the "low complexity" or "medium complexity" class regarding this aspect can be used instead of the default mode, see Table 3. In case that it is known that the model shall be applied to games with low sensitivity towards frame losses, the "low sensitivity" class regarding this aspect should be used instead of the default mode, see Tables 4 and 5. In case that it is known that the model shall be applied to games with low sensitivity towards delay, the "low sensitivity" class regarding this aspect can be used instead of the default mode, see Table 6.
+
+The classification of game types according to the cited characteristics is currently left at the discretion of the model user, and examples can be found in [b-Zadtootaghaj]. The development of a method for accurately classifying games into these classes is an object of further study by Study Group 12 of ITU-T.
+
+# Bibliography
+
+- [b-ITU-T G.107] Recommendation ITU-T G.107 (2015), *The E-model: a computational model for use in transmission planning*.
+- [b-ITU-T P.1203] Recommendation ITU-T P.1203 (2017), *Parametric bitstream-based quality assessment of progressive download and adaptive audiovisual streaming services over reliable transport*.
+- [b-Juul] Juul, J. (2005), Video games between real rules and fictional worlds, *Video Games Real Rules Fict. Worlds*.
+- [b-Schiffner] Schiffner, F. and Möller, S. (2018), Direct Scaling & Quality Prediction for perceptual Video Quality Dimensions, *Tenth International Conference on Quality of Multimedia Experience (QoMEX)*, pp. 1-3, IEEE.
+- [b-Zadtootaghaj] Zadtootaghaj, S., Schmidt, S., Barman, N., Möller, S., and Martini, M. G. (2018), A classification of video games based on game characteristics linked to video coding complexity, *16th Annual Workshop on Network and Systems Support for Games (NetGames)*, pp. 1-6.
+
+
+
+# **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 logo: A globe with a lightning bolt and the letters ITU.](2dfa6ac3edfe874f68aa0cbccaa42322_img.jpg)
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+ITU logo: A globe with a lightning bolt and the letters ITU.
+
+INTERNATIONAL TELECOMMUNICATION UNION
+
+**ITU-T**
+
+TELECOMMUNICATION  
+STANDARDIZATION SECTOR  
+OF ITU
+
+**G.131**
+
+(11/2003)
+
+SERIES G: TRANSMISSION SYSTEMS AND MEDIA,  
+DIGITAL SYSTEMS AND NETWORKS
+
+International telephone connections and circuits – General  
+characteristics of the 4-wire chain formed by the  
+international circuits and national extension circuits
+
+---
+
+**Talker echo and its control**
+
+ITU-T Recommendation G.131
+
+---
+
+# 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        |
+| 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        |
+| <b>General characteristics of the 4-wire chain formed by the international circuits and national extension circuits</b>                      | <b>G.130–G.139</b> |
+| 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.131
+
+# Talker echo and its control
+
+## Summary
+
+This Recommendation provides guidance on the effect of talker echo and its control. Talker echo is considered independently of all other impairments. Furthermore, the conjunction of talker echo and the E-model of ITU-T Rec. G.107 is explained as well as the reference to ITU-T Rec. G.108.2 on transmission planning aspects of echo cancellers is provided.
+
+Previous versions of this Recommendation included a clause on stability that has been deleted because modern networks are largely all four-wire.
+
+Earlier versions of this Recommendation contained several planning rules for connections with echo control devices. As many of those rules are now obsolete, they are not reproduced here.
+
+A new Appendix III on the Combined effects of talker echo in the presence of absolute delay has been added.
+
+###### Source
+
+ITU-T Recommendation G.131 was approved by ITU-T Study Group 12 (2001-2004) under the ITU-T Recommendation A.8 procedure on 13 November 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 2004
+
+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 Effect of talker echo.....                                                                                                                           | 2    |
+| 4 Effect of talker echo on overall speech transmission quality .....                                                                                   | 4    |
+| 5 Active echo control devices.....                                                                                                                     | 5    |
+| 6 Rules for connections with echo control devices .....                                                                                                | 5    |
+| Appendix I – Assessment of talker echo effects .....                                                                                                   | 6    |
+| Appendix II – Relation between echo disturbances under single talk and double talk conditions (evaluated for one-way transmission time of 100 ms)..... | 7    |
+| II.1 Introduction .....                                                                                                                                | 7    |
+| II.2 Echo assessment for the test conditions according to ITU-T Rec. G.131 .....                                                                       | 8    |
+| II.3 Correlation between the results under single and double talk conditions .....                                                                     | 9    |
+| Appendix III – Combined effects of talker echo in the presence of absolute delay .....                                                                 | 10   |
+
+
+
+## Talker echo and its control
+
+## 1 Introduction
+
+This Recommendation provides guidance on the effect of talker echo, and some general rules for the insertion of network echo cancellers. (Talker echo is considered independently of all other impairments.)
+
+In a telephone conversation a talker sometimes can hear his own voice as a delayed echo.
+
+This phenomenon is referred to as talker echo; it is caused by signal reflections in the transmission path; such can either be caused by 4-wire/2-wire hybrids, or by an acoustic feedback via the airpath at the listener side, i.e. from the earpiece (or loudspeaker) to the microphone; other causes include crosstalk in the handset cord.
+
+In cases where the reflected voice signal has a delay close to zero it is referred to as sidetone, see ITU-T Rec. G.121 [7].
+
+NOTE – Previous versions of this Recommendation included a clause on stability that has been deleted because modern networks are largely all four-wire.
+
+## 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.107 (2003), *The E-model, a computational model for use in transmission planning*.
+- [3] ITU-T Recommendation G.108 (1999), *Application of the E-model: A planning guide*.
+- [4] ITU-T Recommendation G.108.2 (2003), *Transmission planning aspects of echo cancellers*.
+- [5] ITU-T Recommendation G.109 (1999), *Definition of categories of speech transmission quality*.
+- [6] ITU-T Recommendation G.114 (2003), *One-way transmission time*.
+- [7] ITU-T Recommendation G.121 (1993), *Loudness ratings (LRs) of national systems*.
+- [8] ITU-T Recommendation G.122 (1993), *Influence of national systems on stability and talker echo in international connections*.
+- [9] ITU-T Recommendation G.164 (1988), *Echo suppressors*.
+- [10] ITU-T Recommendation G.165 (1993), *Echo cancellers*.
+- [11] ITU-T Recommendation G.168 (2002), *Digital network echo cancellers*.
+
+- [12] ITU-T Recommendation P.310 (2003), *Transmission characteristics for telephone band (300-3400 Hz) digital telephones.*
+- [13] ITU-T Recommendation Q.115.1 (2002), *Logic for the control of echo control devices and functions.*
+
+# 3 Effect of talker echo
+
+The degree of annoyance of talker echo depends both on the amount of delay and on the level difference between the original voice and the received echo signal. This level difference is characterized by the measure "Talker Echo Loudness Rating" (TELR).
+
+ITU-T Rec. G.122 [8] describes how TELR can be determined from the Echo Loss (EL) of a 4-wire/2-wire hybrid and from the weighted Terminal Coupling Loss (TCLw) of a telephone set respectively.
+
+Recommended limits for TCLw of telephone sets can be found in the P.300-series of Recommendations; e.g., ITU-T Rec. P.310 [12] provides specifications for limits of acoustic feedback for standard digital telephone sets.
+
+Furthermore, delay estimations for various connection elements are given in ITU-T Rec. G.114 [6].
+
+Figure 1 shows the minimum requirements on TELR as a function of the mean one-way transmission time T (half the value of the total round-trip delay from the talker's mouth to the talker's ear). In general, the "acceptable" curve is the one to follow. Only in exceptional circumstances should values for the "limiting case" be allowed.
+
+![Figure 1/G.131 – Talker echo tolerance curves. A line graph showing two curves, 'Acceptable' and 'Limiting case', plotting TELR (dB) against T (ms).](aa14b9ec884bf40ce06c161be468cd84_img.jpg)
+
+The graph plots TELR (dB) on the y-axis (0 to 60) against T (ms) on the x-axis (logarithmic scale: 5, 10, 20, 30, 50, 100, 200, 300). Two curves are shown:
+ 
+
+- The "Acceptable" curve starts at approximately 21 dB at 5 ms and rises to about 54 dB at 300 ms.
+- The "Limiting case" curve starts at approximately 15 dB at 5 ms and rises to about 48 dB at 300 ms.
+
+ The graph includes a grid with major lines every 10 dB and major ticks every 10 ms on the logarithmic scale. The label "G.131\_F01" is in the bottom right corner.
+
+T Mean one-way transmission time  
+ TELR Talker Echo Loudness Rating
+
+Figure 1/G.131 – Talker echo tolerance curves. A line graph showing two curves, 'Acceptable' and 'Limiting case', plotting TELR (dB) against T (ms).
+
+Figure 1/G.131 – Talker echo tolerance curves
+
+Previous versions of this figure (see Figure 2/G.131 (1988)) included curves labelled "1%" and "10%", which sometimes caused confusion as to what these terms meant; these percentages refer to the probability of encountering objectionable echo. Transmission planning experience, corroborated by computational modelling results, has shown that the earlier "1%" curve for all-digital networks corresponds to the limit for acceptable talker echo performance (with some margins), so it is retained and labelled "Acceptable". The "limiting case" curve corresponds to a TELR of 6 dB lower (than that of the new acceptable curve) and should only be used in exceptional circumstances, as it corresponds to a 10% probability of encountering objectionable echo.
+
+It must be mentioned that the Transmission Rating Model of ITU-T Rec. G.107 [2] (the E-model) takes into account the effect of echo on speech transmission quality based on these graphs. Thus, if the E-model is used considering the effects of talker echo only (i.e., with nominal values for all other parameters) the upper graph, labelled "Acceptable", corresponds to an E-model Rating of  $R = 74$ , whereas the lower graph, labelled "Limiting case", corresponds to  $R = 60$ .
+
+Figures 2a and 2b provide the requirements for talker echo derived from the E-model. The dashed graphs in Figures 2a and 2b are representing  $R = 74$  and  $R = 60$ .
+
+![Figure 2a/G.131 – Effects of talker echo based on E-model. A line graph showing Talker Echo Loudness Rating TELR (dB) on the y-axis (25 to 75) versus Mean one-way delay of the echo path (ms) on the x-axis (0 to 500). Seven curves are plotted for different R values: R=90 (solid black), R=80 (dashed magenta), R=74 (dotted red), R=70 (dash-dot cyan), R=60 (dash-dot-dot purple), R=50 (dash-dot-dot-dot dark purple), and R=60 (solid purple). The R=90 curve is the highest, starting at ~40 dB at 0 ms and rising to ~75 dB at 500 ms. The R=60 (solid purple) curve is the lowest, starting at ~25 dB at 0 ms and rising to ~46 dB at 500 ms.](bedcca5cdf168e3508ef511d94ec514c_img.jpg)
+
+Figure 2a/G.131 is a line graph showing the relationship between the Mean one-way delay of the echo path (ms) on the x-axis (ranging from 0 to 500) and the Talker Echo Loudness Rating TELR (dB) on the y-axis (ranging from 25 to 75). The graph displays several curves representing different E-model Ratings ( $R$ ).
+
+The curves are:
+
+- $R = 90$  (Solid black line)
+- $R = 80$  (Dashed magenta line)
+- $R = 74$  (Dotted red line)
+- $R = 70$  (Dash-dot cyan line)
+- $R = 60$  (Dash-dot-dot purple line)
+- $R = 60$  (Solid purple line)
+- $R = 50$  (Dash-dot-dot-dot dark purple line)
+
+The graph shows that as the mean one-way delay increases, the Talker Echo Loudness Rating (TELR) also increases. Higher  $R$  values correspond to higher TELR values for a given delay. The  $R = 90$  curve is the highest, starting at approximately 40 dB at 0 ms and reaching about 75 dB at 500 ms. The  $R = 60$  (solid purple) curve is the lowest, starting at approximately 25 dB at 0 ms and reaching about 46 dB at 500 ms.
+
+Figure 2a/G.131 – Effects of talker echo based on E-model. A line graph showing Talker Echo Loudness Rating TELR (dB) on the y-axis (25 to 75) versus Mean one-way delay of the echo path (ms) on the x-axis (0 to 500). Seven curves are plotted for different R values: R=90 (solid black), R=80 (dashed magenta), R=74 (dotted red), R=70 (dash-dot cyan), R=60 (dash-dot-dot purple), R=50 (dash-dot-dot-dot dark purple), and R=60 (solid purple). The R=90 curve is the highest, starting at ~40 dB at 0 ms and rising to ~75 dB at 500 ms. The R=60 (solid purple) curve is the lowest, starting at ~25 dB at 0 ms and rising to ~46 dB at 500 ms.
+
+**Figure 2a/G.131 – Effects of talker echo based on E-model**
+
+![Figure 2b/G.131: Effects of talker echo based on E-model. A line graph showing Talker Echo Loudness Rating TELR (dB) on the y-axis (10 to 30) versus Mean one-way delay of the echo path (ms) on the x-axis (0 to 50). Six curves are plotted for different values of R: R=90 (solid black), R=80 (dashed magenta), R=74 (dotted red), R=70 (dash-dot cyan), R=60 (dash-dot-dot purple), and R=50 (dash-dot-dot-dot dark purple). All curves show that TELR increases with delay and decreases as R increases. The R=90 curve is the steepest, while the R=50 curve is the least steep.](adb1f42239329fa8283d1a40005f989f_img.jpg)
+
+Figure 2b/G.131: Effects of talker echo based on E-model. A line graph showing Talker Echo Loudness Rating TELR (dB) on the y-axis (10 to 30) versus Mean one-way delay of the echo path (ms) on the x-axis (0 to 50). Six curves are plotted for different values of R: R=90 (solid black), R=80 (dashed magenta), R=74 (dotted red), R=70 (dash-dot cyan), R=60 (dash-dot-dot purple), and R=50 (dash-dot-dot-dot dark purple). All curves show that TELR increases with delay and decreases as R increases. The R=90 curve is the steepest, while the R=50 curve is the least steep.
+
+**Figure 2b/G.131 – Effects of talker echo based on E-model**
+
+In order to use the curves of Figure 1 without using the whole E-model from ITU-T Rec. G.107 [2], the following rule, derived from formula 3-22/G.107, can be used:
+
+- if  $x$  and  $y$  are respectively the values of the mean one-way transmission time and of the talker echo return loss (i.e., the coordinates of the corresponding plot on Figure 1) as evaluated for a given link or communication, then:
+
+$$f(x, y) = y - 40 \log \left( \frac{1 + \frac{x}{10}}{1 + \frac{x}{150}} \right) + 6e^{-0.3x^2}$$
+
+- if  $f(x,y) \leq 8$  (i.e., below the "limiting case" curve), the echo will be annoying, and needs to be cancelled;
+- if  $8 < f(x,y) < 14$  (i.e., between both curves), the echo will be probably annoying;
+- if  $f(x,y) \geq 14$  (i.e., above the "acceptable" curve), the echo will not be annoying, and does not need to be cancelled.
+
+# 4 Effect of talker echo on overall speech transmission quality
+
+For general transmission planning purposes, the total effect of all transmission impairments may be estimated with the E-model of ITU-T Rec. G.107 [2]. For telephone connections incurring a higher amount of absolute end-to-end delay, it is very important to consider the combined effects of talker echo and absolute delay in order to cover both the single talk situation of either talker and the interactive situation between both parties involved in the call. For the convenience of the readers of this Recommendation, Appendix III provides a figure with respective graphs for tutorial purposes.
+
+ITU-T Rec. G.108 [3] gives detailed examples on how to use the E-model to assess the transmission performance of connections involving various impairments, including talker echo; 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.131 provides useful information regarding talker echo as a parameter by itself, ITU-T Rec. G.107 [2] (and its ITU-T Recs G.108 [3] and G.109 [5] companions) should be used to assess the effects of talker echo in conjunction with other impairments (e.g., distortions due to speech processing).
+
+# **5 Active echo control devices**
+
+For connections where the effects of talker echo are responsible for an undesirable decrease of transmission quality, the deployment of active echo control devices, such as echo cancellers, is a valid choice. Echo cancellers detect the echo portion contained in the receive signal of the talker and (attempt to) remove it from the receive signal; this is mainly based on a process of permanently estimating the echo path transfer function.
+
+In former versions of this Recommendation, it was recommended that the active echo control devices be deployed on all connections which exceed the total one-way talker echo transmission path time of 25 ms. This guideline was intended to ensure the acceptable echo performance on international connections terminated by analogue subscriber lines.
+
+However, echo control devices may be deployed in connections with less or more transmission time for reasons such that low or high values of TELR are to be expected in the network. In such cases, curves in Figure 1 can be used as the guidance for the desirable performance (see Appendix I). The threshold of 25 ms remains valid for networks using 600  $\Omega$  hybrids.
+
+When TELR is much greater than 65 dB, the "no echo" curve from Appendix III may be applied.
+
+For details on transmission planning aspects of echo cancellers, see ITU-T Rec. G.108.2 [4] and for details on echo canceller control logic, see ITU-T Rec. Q.115.1 [13].
+
+In general it is recommended to only deploy echo cancellers into the network that conform to ITU-T Rec. G.168 [11]. Echo suppressors according to ITU-T Rec. G.164 [9] and echo cancellers according to ITU-T Rec. G.165 [10] may still be in use, but are not recommended for any new deployment.
+
+A general rule for echo control devices is that they should ensure the returned echo from any device is less than  $-65$  dBm0.
+
+A trade-off between additional delay and talker echo is possible; see ITU-T Rec. G.108 [3] for planning examples and guidance also in this respect.
+
+In some specific cases, such as interconnections between public and other networks (e.g., private networks), the public network may not provide sufficient echo control. In such cases, the need of providing echo control in the connection segment attached to the public network has to be considered by the private network provider.
+
+# **6 Rules for connections with echo control devices**
+
+Earlier versions of this Recommendation contained several planning rules for connections with echo control devices. As many of those rules are now obsolete, they are not reproduced here. However, some rules still apply, for example:
+
+- 1) Circuits with properly designed and thoroughly tested echo cancellers (meeting or exceeding the requirements of ITU-T Rec. G.168) can be connected in tandem without significant performance degradation.
+- 2) Circuits with echo suppressors can be connected with circuits equipped with echo cancellers without additional performance degradation caused by the canceller; however, the overall performance will be limited by that provided by the poorer performing device.
+
+**Note** that much new detail regarding the use of echo cancellers is provided in ITU-T Rec. G.108.2.
+
+## Appendix I
+
+## Assessment of talker echo effects
+
+Figure I.1 illustrates the typical talker echo caused by a reflection at the 2-wire/4-wire hybrid at the far end of a connection.
+
+![Diagram of talker echo at side a, caused by reflections at side b. The diagram shows a 2-wire/4-wire hybrid at the far end (Side b) with a reflection coefficient L_r. The echo path is shown with arrows indicating the signal flow from Side a, through the hybrid, and back to Side a. The signal loss components are labeled: SLR (Sending Loss Ratio), RLR (Receiving Loss Ratio), and L_e (Echo Loss). The diagram is labeled G.131_FI.1.](27b06ec9f42b5d727a2630f61a5f1861_img.jpg)
+
+Diagram of talker echo at side a, caused by reflections at side b. The diagram shows a 2-wire/4-wire hybrid at the far end (Side b) with a reflection coefficient L\_r. The echo path is shown with arrows indicating the signal flow from Side a, through the hybrid, and back to Side a. The signal loss components are labeled: SLR (Sending Loss Ratio), RLR (Receiving Loss Ratio), and L\_e (Echo Loss). The diagram is labeled G.131\_FI.1.
+
+Figure I.1/G.131 – Talker echo at side a, caused by reflections at side b
+
+With the designations of the figure:
+
+$$TELR = SLR + RLR + L_e$$
+
+and:
+
+$$L_e = R + T + L_r$$
+
+where  $L_r$  is the weighted average of the return loss at the hybrid, weighting according to ITU-T Rec. G.122 [8].
+
+This Recommendation specifies that no special echo control devices are needed if  $T < 25$  ms. According to Figure 1, this corresponds to  $TELR = 33$  dB at the limit  $T = 25$  ms. In many networks,  $(T + R) = 6$  dB and  $SLR_{nom} = 7$ ,  $RLR_{nom} = 3$ . Thus one should have  $L_r > 17$  dB which is not unreasonable for an average length of subscriber cable and if the impedance of the terminals can be specified with fairly tight tolerances. However, this may not be the case for all networks as described in the examples below.
+
+In some networks, the average return loss of the terminating impedances against a nominal balance impedance is 14 dB, with a standard deviation of 3 dB. Very short subscriber lines are also common. According to ITU-T Rec. G.121, the loudness ratings of the telephone sets are:
+
+$$SLR_{nom} = 7, SLR_{min} = 2; RLR_{nom} = 3, RLR_{min} = 1$$
+
+### Example 1
+
+Nominal loudness ratings, nominal return loss  $L_r = 14$ , zero length line.
+
+$$TELR = 7 + 3 + 6 + 14 = 30$$
+
+This corresponds to an "acceptable" limit  $T < 18$  ms, "limiting case"  $T < 33$  ms.
+
+### Example 2
+
+Nominal loudness ratings, lowest "2-sigma" return loss  $L_r = 8$  dB, zero line.
+
+$$TELR = 7 + 3 + 6 + 8 = 24$$
+
+This corresponds to an "acceptable" limit  $T < 9$  ms, "limiting case"  $T < 19$  ms.
+
+### Example 3
+
+Loud telephone set, lowest "2-sigma" return loss  $L_r = 8$  dB, zero line length.
+
+$$\text{TELR} = 2 + 1 + 6 + 8 = 17$$
+
+This corresponds to a "limiting case" of 7 ms.
+
+## Appendix II
+
+## Relation between echo disturbances under single talk and double talk conditions (evaluated for one-way transmission time of 100 ms)
+
+### II.1 Introduction
+
+The telephone situation using a handset was reproduced in a third party listening test (LOT). The listening examples were generated by a computer simulation considering two double talk periods:
+
+- sequence 1: a long double talk (a whole sentence); and
+- sequence 2: a short double talk represented by a single word.
+
+The structure of the listening examples can be subdivided into three periods:
+
+- period A: listening to the far-end speech (male voice);
+- period B: double talk period (sequence 1 or sequence 2, female voice);
+- period C: listening again to the far-end speech.
+
+In addition to the double talk conditions these two sequences were also judged under single talk conditions (no far-end speech was present). The test conditions were as follows:
+
+- average speech level on both sides of the connection was adjusted to  $-4.7$  dBPa;
+- simulated characteristics of a standard German handset (FEAP 7);
+- the connection was simulated by different TELR values;
+- TELRs representing the "acceptable curve" and "limiting case" were included;
+- variable TELRs in combination with a one-way transmission time of 100 ms were included;
+- 24 naive subjects were used as test persons;
+- the parameters overall quality and echo were judged on a 5-point scale.
+
+The different TELRs were adjusted by a digital attenuation in the (simulated) echo path. This does not influence the loudness of the far-end speech under double talk conditions. If variations of TELRs are simulated by a variable sensitivity in the sending direction of a far-end terminal, the loudness of far-end speech is affected too. Consequently, the masking effect during double talk would be lower and would influence the echo judgement. This influence was excluded.
+
+### II.2 Echo assessment for the test conditions according to ITU-T Rec. G.131
+
+The results are given in Figures II.1 and II.2.
+
+![Bar chart showing MOS results for single talk conditions across various echo delay and level settings. The chart compares 'Acceptable' (black) and 'Limiting case' (grey) curves against a reference.](c2b98986bdf45e15707f6b2bd7ade2bd_img.jpg)
+
+This bar chart displays the Mean Opinion Score (MOS) for single talk conditions. The y-axis represents MOS from 1.0 to 5.0. The x-axis shows different test conditions. For each condition, two bars are shown: a black bar for the "Acceptable" curve and a grey bar for the "Limiting case" curve. Error bars are included for each data point.
+
+| Condition       | "Acceptable" curve (MOS) | "Limiting case" curve (MOS) |
+|-----------------|--------------------------|-----------------------------|
+| Reference       | -                        | ~4.6                        |
+| 10 ms 25/19 dB  | ~4.1                     | ~3.6                        |
+| 25 ms 33/27 dB  | ~4.2                     | ~3.6                        |
+| 50 ms 40/34 dB  | ~4.4                     | ~3.8                        |
+| 100 ms 47/41 dB | ~4.6                     | ~4.0                        |
+| 125 ms 48/42 dB | ~4.4                     | ~3.6                        |
+
+Echo (single talk)
+
+- "Acceptable" curve
+- "Limiting case" curve
+
+G.131\_FII.1
+
+Bar chart showing MOS results for single talk conditions across various echo delay and level settings. The chart compares 'Acceptable' (black) and 'Limiting case' (grey) curves against a reference.
+
+Figure II.1/G.131 – Results under single talk conditions
+
+![Bar chart showing MOS results for double talk conditions across various echo delay and level settings. The chart compares 'Acceptable' (black) and 'Limiting case' (grey) curves against a reference.](411fa16c3211377525ba37c57784fee0_img.jpg)
+
+This bar chart displays the Mean Opinion Score (MOS) for double talk conditions. The y-axis represents MOS from 1.0 to 5.0. The x-axis shows different test conditions. For each condition, two bars are shown: a black bar for the "Acceptable" curve and a grey bar for the "Limiting case" curve. Error bars are included for each data point.
+
+| Condition       | "Acceptable" curve (MOS) | "Limiting case" curve (MOS) |
+|-----------------|--------------------------|-----------------------------|
+| Reference       | -                        | ~4.5                        |
+| 10 ms 25/19 dB  | ~4.4                     | ~3.6                        |
+| 25 ms 33/27 dB  | ~4.5                     | ~4.3                        |
+| 50 ms 40/34 dB  | ~4.4                     | ~4.3                        |
+| 100 ms 47/41 dB | ~4.5                     | ~4.5                        |
+| 125 ms 48/42 dB | ~4.5                     | ~4.2                        |
+
+Echo (single talk)
+
+- "Acceptable" curve
+- "Limiting case" curve
+
+G.131\_FII.2
+
+Bar chart showing MOS results for double talk conditions across various echo delay and level settings. The chart compares 'Acceptable' (black) and 'Limiting case' (grey) curves against a reference.
+
+Figure II.2/G.131 – Results under double talk conditions
+
+The ratings from Figures II.1 and II.2 are given again in Table II.1.
+
+**Table II.1/G.131 – Echo assessment in LOT**
+
+| Conditions                | MOS single talk | MOS double talk |
+|---------------------------|-----------------|-----------------|
+| Reference (infinite TELR) | 4.62            | 4.60            |
+| "acceptable curve"        | 4.0-4.6         | 4.0-4.5         |
+| "limiting case"           | 3.5-4.0         | 4.0-4.5         |
+
+## II.3 Correlation between the results under single and double talk conditions
+
+Variable TELRs in combination with a transmission time of 100 ms were judged under single and double talk conditions. The correlation between the MOS under both conditions is demonstrated graphically in Figure II.3 for the parameters' overall quality and echo. The echo level offset under double talk condition is given as a function of MOS under single talk condition. It indicates the possible echo level offset under double talk condition to still achieve the same rating compared to the single talk condition.
+
+![Figure II.3/G.131: A line graph showing the relationship between MOS under single talk conditions (x-axis) and Offset (dB) (y-axis) for Overall quality and Echo. The x-axis ranges from MOS 1.0 to 5.0. The y-axis ranges from 2.0 to 18.0 dB. Two lines are plotted: Overall quality (squares) and Echo (triangles). The data points are: (MOS 2.5, 12.0 dB) for Overall quality and (MOS 2.5, 13.5 dB) for Echo; (MOS 3.0, 8.0 dB) for Overall quality and (MOS 3.0, 11.0 dB) for Echo; (MOS 3.5, 5.0 dB) for Overall quality and (MOS 3.5, 7.5 dB) for Echo; (MOS 4.0, 2.0 dB) for Overall quality and (MOS 4.0, 4.0 dB) for Echo.](1145fc59efdc7dacc8d3c715d7ff3248_img.jpg)
+
+Offset (dB)
+
+| MOS under single talk conditions | Overall quality Offset (dB) | Echo Offset (dB) |
+|----------------------------------|-----------------------------|------------------|
+| 2.5                              | 12.0                        | 13.5             |
+| 3.0                              | 8.0                         | 11.0             |
+| 3.5                              | 5.0                         | 7.5              |
+| 4.0                              | 2.0                         | 4.0              |
+
+Ratings under single talk conditions
+
+G.131\_FII.3
+
+—■— Overall quality  
+ —▲— Echo
+
+Figure II.3/G.131: A line graph showing the relationship between MOS under single talk conditions (x-axis) and Offset (dB) (y-axis) for Overall quality and Echo. The x-axis ranges from MOS 1.0 to 5.0. The y-axis ranges from 2.0 to 18.0 dB. Two lines are plotted: Overall quality (squares) and Echo (triangles). The data points are: (MOS 2.5, 12.0 dB) for Overall quality and (MOS 2.5, 13.5 dB) for Echo; (MOS 3.0, 8.0 dB) for Overall quality and (MOS 3.0, 11.0 dB) for Echo; (MOS 3.5, 5.0 dB) for Overall quality and (MOS 3.5, 7.5 dB) for Echo; (MOS 4.0, 2.0 dB) for Overall quality and (MOS 4.0, 4.0 dB) for Echo.
+
+**Figure II.3/G.131 – Echo level offset during double talk to achieve the same MOS values compared to the single talk condition (transmission time: 100 ms)**
+
+It can be assumed that a similar functional relation exists for other combinations of transmission time and TELR values although the exact curves might be slightly different.
+
+## Appendix III
+
+### Combined effects of talker echo in the presence of absolute delay
+
+Figure III.1 provides an overview of the combined effects of talker echo in the presence of absolute delay. It has been derived from the E-model of ITU-T Rec. G.107 [2] (see ITU-T Rec. G.114 [6] for a similar exhibit).
+
+![Line graph showing E-Model Rating R vs Mouth-to-ear delay (ms) for various TELR values and no talker echo.](cbc4516eb885829fe8c9dabc0946dcbe_img.jpg)
+
+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 legend indicates the following data series:
+
+- TELR = 25 dB (solid black line)
+- TELR = 35 dB (dashed magenta line)
+- TELR = 45 dB (dotted red line)
+- TELR = 55 dB (dash-dot cyan line)
+- TELR = 65 dB (dash-dot-dot purple line)
+- No talker echo (dash-dot-dot dark purple line)
+
+The graph shows that as the mouth-to-ear delay increases, the E-Model Rating R decreases for all conditions. The 'No talker echo' condition maintains the highest rating, while the 'TELR = 25 dB' condition shows the steepest decline, reaching a rating of 50 at approximately 30 ms delay.
+
+| Mouth-to-ear delay (ms) | TELR = 25 dB | TELR = 35 dB | TELR = 45 dB | TELR = 55 dB | TELR = 65 dB | No talker echo |
+|-------------------------|--------------|--------------|--------------|--------------|--------------|----------------|
+| 0                       | 93           | 93           | 93           | 93           | 93           | 93             |
+| 50                      | 60           | 80           | 85           | 90           | 92           | 93             |
+| 100                     | 50           | 65           | 70           | 85           | 90           | 93             |
+| 150                     | 50           | 55           | 60           | 80           | 88           | 92             |
+| 200                     | 50           | 50           | 55           | 75           | 85           | 90             |
+| 250                     | 50           | 50           | 50           | 68           | 80           | 85             |
+| 300                     | 50           | 50           | 50           | 60           | 75           | 80             |
+| 350                     | 50           | 50           | 50           | 55           | 70           | 75             |
+| 400                     | 50           | 50           | 50           | 50           | 65           | 70             |
+| 450                     | 50           | 50           | 50           | 50           | 60           | 65             |
+| 500                     | 50           | 50           | 50           | 50           | 55           | 62             |
+
+Line graph showing E-Model Rating R vs Mouth-to-ear delay (ms) for various TELR values and no talker echo.
+
+Figure III.1/G.131 – Combined effects of talker echo in the presence of absolute delay
+
+
+
+## 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, Internet protocol aspects and Next Generation Networks                                      |
+| Series Z        | Languages and general software aspects for telecommunication systems                                                           |