1、 INTERNATIONAL TELECOMMUNICATION UNION ITU-T G.114TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (05/2003) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits General Recommendations on the transmission quality for an entire intern
2、ational telephone connection One-way transmission time ITU-T Recommendation G.114 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199 General definitions G.100G.109 General Recommendations on the tran
3、smission quality for an entire international telephone connection G.110G.119 General characteristics of national systems forming part of international connections G.120G.129 General characteristics of the 4-wire chain formed by the international circuits and national extension circuits G.130G.139 Ge
4、neral characteristics of the 4-wire chain of international circuits; international transit G.140G.149 General characteristics of international telephone circuits and national extension circuits G.150G.159 Apparatus associated with long-distance telephone circuits G.160G.169 Transmission plan aspects
5、 of special circuits and connections using the international telephone connection network G.170G.179 Protection and restoration of transmission systems G.180G.189 Software tools for transmission systems G.190G.199 GENERAL CHARACTERISTICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299
6、 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON METALLIC LINES G.300G.399 GENERAL CHARACTERISTICS OF INTERNATIONAL CARRIER TELEPHONE SYSTEMS ON RADIO-RELAY OR SATELLITE LINKS AND INTERCONNECTION WITH METALLIC LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHON
7、Y G.450G.499 TESTING EQUIPMENTS G.500G.599 TRANSMISSION MEDIA CHARACTERISTICS G.600G.699 DIGITAL TERMINAL EQUIPMENTS G.700G.799 DIGITAL NETWORKS G.800G.899 DIGITAL SECTIONS AND DIGITAL LINE SYSTEM G.900G.999 QUALITY OF SERVICE AND PERFORMANCE GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSIO
8、N MEDIA CHARACTERISTICS G.6000G.6999 DIGITAL TERMINAL EQUIPMENTS G.7000G.7999 DIGITAL NETWORKS G.8000G.8999 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. G.114 (05/2003) i ITU-T Recommendation G.114 One-way transmission time Summary This Recommendation provides g
9、uidance on the effect of end-to-end one-way delay (sometimes termed latency), and an upper bound one-way network delay. While it is recommended that a one-way delay of 400 ms should not be exceeded for general network planning, it is important to appreciate that highly interactive tasks (e.g., many
10、voice calls, interactive data applications, video conferencing) can be affected by much lower delays. The effects of delays below 500 ms on conversational speech are estimated using a curve derived from the E-model (ITU-T Rec. G.107). This version constitutes a major revision of this Recommendation
11、in order to align with other ITU-T Recommendations of the G.100 series. Source ITU-T Recommendation G.114 was approved by ITU-T Study Group 12 (2001-2004) under the ITU-T Recommendation A.8 procedure on 6 May 2003. It includes the modifications introduced by ITU-T Rec. G.114 (2003) Appendix II appro
12、ved on 30 September 2003. ii ITU-T Rec. G.114 (05/2003) 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
13、 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
14、 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-Ts purview, the necessary standards are prepared on a collaborative basis w
15、ith 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 pro
16、visions (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 s
17、uch 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 posit
18、ion 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, protec
19、ted by patents, which may be required to implement this Recommendation. However, implementors are cautioned that this may not represent the latest information and are therefore strongly urged to consult the TSB patent database. ITU 2003 All rights reserved. No part of this publication may be reprodu
20、ced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. G.114 (05/2003) iii CONTENTS Page 1 Introduction 1 2 References. 1 3 Applicability to speech transmission quality Use of the E-model. 2 4 Recommendations for one-way transmission time. 2 5 Estimating end-to-end del
21、ay based on assemblies of transmission elements. 3 Annex A End-to-end delay estimation 4 A.1 Planning values for the delay of transmission elements. 4 A.2 Codec delay 5 A.3 Delay due to IP delay variation buffer . 7 Appendix I Delay introduced by coder-related processing. 8 Appendix II Guidance on o
22、ne-way delay for voice over IP . 12 II.1 Introduction 12 II.2 Achieving satisfactory delay 12 ITU-T Rec. G.114 (01/2003) 1 ITU-T Recommendation G.114 One-way transmission time 1 Introduction This Recommendation provides guidance on the effect of end-to-end one-way delay (sometimes termed latency), a
23、nd an upper bound on one-way network delay. The effect of delay on speech transmission quality can be estimated by the use of a curve derived from the Transmission Rating Model of ITU-T Rec. G.107 3, which is the recommended ITU-T method for end-to-end speech transmission planning. ITU-T Rec. G.108
24、4 gives detailed examples on how to use the model to assess the transmission performance of connections involving various impairments, including one-way delay; and ITU-T Rec. G.109 5 maps transmission rating predictions of the model into categories of speech transmission quality. Thus, while ITU-T R
25、ec. G.114 provides useful information regarding one-way delay as a parameter by itself, ITU-T Rec. G.107 3 (and its ITU-T Rec. G.108 4 and ITU-T Rec. G.109 5 companions) should be used to assess the effects of delay in conjunction with other impairments (e.g., distortions due to speech processing).
26、Highly interactive tasks (e.g., some speech, video conferencing and interactive data applications) may be affected by delays below 100 ms, as per test result documented in Annex B of previous versions of ITU-T Rec. G.114. For this reason, previous versions of this Recommendation noted that if delays
27、 were kept below 150 ms, then most applications would not be significantly affected. Additionally, an upper limit of 400 ms for network planning purposes was always a part of ITU-T Rec. G.114. However, this parallel treatment of network delays on one hand, with application (“mouth-to-ear“) level del
28、ays on the other hand, led to confusion in how ITU-T Rec. G.114 should be applied. Fortunately, with the development and approval of the E-model (ITU-T Rec. G.107 3), which is based on subjective tests of delay (among other parameters), there now exists an agreed way of estimating the effects of del
29、ay on mouth-to-ear speech transmission quality. Accordingly, simple and straightforward guidance can now be provided for the effects of delay on speech transmission and is given in the present Recommendation. The lack of similar tools for non-speech applications is a subject for further study, so th
30、is Recommendation can only provide general planning guidance. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At the time of publication, the editions indicated were valid. A
31、ll 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 r
32、egularly 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 circu
33、its. 2 ITU-T Recommendation G.101 (1996), The transmission plan. 3 ITU-T Recommendation G.107 (2003), The E-Model, a computational model for use in transmission planning. 4 ITU-T Recommendation G.108 (1999), Application of the E-model: A planning guide. 2 ITU-T Rec. G.114 (05/2003) 5 ITU-T Recommend
34、ation G.109 (1999), Definition of categories of speech transmission quality. 6 ITU-T Recommendation G.131 (1996), Control of talker echo. 7 ITU-T Recommendation G.168 (2002), Digital network echo cancellers. 8 ITU-T Recommendation G.763 (1998), Digital circuit multiplication equipment using G.726 AD
35、PCM and digital speech interpolation. 9 ITU-T Recommendation G.764 (1990), Voice packetization Packetized voice protocols. 10 ITU-T Recommendation G.766 (1996), Facsimile demodulation/remodulation for digital circuit multiplication equipment. 11 ITU-T Recommendation G.767 (1998), Digital circuit mul
36、tiplication equipment using 16 kbit/s LD-CELP, digital speech interpolation and facsimile demodulation/remodulation. 12 ITU-T Recommendation Q.551 (2002), Transmission characteristics of digital exchanges. 13 ITU-T Recommendation Y.1541 (2002), Network performance objectives for IP-based services. 3
37、 Applicability to speech transmission quality Use of the E-model This Recommendation provides end-to-end limits for mean one-way delay, independent of other transmission impairments. The need to consider the combined effects of all impairments on speech transmission quality is addressed by the Trans
38、mission Rating Model of ITU-T Rec. G.107 3, which is the recommended ITU-T method for end-to-end speech transmission planning. ITU-T Rec. G.108 4 gives detailed examples on how to use the model to assess the transmission performance of connections involving various impairments, including delay; and
39、ITU-T Rec. G.109 5 maps transmission rating predictions of the model into categories of speech transmission quality. Thus, while this Recommendation provides useful information regarding mean one-way delay as a parameter by itself, ITU-T Rec. G.107 3 (and its ITU-T Rec. G.108 4 and ITU-T Rec. G.109
40、5 companions) should be used to assess the effects of delay in conjunction with other impairments (e.g., distortions due to speech processing). 4 Recommendations for one-way transmission time Regardless of the type of application, it is recommended to not exceed a one-way delay of 400 ms for general
41、 network planning (i.e., UNI to UNI, as illustrated, for example, in ITU-T Rec. Y.1541 13), a value that allows flexibility in deploying global networks, without making an excessive number of user experiences unacceptable. However, it is desirable to keep the delays seen by user applications as low
42、as possible. The E-model should be used to estimate the effect of one-way delay (including all delay sources, i.e., “mouth-to-ear“) on speech transmission quality for conversational speech as shown below. For non-speech applications such as interactive data or video, there are no agreed-upon assessm
43、ent tools like the E-model, so the effects of delay on such applications must be carefully monitored. Although a few applications may be slightly affected by end-to-end (i.e., “mouth-to-ear“ in the case of speech) delays of less than 150 ms, if delays can be kept below this figure, most applications
44、, both speech and non-speech, will experience essentially transparent interactivity. While delays above 400 ms are unacceptable for general network planning purposes, it is recognized that in some exceptional cases this limit will be exceeded. An example of such an exception is an unavoidable double
45、 satellite hop for a hard-to-reach location, the impact of which can be estimated by use of the advantage factor in the E-model. ITU-T Rec. G.114 (01/2003) 3 Regarding the use of the E-model for speech applications, the effect of delay can be seen in the following graph of Transmission Rating, R, ve
46、rsus delay. Also shown are the speech quality categories of ITU-T Rec. G.109 5, which translate the R values to levels of user acceptance. G.114_F010 100 200 300 400 5005060708090100Nearly allusersdissatisfiedMany usersdissatisfiedSome usersdissatisfiedUserssatisfiedUsersvery satisfiedMouth-to-ear-d
47、elay/msE-modelratingRFigure 1/G.114 Determination of the effects of absolute delay by the E-model NOTE 1 The curve in Figure 1 is based on the effect of pure delay only, i.e., in the complete absence of any echo. This is calculated by setting the G.107 E-model parameter Ta equal to the total value o
48、f one-way delay from mouth-to-ear, with all other E-model input parameter values set to their default values. The effect of echo, as would be incurred due to imperfect echo control, will result in lower speech quality for a given value of one-way delay. NOTE 2 The calculation also assumes an Equipme
49、nt Impairment Factor (Ie) of zero. Non-zero values, as would be incurred due to speech coding/processing, will result in lower speech quality for a given value of one-way delay. NOTE 3 For one-way delay values exceeding 500 ms, the graph is continued as a dashed line to indicate that these results are not fully validated, but is the best estimate of what should be expected, and, therefore, provides useful guidance. 5 Estimating end-to-end delay based on assemblies of transmission elements
