1、 International Telecommunication Union ITU-T G.108.2TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU (03/2007) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits General definitions Transmission planning aspects of echo cancellers I
2、TU-T Recommendation G.108.2 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 transmission quality for an entire international telephone
3、 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 General characteristics of the 4-wire chain of internati
4、onal 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 of special circuits and connections using the interna
5、tional 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 INDIVIDUAL CHARACTERISTICS OF INTERNATIONAL CARRIER T
6、ELEPHONE 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 TELEPHONY G.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS CH
7、ARACTERISTICS 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 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC
8、 ASPECTS G.7000G.7999 PACKET OVER TRANSPORT ASPECTS G.8000G.8999ACCESS NETWORKS G.9000G.9999 For further details, please refer to the list of ITU-T Recommendations. ITU-T Rec. G.108.2 (03/2007) i ITU-T Recommendation G.108.2 Transmission planning aspects of echo cancellers Summary ITU-T Recommendati
9、on G.108.2 provides guidance for the transmission planning aspects in conjunction with the deployment of echo cancellers in the network. Recognizing that echo cancellers are required on an increasing number of connections due to the increase of end-to-end delay by the introduction of speech processi
10、ng techniques as well as of packet-based transportation mechanisms, this Recommendation is intended to assist network operators and transmission planners as well as equipment manufacturers and application designers in controlling the effects of echo cancellers on end-to-end speech transmission perfo
11、rmance. Source ITU-T Recommendation G.108.2 was approved on 1 March 2007 by ITU-T Study Group 12 (2005-2008) under the ITU-T Recommendation A.8 procedure. ii ITU-T Rec. G.108.2 (03/2007) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the field of
12、 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 T
13、elecommunication 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 som
14、e areas of information technology which fall within ITU-Ts 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 recognize
15、d 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 “
16、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
17、 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 d
18、evelopment 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 th
19、erefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2007 All rights reserved. No part of this publication may be reproduced, by any means whatsoever, without the prior written permission of ITU. ITU-T Rec. G.108.2 (03/2007) iii CONTENTS Page 1 Introduction
20、1 2 References. 1 3 Definitions 2 4 Abbreviations and acronyms 2 5 Conventions 2 6 Echo control in telecommunication networks 2 6.1 Echo control devices. 3 6.2 General echo control considerations from a PSTN perspective. 4 6.3 Additional echo control considerations from the perspective of interconne
21、cted networks (e.g., private networks, the Internet) 4 6.4 Application-related echo control considerations 5 6.5 Voiceband data modem constraints 5 7 Application rules and operational constraints. 5 7.1 Public network transmission planning 5 7.2 Delay considerations 5 7.3 Provisioning of the echo-pa
22、th capacity and echo-path characteristics. 6 7.4 Echo canceller transmission planning for multiple interconnected networks (e.g., public, private, Internet) 7 8 Network and service evolutionary considerations 7 8.1 Bit transparency of echo cancellers 7 8.2 Non-linearities and time-variant effects in
23、 the echo path. 7 8.3 Low bit-rate coding between tandem cancellers 8 8.4 Tandeming of echo cancellers 8 8.5 Convergence speed. 10 8.6 Acoustic echo control and environments . 10 8.7 New circuit-switched service . 11 8.8 Comfort noise . 11 9 Special CME networking considerations 12 9.1 Detailed inte
24、raction 12 9.2 Possible solutions . 13 Appendix I Echo-path dispersion time 14 I.1 Echo-path characteristics from measurements in North America 14 Appendix II Multiple tail circuits 15 II.1 Multiple tails in a typical bridged telephone call (North America) 15 ITU-T Rec. G.108.2 (03/2007) 1 ITU-T Rec
25、ommendation G.108.2 Transmission planning aspects of echo cancellers 1 Introduction Echo cancellers are adaptive signal processors used to control echo1. Echo cancellers are present on nearly every long-distance connection and may today and in future be required on an increasing number of shorter co
26、nnections which gather delay from sources other than propagation (e.g., coding, signal processing, packetization). Unintended tandem operation of echo cancellers also is an issue which needs increasing consideration. The purpose of this Recommendation is to: provide guidance on the general principle
27、s of operation of echo cancellers; identify application rules and constraints under which echo cancellers operate properly; provide guidance on the different tasks of transmission planners regarding the control of echo. The user of this Recommendation is advised on the availability of ITU-T G.161, “
28、Interaction aspects of signal processing network equipment“, parts of which cover issues which are also dealt with here. 2 References The following ITU-T Recommendations and other references contain provisions which, through reference in this text, constitute provisions of this Recommendation. At th
29、e 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 belo
30、w. 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.122 ITU-T Recommendation G.122 (1993), Influence of national systems on stability
31、 and talker echo in international connections. ITU-T G.131 ITU-T Recommendation G.131 (2003), Talker echo and its control. ITU-T G.161 ITU-T Recommendation G.161 (2004), Interaction aspects of signal processing network equipment. ITU-T G.164 ITU-T Recommendation G.164 (1988), Echo suppressors. ITU-T
32、 G.165 ITU-T Recommendation G.165 (1993), Echo cancellers. ITU-T G.168 ITU-T Recommendation G.168 (2007), Digital network echo cancellers. ITU-T G.961 ITU-T Recommendation G.961 (1993), Digital transmission system on metallic local lines for ISDN basic rate access. ITU-T P.10/G.100 ITU-T Recommendat
33、ion P.10/G.100 (2006), Vocabulary for performance and quality of service. ITU-T P.300 ITU-T Recommendation P.300 (2001), Transmission performance of group audio terminals (GATs). ITU-T P.310 ITU-T Recommendation P.310 (2003), Transmission characteristics for telephone-band (300-3400 Hz) digital tele
34、phones. _ 1Echo cancellers have in modern telecommunication networks replaced echo suppressors. 2 ITU-T Rec. G.108.2 (03/2007) ITU-T P.340 ITU-T Recommendation P.340 (2000), Transmission characteristics and speech quality parameters of hands-free telephones. ITU-T P.341 ITU-T Recommendation P.341 (2
35、005), Transmission characteristics for wideband (150-7000 Hz) digital hands-free telephony terminals. ITU-T P.342 ITU-T Recommendation P.342 (2000), Transmission characteristics for telephone band (300-3400 Hz) digital loudspeaking and hands-free telephony terminals. ITU-T P.561 ITU-T Recommendation
36、 P.561 (2002), In-service non-intrusive measurement device Voice service measurements. ITU-T V.32 ITU-T Recommendation V.32 (1993), A family of 2-wire, duplex modems operating at data signalling rates of up to 9600 bit/s for use on the general switched telephone network and on leased telephone-type
37、circuits. ITU-T V.34 ITU-T Recommendation V.34 (1998), A modem operating at data signalling rates of up to 33 600 bit/s for use on the general switched telephone network and on leased point-to-point 2-wire telephone-type circuits. 3 Definitions This Recommendation defines the following terms: 3.1 ac
38、tivated NLP: A non-linear processor (NLP) that is performing signal processing on the speech path and is non-transparent to signals. NOTE The definition of an NLP is given in ITU-T G.168. 3.2 deactivated NLP: A non-linear processor (NLP) that is not performing signal processing on the speech path an
39、d is transparent to signals. 3.3 echo cancellers in tandem: Multiple echo cancellers in a connection that are meant to cancel echo from the same source. 4 Abbreviations and acronyms This Recommendation uses the following abbreviations and acronyms: DTDT Double-Talk Detection Threshold NEST Near-End
40、Speech Threshold NLP Non-Linear Processor PCM Pulse Code Modulation SLA Service Level Agreement 5 Conventions None. 6 Echo control in telecommunication networks In the light of liberalization, responsibility of the PSTN transmission planner for the totality of the national section of the worldwide t
41、elecommunication plant can no longer be assumed. Therefore, shared responsibilities have to be implemented. ITU-T Rec. G.108.2 (03/2007) 3 Whereas each and every transmission planner of every and each single telecommunications network may deploy echo cancellers for intra-network connections adequate
42、ly, the taking into account of inter-network connections immediately will bring one or more assumptions into the equation. Such assumptions may be based on either of the following: enforced national or regional transmission plans (where appropriate); historically embedded knowledge about the network
43、 interconnected to; service level agreements (SLAs); in-service non-intrusive measurements (e.g., according to ITU-T P.561); information provided by means of signalling; experience. 6.1 Echo control devices Whereas formerly echo suppressors ITU-T G.164 were in use to control echo on long-haul connec
44、tions, nowadays only echo canceller are recommended. Different combinations of G.164, G.165 and G.168 tone disablers may be found. The following combinations are possible: G.164 echo suppressor with G.164 tone disabler; G.165 echo canceller with G.164 or G.165 tone disabler; G.168 echo canceller wit
45、h G.164, G.165 or G.168 tone disabler. NOTE Echo suppressors are no longer recommended by the ITU-T. Nevertheless they may still be in operation in some places. While for the purpose of transmission planning, the term “echo canceller“ is usually to be understood as referring to digital network echo
46、cancellers for speech signals according to ITU-T G.168 or ITU-T G.165, devices compliant with ITU-T G.168 are to be preferred. Acoustical echo cancellers according to ITU-T P.340 may be considered as part of the terminal and thus are subject to transmission planning in exceptional cases, only. Echo-
47、cancelling devices for digital data transmission circuits (e.g., full-duplex echo canceller modem V.32 ITU-T V.32, xDSL, ISDN) are also not considered for transmission planning; further information in this regard is to be found, e.g., in ITU-T G.961. 6.1.1 Principles of operation A voice-operated de
48、vice placed in the 4-wire portion of a circuit and used for reducing the cancelled end echo present on the send path by subtracting an estimation of that echo from the cancelled end echo. Those reflections are referred to as talker echo; for detailed guidance on these parameters, see ITU-T G.131. Th
49、e echo canceller typically consists of various components: The key component is an adaptive filter which simulates the echo path. The algorithm which has to control the adaptive filter may be realized with different approaches, which results in a variety of different (proprietary) algorithms, having each their individual benefits and drawbacks. In addition, a non-linear processor is found in all echo cancellers which suppresses the residual e
