1、 International Telecommunication Union ITU-T G.108.2TELECOMMUNICATION STANDARDIZATION SECTOR OF ITU Amendment 1(10/2007) SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS International telephone connections and circuits General definitions Transmission planning aspects of echo c
2、ancellers Amendment 1: New Appendix III Guidance for using echo cancellers to prevent low-level echo ITU-T Recommendation G.108.2 (2007) Amendment 1 ITU-T G-SERIES RECOMMENDATIONS TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS INTERNATIONAL TELEPHONE CONNECTIONS AND CIRCUITS G.100G.199
3、 General definitions G.100G.109 General Recommendations on the transmission 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 i
4、nternational circuits and national extension circuits G.130G.139 General 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 lo
5、ng-distance telephone circuits G.160G.169 Transmission plan aspects 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 CHARACTERIS
6、TICS COMMON TO ALL ANALOGUE CARRIER-TRANSMISSION SYSTEMS G.200G.299 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 METALLI
7、C LINES G.400G.449 COORDINATION OF RADIOTELEPHONY AND LINE TELEPHONY G.450G.499 TRANSMISSION MEDIA AND OPTICAL SYSTEMS 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
8、GENERIC AND USER-RELATED ASPECTS G.1000G.1999 TRANSMISSION MEDIA CHARACTERISTICS G.6000G.6999 DATA OVER TRANSPORT GENERIC 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.1
9、08.2 (2007)/Amd.1 (10/2007) i ITU-T Recommendation G.108.2 Transmission planning aspects of echo cancellers Amendment 1 New Appendix III Guidance for using echo cancellers to prevent low-level echo Summary Amendment 1 to ITU-T Recommendation G.108.2 provides guidance for using echo cancellers to pre
10、vent low-level echo. Source Amendment 1 to ITU-T Recommendation G.108.2 (2007) was agreed on 11 October 2007 by ITU-T Study Group 12 (2005-2008). ii ITU-T Rec. G.108.2 (2007)/Amd.1 (10/2007) FOREWORD The International Telecommunication Union (ITU) is the United Nations specialized agency in the fiel
11、d 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 Wor
12、ld 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
13、 some 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 recog
14、nized 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 wor
15、ds “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
16、 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 Recommendati
17、on 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 ar
18、e therefore strongly urged to consult the TSB patent database at http:/www.itu.int/ITU-T/ipr/. ITU 2008 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 (2007)/Amd.1 (10/2007) 1 ITU-T Recommen
19、dation G.108.2 Transmission planning aspects of echo cancellers Amendment 1 New Appendix III Guidance for using echo cancellers to prevent low-level echo (This appendix does not form an integral part of this Recommendation) Echo control has proven to be a major challenge in efforts to deliver PSTN-e
20、quivalent voice quality in the VoIP environment. The primary echo problem encountered early in the VoIP evolution is referred to as “initial echo“ where the customer hears echo on the first few syllables they speak at the beginning of a call. Fortunately, this problem has been significantly reduced
21、by the introduction of network echo cancellers that have improved (i.e., faster) convergence speeds. While customer complaints of initial echo have been significantly reduced, another echo problem has surfaced that involves very low, but detectable, voice reflections that can be audible for the enti
22、re duration of a call. This low-level echo problem occurs on calls terminated in IP phones that have some form of echo control designed into them, and on hybrid IP/PSTN connections where network-based echo cancellers (e.g., in a media gateway) are used to cancel echo reflected from the PSTN. Given t
23、he presence of active echo control, why the echo? The problem with IP phones has pretty much been traced to echo canceller designs that, while reducing the level of echo coming from the set itself, did not insert enough loss and, thus, let leak through a level of echo that was still audible by the c
24、ustomer at the other end of the connection. Low-level echo involving IP phones has been addressed to a large extent by the IP phones being manufactured with improved echo canceller designs. Efforts to address complaints of echo on hybrid IP/PSTN connections with a media gateway echo canceller have b
25、een complicated by a different underlying issue. Some network echo cancellers disable on connections assumed to not need an active echo canceller, with the severity of the echo problem depending on the algorithm the canceller uses to decide it is unneeded. Some cancellers have been observed to disab
26、le when the echo return loss (ERL) seen by the canceller was at least 35 dB; others at 45 dB; and yet another that disabled based on an absolute level of 65 dBm for the echo signal reaching its near-in port. What constitutes an acceptable level of echo signal reduction has become increasingly proble
27、matic with the now-common use of amplified receivers in IP phones and headset adjuncts that allow customers to add gain to the receive signal. This gain can be substantial; for example, with one popular IP phone design, 21 dB of gain can be applied if the customer uses the maximum setting of the rec
28、eive amplifier. Thus in the case where an echo canceller disables for an ERL of 45 dB, a loud-speaking terminal whose speech level reaches the canceller at 10 dBm will be reflected back to that customer at 55 dBm (reduced by the 45 dB of ERL). But if the customer adds 10 dB of gain to the connection
29、 with their amplified receive feature, the echo level would then be 45 dBm, a low but readily audible echo on calls with the long delays of VoIP. 2 ITU-T Rec. G.108.2 (2007)/Amd.1 (10/2007) With a design that uses the absolute 65 dBm level as the disabling threshold, the likelihood of audible low-le
30、vel echo is significantly reduced. Additionally, by controlling echoes down to, that design meets the ITU-T G.168 Test 2A requirement that the return echo level of the canceller be 65 dBm or greater for inputs in the 10 to 30 dBm range. Even so, we have found that some echo problems may occur with s
31、uch a canceller design, because actual speech signals are often more dynamic than the G.168-defined test signal (the composite source signal), with spikes of speech leaking through that are approximately 60 dBm in level. It is recognized that processing and memory resources can be saved by disabling
32、 echo cancellers where it is assumed they are not needed. However, problems experienced with low-level echo in actual VoIP deployments lead to the conclusion that this practice introduces an impairment (with associated customer complaints) that more than offsets any benefit. Accordingly, network can
33、cellers should not be disabled because it is assumed there is sufficient ERL without them, as the factors described here show that such an assumption may be flawed. Printed in Switzerland Geneva, 2008 SERIES OF ITU-T RECOMMENDATIONS Series A Organization of the work of ITU-T Series D General tariff
34、principles Series E Overall network operation, telephone service, service operation and human factors Series F Non-telephone telecommunication services Series G Transmission systems and media, digital systems and networks Series H Audiovisual and multimedia systems Series I Integrated services digit
35、al network Series J Cable networks and transmission of television, sound programme and other multimedia signals Series K Protection against interference Series L Construction, installation and protection of cables and other elements of outside plant Series M Telecommunication management, including T
36、MN 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 Series R Telegrap
37、h transmission Series S Telegraph services terminal equipment Series T Terminals for telematic services Series U Telegraph switching Series V Data communication over the telephone network Series X Data networks, open system communications and security Series Y Global information infrastructure, Internet protocol aspects and next-generation networks Series Z Languages and general software aspects for telecommunication systems
