ITU-T Recommendations

Echo has a major effect on voice quality in telecommunication networks. The objectionable effect of echo results from a combination of reflections from network components such as 2- to 4-wire converters, together with signal processing and transmission delay. Echo may cause users difficulty in talking or listening over a telephone connection. It may also affect the transmission of voiceband data, fax and text telephones.Digital network echo cancellers are designed to eliminate echo for the user and to allow successful transmission of voiceband data and fax. This Recommendation describes the characteristics of an echo canceller, including the requirement for in-band tone disabling and other control mechanisms. It also describes a number of laboratory tests that should be performed on an echo canceller to assess its performance under conditions likely to be experienced in the network.This version of G.168 is a revision of the previously published version of June 2002. All of the ambiguities and errors contained in this previous version have been fixed. A re-convergence test has been added to Test No. 2 and requirements have been specified for Test No. 2C part (c). Further clarification is given on the definition of "convergence time", since this is often mis-quoted in echo canceller literature. Test No. 3 (double-talk) has been modified to examine both convergence and stability during periods of double-talk and some new requirements have been added to Test No. 3B. New Appendix IV gives some guidelines on the use of parameters for anyone testing echo cancellers to this Recommendation. Some tests remain under study, namely Test No. 10C (fax), Test No. 11 (tandem echo cancellers), Test No. 12 (residual acoustic echo) and Test No. 13 (low bit rate coders). It is the intention that these tests will be addressed in a subsequent revision to this Recommendation.