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. ITU-T Recommendation G.168 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 ITU-T Recommendation G.168 is a revision of the previously published version of August 2004. Any ambiguities and errors contained in this previous version have been fixed. The specification of delay through an echo canceller defined in clause 6.4.1.1 has been clarified. The requirements of the fax test (Test 10) have been clarified. A structure for Test No. 11 (tandem echo canceller test) has been defined. A major change in this revision of ITU-T Recommendation G.168 is the revision of Test No. 14 (data modem test) that has been re-worked and now includes a mandatory set of modems that should be used to perform this test. For users wishing to demonstrate compliance against a wider set of modems, a more comprehensive list is given in new Appendix V. Appendix II has been enhanced with an additional method of measuring the response of an echo path.