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 April 2000. Many of the tests have been updated and improved. A PCM offset test has been added. Annex A has been deleted and transferred to clause 7. Annex D has been revised and a new double reflection echo-path model has been added. Appendix I has been shortened, with the planning and interaction aspects of echo cancellers moved to other ITU-T Recommendations. Appendix II has also been updated with some examples of European echo path characteristics.