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. Recommendation ITU-T 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.