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.
The revision of ITU-T G.168 approved 02/2012 contains enhancements to existing tests, including new Test 5B that measures performance during echo path changes (e.g., due to call forwarding) and new Test 9B that measures the dynamic performance of comfort noise algorithms. Further clarification is provided around the use of echo cancellers in conference bridges.