A voice signal that is reflected back to the speaker at an audible level so that it interferes with the ability to have a normal conversation with another party is called echo. In recent years, echo has mostly been imperceptible in circuit-switched networks due to their low delay and the deployment of echo cancellers. IP calls can experience a much larger delay, and therefore echo can be much more noticeable.
Echo can be created in two ways:
Acoustically, in a telephone handset, a telephone that is operating in speakerphone mode, a speakerphone, a headset, or a multimedia laptop computer or desktop computer with a headset or an integrated or separate microphone and speaker. In particular, speakerphones or telephones that are operating in speakerphone mode provide a high level of acoustical echo return signal. The level of acoustic echo is determined by the acoustics of the environment (such as wall and ceiling reflection), the degree to which loudspeaker and microphone are directed towards each other, and the directional acoustic characteristics of the microphone.
Electrically, by impedance mismatches in 2-to-4 wire hybrids on analog line or trunk cards, or electrical cross-talk interference in wires or headset adapters.
In general, the perception of echo is call dependent. The perceived echo problems for calls that are made over a WAN are normally much larger compared with calls that are made over a LAN because of the larger delay in WAN-connected systems.
As echo is not caused by an IP network (although it is exacerbated by delay), so its resolution will not be covered in detail in this document. In general, there are three strategies for dealing with echo:
When echo is experienced, the problem is generally resolved at the far-end of the link. For more information, see Avaya IP Voice Quality Network Requirements.
