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Echo often happens when a VoIP call leaves the LAN through a mis-administered analog trunk into the PSTN. Echo can result when there is an impedance mismatch between four-wire and two wire systems. An impedance mismatch in the conversion between the TDM bus and the LAN or a headset and its adapter may also cause echo. Impedance mismatch results in an inefficient energy transfer; the energy imbalance must go somewhere and is reflected back in the form of an echo. Usually the speaker hears an echo but the receiver does not.
Echo cancellers compare the received voice with the current voice patterns. If the patterns match, the canceller cancels the echo. However, echo cancellers aren't perfect, and under some circumstances the echo may be greater than the canceller can adjust. The problem is exacerbated in VoIP systems. If the one-way trip delay between endpoints is larger than the echo canceller memory, the echo canceller can't discover the pattern to cancel.
G700 Media Gateways and other Avaya IP Solutions incorporate echo cancellers designed for VoIP to improve voice quality.
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