Echo is the sound of your own voice reverberating in the telephone receiver while you are talking. When timed properly, echo is not a problem in the conversation; however, if the echo interval exceeds approximately 25 milliseconds (ms), it can be distracting to the speaker. In the traditional telephony network, echo is generally caused by an impedance mismatch when the four-wire network is converted to the two-wire local loop. Echo is controlled by echo cancellers (ECs).
A packet voice gateway, which operates between a digital packet network and the PSTN, can include both digital (time division multiplexing [TDM]) and analog links. The analog circuit is known as the tail circuit. It forms the tail or termination of the call from the perspective of the person experiencing the echo. The tail circuit is everything connected to the PSTN side of a packet voice gateway--all the switches, multiplexers, cabling, and PBXs between the voice gateway and the telephone.
The figure below shows a common voice network where echo cancellation might be used.
Figure 2. Echo Cancellation Network
An echo canceller reduces the level of echoes that leak from the Rx path (from the gateway out into the tail circuit) into the Tx path (from the tail circuit into the gateway). From the perspective of the echo canceller in a voice gateway, the Rx signal is a voice coming across the network from another location. The Tx signal is a mixture of the voice call in the other location and the echo of the original voice, which comes from the tail circuit on the initiating end and is sent to the receiving end.
Echo cancellers face into the PSTN tail circuit. They eliminate echoes in the tail circuit on its side of the network.The echo canceller in the originating gateway looks out into the tail circuit and is responsible for eliminating the echo signal from the initiation Tx signal and allowing a voice call to go through unimpeded. By design, ECs are limited by the total amount of time they wait for the reflected speech to be received, which is known as an echo tail. The echo tail is normally 32 ms.
Delay and jitter in the WAN do not affect the operation of the echo canceller because the tail circuit, where the echo canceller operates, is static.
Echo cancellation is implemented in digital signal processor (DSP) firmware (DSPWare) on Cisco voice gateways and is independent of other functions implemented in the DSP (the DSP protocol and compression algorithm). In voice packet-based networks, ECs are built into the low-bit-rate codecs and are operated on each DSP.
The figure below shows a typical DSP channel configured for voice processing.
Figure 3. DSP Channel Configured for Voice Processing