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If echo usually occurs at the far end of a call, echo cancellation is ideally done at the far end of the call as well. However, it can be done at any analog boundary in the network, with varying degrees of success. In one sense, you cannot eliminate an echo originating at the far end of the circuit if you do not control the equipment at the far end. However, you can minimize it using echo cancellation (ECAN) devices.
One place where telecom or network engineers try to minimize echo in a VoIP system is at the gateway that connects an IP network to the PSTN. Although the echo originates at the far end of the call, echo cancellation here can work, within limits.
An echo cancellation device compares the signal going into the tail circuit with the signal coming back out. The ECAN remembers the signal pattern of the signal entering the tail circuit and examines the data exiting the circuit to see if it contains this pattern. If I say, “Is that good?” and you reply, “Fine” the ECAN is remembering “Is that good?” and looking for it mixed in with “Fine.”
Two dimensions of the ECAN’s work are important: echo strength, or volume, and echo delay.
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