It is called Near End because the echo is cancelled nearest to the echo source. It is possible to perform echo cancellation at the far end, Gateway A, but the echo path will be much longer, normally requiring more processing power. The echo canceller compares the signal similarity and power level coming from the hybrid switch to the previously stored signals passed from the packet network. The comparison is made using a model of the hybrid circuit, which was created by an adaptive algorithm inside the echo canceller. However, identifying echo is a difficult task for the adaptive filter because of double-talk issues.
Double talk occurs when both sides of a conversation attempt to speak simultaneously.
The near end speaker, phone B in Figure 1, confuses the echo identification process, thus limiting the effectiveness of the echo canceller. Also, The echo canceller must not clip the beginning of the double talk session. The echo canceller also needs to identify and remove background noise. Once an echo is identified, the echo canceller subtracts the echo from the returning signal. Echo cancellation can be very resource intensive.