
308 ◾ IP Communications and Services for NGN
Echo
Echo is hearing your own voice reflected back to you in the telephone receiver
while you are talking. If the duration of echo exceeds approximately 25 ms, it can
be distracting and cause breaks in the conversation. Echo in a traditional telephony
network is caused by impedance mismatch from the four-wire network switch con-
version to the two-wire local loop and can be controlled by echo cancellers. Echo
cancellers are set by design by the total amount of time they will wait to receive the
reflected speech. is is known as an echo trail and is normally about 32 ms. e
ITU-T echo cancellation sta ...