B.5. Echo

You may not realize it, but echo has been a problem in the PSTN for as long as there have been telephones. You probably haven’t often experienced it, because the telecom industry has spent large sums of money designing expensive echo-cancellation devices. Also, when the endpoints are physically close—e.g., when you phone your neighbor down the street—the delay is so minimal that anything you transmit will be returned back so quickly that it will be indistinguishable from the sidetone^[259] normally occurring in your telephone. So, the fact of the matter is that there is echo on your local calls much of the time, but you cannot perceive it with a regular telephone because it happens almost instantaneously. It may help you to understand this if you consider that when you stand in a room and speak, everything you say echos back to you off of the walls and ceiling (and possibly the floor, if it’s not carpeted), but this does not cause any problems because it happens so fast you do not perceive a delay.

^[259] As discussed in Appendix A, sidetone is a function in your telephone that returns part of what you say back to your own ear, to provide a more natural-sounding conversation.

The reason that VoIP telephone systems such as Asterisk can experience echo is that the addition of a VoIP telephone introduces a slight delay. It takes a few milliseconds for the packets to travel from your phone to the server (and vice versa). Suddenly there is an appreciable delay, which allows you to perceive the echo that was always there, but never really noticeable.