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‡ 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 be helpful 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 floor if it’s not carpeted), but does not cause any problems because it happens so fast you do not perceive a delay. 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 and the server (and vice versa). Suddenly there is an appreciable delay, which allows you to perceive the echo that was always there, but never had a delay before. |
Why Echo Occurs |
Before we discuss measures to deal with echo, let’s first take a look at why echo occurs in the analog world. If you hear echo, it’s not your phone that’s causing the problem; it’s the far end of the circuit. Conversely, echo heard on the far end is being generated at your end. Echo can be caused by the fact that an analog local loop circuit has to transmit and receive on the same pair of wires. If this circuit is not electrically balanced, or if a low-quality telephone is connected to the end of the circuit, signals it receives can be reflected back, becoming part of the return transmission. When this reflected circuit gets back to you, you will hear the words you spoke just moments before. Humans will perceive an echo |