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[215] 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.

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.

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 beyond a certain amount of delay (possibly as low as 20 milliseconds for some people). This echo will become annoying as the delay increases.

In a cheap telephone, it is possible for echo to be generated in the body of the handset. This is why some cheap IP phones can cause echo even when the entire end-to-end connection does not contain an analog circuit.[216] In the VoIP world, echo is usually introduced either by an analog circuit somewhere in the connection, or by a cheap endpoint reflecting back some of the signal (e.g., feedback through a hands-free or poorly designed handset or headset). The greater the latency on the network, the more annoying this echo can be.

Managing Echo on DAHDI Channels

You can enable and disable echo cancellation for DAHDI interfaces in the chan_dahdi.conf file. The default configuration enables echo cancellation with echocancel=yes. echocancelwhenbridged=yes will enable echo cancellation for TDM bridged calls. While bridged calls should not require echo cancellation, this may improve call quality.

When echo cancellation is enabled, the echo canceller learns of echo on the line by listening for it throughout the duration of the call. Consequently, echo may be heard at the beginning of a call and lessen after a period of time. To avoid this situation, you can employ a method called echo training, which will mute the line briefly at the beginning of a call, and send a tone from which the amount of echo on the line can be determined. This allows Asterisk to deal with the echo more quickly. Echo training can be enabled with echotraining=yes.

Hardware Echo Cancellation

The most effective way to handle echo cancellation is not in software. If you are planning on deploying a good-quality system, spend the extra money and purchase cards for the system that have onboard hardware echo cancellation. These cards are a bit more expensive, but they quickly pay for themselves in terms of reduced load on the CPU, as well as reduced load on you due to fewer user complaints.

[215] As discussed in Appendix A, Understanding Telephony, 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.

[216] Actually, the handset in any phone, be it traditional or VoIP, is an analog connection.