Page 228 - Asterisk™: The Future of Telephony

Best Effort
The simplest, least expensive approach to QoS is not to provide it at all—the “best
effort” method. While this might sound like a bad idea, it can in fact work very well.
Any VoIP call that traverses the public Internet is almost certain to be best-effort, as
QoS mechanisms are not yet common in this environment.

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 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

‡ As discussed in Chapter 7, 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.

200 | Chapter 8: Protocols for VoIP