System information

dynamic by distributing control of labels to the routers. This enables the network to
become more responsive to changing conditions, because it can be set up to change the
paths based on certain conditions, such as a certain path going down (perhaps due to
a faulty router). The configuration within the router will then be able to use RSVP to
distribute new labels to the routers in the MPLS network, with no (or minimal) human
intervention.
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 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.
‡ 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.
632 | Appendix B: Protocols for VoIP