System information

CHAPTER 7
Outside Connectivity
You cannot always control what goes on outside. But
you can always control what goes on inside.
—Wayne Dyer
In the previous chapters, we have covered a lot of important information that is essential
to a working Asterisk system. However, we have yet to accomplish the one thing that
is vital to any useful PBX: namely, connecting it to the outside world. In this chapter
we will rectify that situation.
The architecture of Asterisk is significant, due in large part to the fact that it treats all
channel types as equal. This is in contrast to a traditional PBX, where trunks (which
connect to the outside world) and extensions (which connect to users and resources)
are very different. The fact that the Asterisk dialplan treats all channels in a similar
manner means that in an Asterisk system you can accomplish very easily things that
are much more difficult (or impossible) to achieve on a traditional PBX.
This flexibility does come with a price, however. Since the system does not inherently
know the difference between an internal resource (such as a telephone set) and an
external resource (for example, a telco circuit), it is up to you to ensure that your
dialplan handles each type of resource appropriately.
The Basics of Trunking
The purpose of trunking is to provide a shared connection between two entities. For
example, a trunk road would be a highway that connects two towns together. Railroads
used the term “trunk” extensively, to refer to a major line that connected feeder lines
together.
Similarly, in telecom, trunking is used to connect two systems together. Carriers use
telecom trunks to connect their networks together, and in a PBX, the circuits that con-
nect the PBX to the outside world are commonly referred to as trunks (although the
carriers themselves do not generally consider these to be trunks). From a technical
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