Asterisk: The Definitive Guide, 5th Edition

Chapter 2. Asterisk Architecture

First things first, but not necessarily in that order.

Doctor Who

Asterisk is very different from other, more traditional, PBXs in that the dialplan in Asterisk treats all incoming channels in essentially the same manner, rather than separating them into stations, trunks, peripheral modules, and so forth.

In a traditional PBX, there is a logical difference between stations (telephone sets) and trunks (resources that connect to the outside world). This limitation makes creative routing in traditional PBXs very difficult or impossible.

Asterisk, on the other hand, does not have an internal concept of trunks or stations. In Asterisk, everything that comes into or goes out of the system passes through a channel of some sort. There are many different kinds of channels; however, the Asterisk dialplan handles all channels in a similar manner, which means that, for example, an internal user can exist on the end of an external trunk (e.g., a cell phone) and be treated by the dialplan in exactly the same manner as that user would be if they were on an internal extension. Unless you have worked with a traditional PBX,¹ it may not be immediately obvious how powerful and liberating this is. Figure 2-1 illustrates the differences between the two architectures.

Figure 2-1. Asterisk versus PBX architecture

Modules

Asterisk is built on modules. A module is a loadable component ...