Asterisk: The Definitive Guide, 3rd Edition

Chapter 5. User Device Configuration

I don’t always know what I’m talking about, but I know I’m right.

—Muhammad Ali

In this chapter we’ll delve into the user devices that you might want to connect to Asterisk, typically VoIP telephones of some sort. Configuring a channel in Asterisk for the device to connect through is relatively straightforward, but you also need to configure the device itself so it knows where to send its calls.^[42] In other words, there are two parts to configuring a device on Asterisk: 1) telling Asterisk about the device, and 2) telling the device about Asterisk.

How Asterisk Relates to the SIP Protocol

SIP is a peer-to-peer protocol, and while it is common to have a setup where endpoints act as clients and some sort of gateway acts as a server, the protocol still thinks in terms of peer-to-peer relationships. What this means is that a SIP telephone expects to make a direct connection to another SIP telephone, without a PBX in between.

The reality is that many SIP transactions happen through a server, and in the case of Asterisk, it is common to have the PBX in the middle of all connections. When a SIP call is made from a telephone to another telephone through Asterisk, there are actually two calls happening: one from the originating set to Asterisk, and another separate call from Asterisk to the destination set. Asterisk bridges the two channels together.

From the perspective of the SIP telephone, therefore, you need to configure it to send all its calls to Asterisk, ...