Asterisk basic configuration

Asterisk basic configuration principles

In this section we'll have a look at the basic configuration of an Asterisk PBX. Contrary to the TFOT-book, we'll be looking at it from the theory side, and from there go to an example configuration. For this example configuration, we'll start from a server with on the "outside" a POTS line and a SIP account with an Internet provider, and on the "inside" two analogue telephones and a SIP softphone. Our goal is to let the PBX behave as if we don't even have a PBX, but a standard home situation: for an incoming call, all phones ring; and every internal phone can make calls to the outside.

Asterisk channel configuration basics

The first thing to realise is that Asterisk sees each connected telephony device as a "channel". But what is a Channel? To Asterisk, a channel is a connection which brings in a call to the Asterisk PBX. A channel could be a connection to an ordinary telephone handset or an ordinary telephone line, or to a logical call (like an Internet phone call). Asterisk makes no distinction between "FXO" and "FXS" style channels (that is, it doesn't distinguish between telephone lines and telephones). Every call is placed or received on a distinct channel.

The second thing to realise, is that every type of device attached to the Asterisk PBX have their own specific configuration files. Correctly setting up each (set of) telephony devices makes them available in our Asterisk PBX as channels. Thus, the device configuration files serve as a kind of abstraction layer, and simplify our final Asterisk configuration by putting "channel-specific" information in separate files.

If you've followed the previous Asterisk sections in this Wiki, or followed the first four chapters of the TFOT-book, then you've encountered the following configuration files:

• zaptel.conf - in this file, you can set up the most basic parameters for your Zap-type hardware channels, most notably Digium telephony cards. You're configuring the hardware itself, via its driver. You could think of this file as belonging to the hardware, not Asterisk. If you'd have other software on your machine besides Asterisk that could make use of your telephony hardware, then that'd be influenced as well when you change zaptel.conf.
• zapata.conf (and possibly zapata-channel.conf if you're on Debian) - this file handles the Asterisk-specific configuration of the hardware telephony channels. It thus "sets up the Zap channels" that Asterisk sees - for incoming and outgoing calls. All parameters that set up the channel are specified here.
• vpb.conf - this file is used to configure Voicetronix cards with Asterisk. If you've a Voicetronix card in use, then you've also installed the Voicetronix drivers. This makes the card(s) useable under Linux. But to have Asterisk use the card(s), you need this vpb.conf file. It thus serves the same purpose as zapata.conf.
• sip.conf - this file contains all information on all SIP devices that we want to connect to our Asterisk server; be it one or more SIP accounts with Internet providers, or SIP capable telephones (hardware or software). When sip.conf has been set up correctly, we can refer to a SIP device by a friendly name like [1000] or [Jane].
• iax.conf - this file is just the same as the sip.conf file, only it handles devices that talk IAX2 protocol instead of the SIP protocol.

These five files define all channels that our basic Asterisk PBX might handle (well, it won't even handle the last one, since we're not connecting any other Asterisk boxes or IAX-(soft)phones to our PBX, and it won't handle the third one if you don't have a Voicetronix card).

As a reference, we're going to list the contents of four example files, and explain what they do in the way of preparing channels for our simple Asterisk server.:

zaptel.conf

# Autogenerated by ./genzaptelconf -- do not hand edit
# Zaptel Configuration File
#
# This file is parsed by the Zaptel Configurator, ztcfg
#

# It must be in the module loading order

# Span 1: WCTDM/0 "Wildcard TDM410P Board 1" (MASTER)
fxoks=1
fxoks=2
fxsks=3
# channel 4, WCTDM, no module.

# Global data

loadzone        = nl
defaultzone     = nl

This is a relatively simple file. As you can see, the genzaptelconf command has generated it for us. It belongs with a Digium TDM410P card, which has two FXS modules in positions 1 and 2, and one FXO module in position 3. The first two non-commented lines define the protocol that the FXS modules must "talk" Do not be alarmed by the difference between the

Asterisk dialplans - contexts

A dial plan consists of a number of extensions. Each extensions consists of a number of priorities. Extensions are grouped in contexts. For each priority, an application is called.

[context]
exten => id, priority, command

When Asterisk receives an incoming connection on a channel, Asterisk looks at the context defined for that channel for commands telling Asterisk what it should do. The context defines different sets of commands depending on what extension the user has dialed. For example, a context might provide one set of commands for what to do if the user dials "123", and another set of commands for what to do if the user dials "9", and another set of commands for what to do if the user dials any number beginning with "555".

For some kinds of connections — such incoming calls from an outside telephone line — the user has not dialed an extension. In that case, Asterisk behaves as if the user had dialed a special extension named "s" (for Start). Asterisk will look for an extension "number" s in the definition of the context for that channel for instructions about what it should do to handle the call.

Let's say, for example, that you have a channel "Zap/1" which is a connection to a telephone handset in your building. And let's say that in the configuration file for Zap channels (zapata.conf), you have defined context=john for Zap channel 1. So when you use that handset to dial a number, Asterisk looks for a context with the name "john" in extensions.conf to find out what it should do. You begin the definition of a context in extensions.conf by putting the name of the context in square brackets on a line by itself, like this:

[john]

Extensions

An extension can be one of two types: a literal or a pattern.

A literal extension can be a number, like 123, and it can also contain the standard symbols * and # that appear on ordinary telephones, so 12#89* is a valid extension.

A single extension can also match patterns. In the extensions.conf file, an extension name is a pattern if it starts with the underscore symbol (_). In an extension pattern, the following characters have special meanings:

X          matches any digit from 0-9
Z          matches any digit from 1-9
N          matches any digit from 2-9
[1237-9]   matches any digit or letter in the brackets
           (in this example, 1,2,3,7,8,9)
.          wildcard, matches one or more characters
!          wildcard, matches zero or more characters immediately           

Asterisk uses some extension names for special purposes:

   * i : Invalid
   * s : Start
   * h : Hangup
   * t : Timeout
   * T : AbsoluteTimeout
   * a : Asterisk extension
   * o : Operator

Asterisk dialplans - handling incoming calls

What is a Channel? A channel is a connection which brings in a call to the Asterisk PBX. A channel could be a connection to an ordinary telephone handset or an ordinary telephone line, or to a logical call (like an Internet phone call). Asterisk makes no distinction between "FXO" and "FXS" style channels (that is, it doesn't distinguish between telephone lines and telephones). Every call is placed or received on a distinct channel.