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use AGI and ARI to write their dialplan. |
While there’s certainly something to be said for writing dialplan in an external lan‐ |
guage (and we’ll cover it in later chapters), the Asterisk dialplan is native to it, and |
you will not get better performance than this. Dialplan code executes fast. |
Also, if you want to understand how Asterisk thinks, you need to understand its |
dialplan. |
Priority labels |
Priority labels allow you to assign a name to a priority within an extension. This is to |
ensure that you can refer to a priority by something other than its number (which |
probably isn’t known, given that dialplans now generally use unnumbered priorities). |
Later you will learn that it’s often necessary to send calls from other parts of the dia‐ |
lplan to a particular priority in a particular extension. To assign a text label to a prior‐ |
ity, simply add the label inside parentheses after the priority, like this: |
exten => 123,n(label),application() |
Later, we’ll cover how to jump between different priorities based on dialplan logic. |
You’ll see a lot more of priority labels, and you’ll use them often in your dialplans. |
A very common mistake when writing labels is to insert a comma |
between the n and the (, like this: |
exten => 555,n,(label),application() ;<-- THIS WON'T WORK |
exten => 556,n(label),application() :<-- This is what we want |
This mistake will break that part of your dialplan, and you will get |
an error stating that the application cannot be found. |
Applications |
Applications are the workhorses of the dialplan. Each application performs a specific |
action on the current channel, such as playing a sound, accepting touch-tone input, |
84 |
| |
Chapter 6: Dialplan Basics |
looking something up in a database, dialing a channel, hanging up the call, feeding |
the cat, and so forth.4 In the previous example, you were introduced to two simple |
applications: Answer() and Hangup(). It’s obvious what they do, but it’s also obvious |
that on their own they aren’t terribly useful. |
Some applications, including Answer() and Hangup(), need no other instructions to |
do their jobs. Most applications, however, require more information. These addi‐ |
tional elements, or arguments, are passed on to the applications to affect how they |
perform their actions. To pass arguments to an application, place them between the |
parentheses that follow the application name, separated by commas. |
The Answer(), Playback(), and Hangup() Applications |
The Answer() application is used to answer a channel that is ringing. It seems a sim‐ |
ple thing, but a lot of things happen on the channel with this one command. |
Answer() tells the channel to send a message back to the far end that the call has been |
answered, and also to enable the media paths (the network streams that will carry the |
sound between the caller and the system). As we mentioned earlier, Answer() takes |
no arguments. Answer() is not always required (in fact, in some cases it may not be |
desirable at all), but it is an effective way to ensure a channel is connected before per‐ |
forming further actions. |
The Progress() Application |
Sometimes it is useful to be able to pass information back to the network before |
answering a call. The Progress() application attempts to provide call progress infor‐ |
mation to the originating channel. Some carriers expect this, and thus you may be |
able to resolve strange signaling problems by inserting Progress() into the dialplan |
where your incoming calls arrive. In terms of billing, the use of Progress() lets the |
carrier know you’re handling the call, without starting the billing meter. |
The Playback() application is used for playing a previously recorded sound file over |
a channel. Input from the user is ignored, which means that you would not use Play |
back() in an auto attendant, for example, unless you did not want to accept input at |
that point.5 |
4 OK, so feeding the cat isn’t a common use for a telephone system, but through Asterisk, such things are not |
impossible. Doc Brown would’ve loved this thing. |
5 There is another application called Background() that is very similar to Playback(), except that it does allow |
input from the caller. You can read more about this application in Chapters 14 and 16. |
Dialplan Syntax |
| |
85 |
Asterisk comes with many professionally recorded sound files, |
which should be found in the default sounds directory (usu‐ |
ally /var/lib/asterisk/sounds). When you compile Asterisk, you can |
choose to install various sets of sample sounds that have been |
recorded in a variety of languages and file formats. We’ll be using |
these files in many of our examples. Several of the files in our |
examples come from the Extra Sound Package, which we installed |
in Chapter 3. You can also have your own sound prompts recorded |
in the same voices as the stock prompts by visiting www.theivr‐ |
voice.com. Later in the book, we’ll talk more about how you can |
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