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—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 sep‐ |
arating 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 crea‐ |
tive 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 Aster‐ |
isk 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. |
1 A good indicator that you’ve worked with traditional PBXs is the presence of a large callus on your forehead, |
obtained from smashing your head against a brick wall too many times to count. |
9 |
Figure 2-1. Asterisk versus PBX architecture |
Modules |
Asterisk is built on modules. A module is a loadable component that provides a spe‐ |
cific functionality, such as a channel driver (for example, chan_pjsip.so), or a resource |
that allows connection to an external technology (such as func_odbc.so). Asterisk |
modules are loaded based on the parameters defined in the /etc/asterisk/modules.conf |
file. We will discuss the use of many modules in this book, but at this point we just |
want to introduce the concept of modules, and give you a feel for the types of mod‐ |
ules that are available. |
It is actually possible to start Asterisk without any modules at all, although in this |
state it will not be capable of doing anything. It is useful to understand the modular |
nature of Asterisk in order to appreciate the architecture. |
You can start Asterisk with no modules loaded by default and load |
each desired module manually from the console, but this is not |
something that you’d want to put into production; it would only be |
useful if you were performance-tuning a system where you wanted |
to eliminate everything not required by your specific application of |
Asterisk. |
The types of modules in Asterisk include the following: |
• Applications—The workhorses of the dialplan, such as Dial(), Voicemail(), |
Playback(), Queue(), and so forth |
• Bridging modules—Mechanisms that connect channels (calls) to each other |
10 |
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Chapter 2: Asterisk Architecture |
• Call detail recording (CDR) modules |
• Channel event logging (CEL) modules |
• Channel drivers—Various connections into and out of the system; SIP (Session |
Initiation Protocol)messaging uses the PJSIP channel drivers |
• Codec translators—Convert various codecs such as G729, G711, G722, Speex, |
and so forth |
• Format interpreters—As above, but relating to files stored in the filesystem |
• Dialplan functions—Enhance the capabilities of the dialplan |
• PBX modules |
• Resource modules |
• Add-on modules |
• Test modules |
In the following sections we have curated a list of modules we feel are important |
enough to be discussed in this book. You’ll find many other modules in the Asterisk |
download, but many older modules are either deprecated or have little or no support, |
and are therefore not recommended for production unless you have access to devel‐ |
opers who can maintain them for you. |
There is an official list of support status types included within menuselect.2 |
Applications |
Dialplan applications are used in extensions.conf to define the various actions that can |
be applied to a call. The Dial() application, for example, is responsible for making |
outgoing connections to external resources and is arguably the most important |
dialplan application. The available applications are listed in Table 2-1. |
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