System information

• Richer signaling information (especially if using ISDN)
• Lower cost for carriers
• Lower cost for customers (at higher densities)
In an Asterisk system (or any PBX, for that matter), there are several types of digital
circuits you might want to connect:
T1 (24 channels)
Used in Canada and the United States (mostly for ISDN-PRI)
E1 (32 channels)
Used in the rest of the world (ISDN-PRI or MFC/R2)
BRI (2 channels)
Used for ISDN-BRI circuits (Euro-ISDN)
Note that the physical circuit can be further defined by the protocol running on the
circuit. For example, a T1 could be used for either ISDN-PRI, or CAS, and an E1 could
be used for ISDN-PRI, CAS, or MFC/R2. We’ll discuss the different protocols in the
next section.
Installing PSTN Trunks
Depending on the hardware you have installed, the process for installing your PSTN
cards will vary. We will discuss installation in general terms, which will apply to all
Digium PSTN cards. Other manufacturers tend to provide installation scripts with their
hardware, which will automate much of this for you.
Downloading and installing DAHDI
The Digium Asterisk Hardware Device Interface, a.k.a. DAHDI (DAW-dee)
#
is the
software framework required to enable communication between PSTN cards and
Asterisk. Even if you do not have any PSTN hardware, we recommend installing
DAHDI since it is a simple, reliable way to get a valid timing source.
*
Complete DAHDI
installation instructions can be found in Chapter 3.
Disable Loading Extra DAHDI Modules
By default DAHDI will load all compiled modules into memory. As this is unnecessary,
let’s disable loading any of the hardware modules for now. If no modules are loaded in
the configuration files, DAHDI will load the dahdi_dummy driver, which provides an
interface for Asterisk to get timing from the kernel so that timing-dependent modules
such as MeetMe and IAX2 trunking work correctly.
#Don’t ask.
* There are other ways of getting a timing source, and if you want a really tight system it is possible to run
Asterisk without DAHDI, but it’s not something we’re going to cover here.
136 | Chapter 7: Outside Connectivity