System information

The prior discussion of IP phone configuration also applies to any analog telephone
adaptors (ATAs) you plan to use—specifically, to those supporting an FXS interface.
In addition, you may need to specify some of the electrical characteristics of the tel-
ephony interface, like line voltage and impedance, together with the caller ID format
that will work with local phones. All that differs is the way you obtain the IP address
for the web GUI—this is usually done by dialing a specific code on the attached analog
phone, which results in the IP address being read back to the caller.
Of course, an ATA may also feature an FXO interface, which will also need to be con-
figured to properly interact with the analog line provided in your region. The types of
things that need to be changed are similar to the FXS interface.
What if you are connecting your analog phone or line to a Digium card? We’ll cover
this next.
PSTN Connectivity, DAHDI, Digium Cards, and Analog Phones
Before we get to DAHDI and Asterisk configuration, we need to physically connect to
the PSTN. Unfortunately, there are no worldwide standards for these connections; in
fact, there are often variations from one part of a given country to another.
PRI connections are generally terminated in an RJ45 connection these days, although
the impedance of the connections can vary. In some countries (notably in South Amer-
ica), it is still possible to find PRIs terminated in two BNC connectors, one for transmit
and one for receive.
Generally speaking, a PRI terminated in an RJ45 will be an ISDN connection, and if
you find the connection is made by a pair of BNC connectors (push-and-twist coaxial
connectors), the likelihood is that you are dealing with a CAS-based protocol (like R2).
Figure 9-3 shows the adaptor required if your telco has supplied BNC connectors (the
Digium cards require an RJ45 connection). It is called a balun, as it converts from a
balanced connection (RJ45) to an unbalanced connection (the BNCs), in addition to
changing the connection impedance.
Basic Rate Interfaces (BRIs) are common in continental Europe and are
almost always supplied via an RJ45 connection.
PSTN Connectivity, DAHDI, Digium Cards, and Analog Phones | 185