[Asterisk-doc] Connecting to a PRI

[Gene Naden]

Here is my explanation of the issues involved in connecting to a PRI. I am
open to feedback, corrections, comments. This is a text file with line
breaks.

Gene Naden, MA
Programmer Analyst
GlobalTeldata II, LLC
4700 N. Ravenswood
Chicago, IL 60640
(773) 878-3161 x 223
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WHAT IS A PRI?

A PRI is a 24-channel connection to a telephone network such as the Public 
Switched Telephone Network (PSTN). PRI is a type of ISDN (Integrated 
Services Digital Network) connection. A PRI can carry 23 voice channels. 
The 24-th channel is used for signalling. The voice channels are referred 
to as B channels and the signalling channel is referred to as the D channel.

Because a PRI uses a four-wire local loop, d4 or esf framing, and AMI or B8ZS 
coding, it may also be referred to at T-1. However, T-1 can be used to carry 
either voice or digital data. 

You specify the type of framing and coding when you provision the PRI with the 
telephone company.

CONNECTING TO A PRI

To connect to a PRI you use Zapata technology. Zapata is also used for 
connecting to POTS lines (ordinary, single channel analog phone lines) and 
analog phones. You configure Zapata for Asterisk in the file /etc/zaptel.conf. 
Therefore this file can have information about both PRI connections and POTS 
line connections. See the example below.

You do not need a Channel Service Unit/Data Service Unit (CSU/DSU) to connect
your PRI to the Asterisk computer.

The Digium cards for PRIs accept RJ45 plugs, the same type of plug used
in Category 5 cables.

All the Digium cards for connecting to a PRI are PCI cards. Therefore, you 
cannot use a 1U rack-mounted server if you are connecting to a PRI. The TE405P
card is an example of a PRI interface card. It has four ports and therefore
can connect to up to four PRIs. Alternatively, one or more ports could be 
connected to channel banks for analog phones. However, channel banks are 
beyond the scope of this discussion.

Below is a sample Zapata configuraion file (zaptel.conf). It shows the 
configuration for a single PRI, two analog phones and two POTS lines. 

# zaptel.conf

loadzone = us

#te405p card
span=1,1,0,esf,b8zs
bchan=1-23
dchan=24
fxols=1-23

#tdm400 card
fxoks=97-98
fxsks=99-100

The 23 B channels and the D channel are shown. 

The line "fxols=1-23" reflects the fact that the connection is to a PRI
rather than a channel bank. "ls" refers to "loop start." 

The way the channel numbering works is that the first 96 channels (one 
through 96) are for the TE405P card. The fxo and fxs channels, if any,
must follow, rather than precede, the PRI channels. In other words,
you cannot have fxoks=1-2, fxsks=3-4, bchan=5-27, dchan=28. If you
do this you will get an obscure error message.

Regarding the line beginning with the word "span", there is one span
for each port that is in use. The "span" line has the following format:

span=port number, timing, line build out, framing, coding

The port number is one because the PRI is connected to the first port on the
card. 

The timing should be 1 for one of the ports. The line connected to that
port provides a timing reference. If other ports are used, the timing for those
ports should be two or zero. Two means this line is a secondary timing reference
and zero means this line is not used as a timing reference.

Line build out reflects the length of last leg of the connection and is set to 
zero if the length is less than 133 feet. The specific values for line build
out are specified in the sample zaptel.conf that comes with Asterisk.

The framing and coding are discussed in "WHAT IS A PRI", above. The option "esf"
refers to "extended super frame" and "b8zs" refers to "Bipolar with 8 Zero 
Substitution."

TROUBLE-SHOOTING
When the PRI card was purchased from Digium, Digium has provided free
technical support for configuring the card.

The lights on the PRI card can be red, amber/green or green. When all is
well they are green. 

If Asterisk aborts when you try to start it, there are several things you
can check:

The Linux modules for your Zapata cards have to be loaded in order for the
connections to work. You load the PRI driver, wct4xxp, before you load the
FXS/FXO driver, wctdm. 

You also have to execute the program ztcfg to get things to work. If all is
well, when you run ztcfg it should not produce any output. When you 
do this, the lights should change to green, even if Asterisk has not been 
started.

Loading of the modules may or may not be done automatically by the system.

It is possible to construct a loop-back plug to test your PRI card.
You do this using an RJ45 plug, connecting pin one to pin four and pin 
two to pin five. When the loop-back connector is connected to the card, the 
light should turn green. Again, the modules have to be loaded and possibly 
ztcfg has to have been run.

The B channels listed in zaptel.conf must agree with the channels listed in
/etc/asterisk/zapata.conf, otherwise Asterisk may abort when you try to
start it.