Comments inline:
Many thanks for taking the time to doc this.
Julian.
Gene Naden wrote:
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.
Euro ISDN PRI have 31 channels, 16th used for signalling
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.
or E1 for euroisdn
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
Um, we are - most 1U servers have riser boards allowing for the card to
be mounted sideways. Both our TE405p and TE405P are in 1U servers.
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.
------------------------------------------------------------------------
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