How to connect two Asterisk PBXs using a Dahdi T1 Trunk
This webpage will discuss how to connect two Asterisk PBXs together using a Dahdi T1 trunk. The T1 trunk will be configured as an ISDN PRI rate trunk.
Brief Talk about T1 trunking
A T1 trunk consists of 24 channels of 64 kbps of data operating at 1.544 Mbps. It is a serial connection over UTP and provides full duplex communications. The connectors are RJ45 connectors the same as Ethernet. The pinout is different and a crossover cable pinout can be found here. The assumption at this point is that your T1 card has been tested and appears to be functioning properly.
The data channels can be configured to pretty much anything you want it to be. You can combine the channels to create a channel with 384 kbps bandwidth or you can use only part of the 24 channels. If you use only part of the T1 available bandwidth, it is called fractional T1.
The control of the T1 channels can be in-band or out of band. If it is in-band, bits are robbed from the 64 kbps channel for signaling. The result is 56 kbps for data per channel but you have 24 channels available for data. This robbed bit signaling is not used too much anymore due to the lower bandwidth per channel.
Out of band signaling uses one complete channel for control. This means that 23 channels of 64 kbps are available for data and the 24th is used for signaling. Any channel can be used for control, it doesn't have to be the 24th.
The most common signaling method in North America is ISDN PRI. It has 23 data channels of 64 kbps called B (bearer) channels and one control channel called the D channel. This is the signaling method that will be used in configuring the PBXs for this website. To find out more about ISDN, visit my online book: Chapter 23: Integrated Services Digital Network (ISDN)
Connecting the PBXs together
In this example, we'll be connecting two PBXs together in a building and running a T1 cross-over cable between them. First we'll set down some rules:
- PBX #1 will provide the master timing clock and PBX #2 will be a slave
- Standard North America signaling will be used: extended superframe and b8zs signaling.
- PBX #1 will simulate the network and provide dialtone
- PBX #2 will be the customer premise equipment (CPE)
Configure the Linux kernel hardware drivers:
After physically cabling the two PBXs together, the next step is configure the Linux kernel hardware drivers:
- Configure PBX #1's /etc/dahdi/system.conf
# Span 1: WCT1/0 "Digium Wildcard TE110P T1/E1 Card 0" span=1,0,0,esf,b8zs # Configuration for T1 trunk # channel 1-23 data channels bchan=1-23 echocanceller=mg2,1-23 # channel 24 control dchan=24
Basically, we made the first 23 channels to be data and channel 24 to be signaling. This configuration assumes that you only have one Dahdi card installed. This page has the details for configuring /etc/dahdi/system.conf for a T1 line
- Configure PBX #2's /etc/dahdi/system.conf
# Span 1: WCT1/0 "Digium Wildcard TE110P T1/E1 Card 0" span=1,1,0,esf,b8zs # Configuration for T1 trunk # channel 1-23 data channels bchan=1-23 echocanceller=mg2,1-23 # channel 24 control dchan=24
The main difference is the second "1" in the "span=" line. This makes PBX #2's T1 card a slave (cpe - customer premise equipment/TE - terminal equipment) to PBX #1's master (net - network) timing clock.
