Configuring an ISDN Trunk
Many PBX vendors support either T1/E1 PRI or BRI connections. In Europe, where
ISDN is more popular, many PBX vendors support BRI connections. When designing
how the PBX passes voice to the network, you must ensure the router supports the correct
connection. The first step in provisioning ISDN capabilities for T1 or E1 PRI is to
enter the basic configuration of the controllers. After the clock source, framing, and line
code are configured, ISDN voice functionality requires these configuration commands:
■ isdn switch-type: Configures the ISDN switch type. You can enter this parameter in
global configuration mode or at the interface level. If you configure both, the interface
switch type takes precedence over the global switch type. This parameter must
match the provider ISDN switch. This setting is required for both BRI and PRI
connections.
■ pri-group: Configures time slots for the ISDN PRI group. T1 allows for time slots 1
through 23 to be configured as B channels, with time slot 24 allocated to the data
channel (D channel). E1 allows for time slots 1 through 31, with time slot 16 allocated
to the D channel. You can configure the PRI group to include all available time
slots, or you can configure only a select group of time slots.
■ isdn incoming-voice: Configures the interface to send all incoming calls to the DSP
card for processing.
■ isdn switch-type [primary-qsig | basic-qsig]: Configures the use of QSIG signaling
on the D channel. You typically use this setting when connecting via ISDN to a PBX.
The command to enable QSIG signaling is isdn switch-type primary-qsig for PRI
and isdn switch-type basic-qsig for BRI connections.
Figure 4-17 and Example 4-7 show the configuration for a PBX connection to the Cisco
voice-enabled router. The connection is configured for QSIG signaling across all 23 time
slots.
Chapter 4: Performing Call Signaling over Digital Voice Ports 221
V
PBX T1 PRI
QSIG
Controller T1 0/0
IP Cloud
Figure 4-17 ISDN Configuration Topology Example