Nortel BCM50 Configuration Manual page 245

Figure 73 PSTN fallback diagram
Public or Private
PSTN line
In a network configured for PSTN fallback, there are two connections between a BCM and a
remote system.
• One connection is a VoIP trunk connection through the IP network.
• The fallback line is a PSTN line, which can be the public lines or a dedicated T1, BRI, PRI or
analog line, to the far-end system.
When a user dials the destination code, the system checks first to see if the connection between the
two systems can support an appropriate level of QoS. If it can, the call proceeds as normal over the
VoIP trunk. If the minimum acceptable level of QoS is not met, the call is routed over the second
route, through the PSTN line.
For PSTN fallback to work, you must ensure that the digits the user dials will be the same
regardless of whether the call is going over the VoIP trunk or the PSTN. In many cases, this
involves configuring the system to add and/or absorb digits.
IP network
BCM50 A
PSTN
Public or Private
PSTN line
Chapter 24 Dialing plans
BCM50 B
Networking Configuration Guide
245