User's Manual 498 Document #: LTRT-27045
Mediant 1000B Gateway & E-SBC
23.4.5 PSTN Fallback
The PSTN Fallback feature enables the device to re-route a Tel-to-IP call to the legacy
PSTN using one of its trunks if the IP destination is unavailable. For example, if poor voice
quality is detected over the IP network, the device attempts to re-route the call to the
PSTN.
The following procedure describes how to configure alternative Tel-to-IP routing to the
PSTN.
To configure alternative Tel-to-IP routing to the PSTN:
1. In the Tel-to-IP Routing table (see ''Configuring Tel-to-IP Routing Rules'' on page
475), configure an alternative routing rule with the same call matching characteristics
(e.g., phone number destination) as the "main" routing rule, but where the destination
is the IP address of the device itself.
2. In the IP-to-Tel Routing table (see ''Configuring IP-to-Tel Routing Rules'' on page
484), configure an IP-to-Tel routing rule to route calls received from the device (i.e., its
IP address) to a specific Trunk Group connected to the PSTN. This configuration is
necessary as the re-routed call is now considered an IP-to-Tel call.
Note: The PSTN Fallback feature is applicable only to digital interfaces.
23.5 Alternative Routing for IP-to-Tel Calls
The device supports alternative IP-to-Tel call routing, as described in this section.
23.5.1 Alternative Routing to Trunk upon Q.931 Call Release Cause
Code
You can configure up to 10 ISDN Q.931 release cause codes, which if received from the
Tel side, the device routes the IP-to-Tel call to an alternative Trunk Group, if configured.
Alternative IP-to-Tel routing rules are configured in the IP-to-Tel Routing table. These rules
must be configured anywhere below the "main" routing rule and with identical matching
characteristics (e.g., destination prefix number) to the "main" routing rule. The device uses
the first alternative route that is available. For more information on configuring alternative
IP-to-Tel routing rules in the IP-to-Tel Routing table, see ''Configuring IP-to-Tel Routing
Rules'' on page 484.
A release cause code indicates that the IP-to-Tel call has been rejected or disconnected on
the Tel side. The release cause codes are configured in the Reasons for IP-to-Tel
Alternative Routing table. For example, you can configure alternative IP-to-Tel routing for
scenarios where the initial Tel destination is busy and a Q.931 Cause Code No. 17 is
received (or for other call releases that issue the default Cause Code No. 3).
You can configure a default release cause code that the device issues itself upon the
following scenarios:
The device initiates a call release whose cause is unknown.
No free channels (i.e., busy) in the Trunk Group.
No appropriate routing rule located in the IP-to-Tel Routing table.
Phone number is not located in the IP-to-Tel Routing table.
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