Version 5.8 509 October 2009
SIP User's Manual 9. IP Telephony Capabilities
9.9 Configuring Alternative Routing (Based on
Connectivity and QoS)
The Alternative Routing feature enables reliable routing of Tel-to-IP calls when a Proxy isn’t
used. The device periodically checks the availability of connectivity and suitable Quality of
Service (QoS) before routing. If the expected quality cannot be achieved, an alternative IP
route for the prefix (phone number) is selected.
9.9.1 Alternative Routing Mechanism
When a Tel-to-IP call is routed through the device, the destination number is compared to
the list of prefixes defined in the 'Tel to IP Routing' table (described in ''Configuring the Tel
to IP Routing Table'' on page 201).
The 'Tel to IP Routing' table is scanned for the
destination number’s prefix starting at the top of the table. For this reason, enter the main IP
route above any alternative route. When an appropriate entry (destination number matches
one of the prefixes) is found, the prefix’s corresponding destination IP address is verified. If
the destination IP address is disallowed (or if the original call fails and the device has made
two additional attempts to establish the call without success), an alternative route is
searched in the table, after which an alternative route is used.
9.9.2 Determining the Availability of Destination IP Addresses
To determine the availability of each destination IP address (or host name) in the routing
table, one (or all) of the following (configurable) methods are applied:
îš„ QoS: The QoS of an IP connection is determined according to RTCP statistics of
previous calls. Network delay (in msec) and network packet loss (in percentage) are
separately quantified and compared to a certain (configurable) threshold. If the
calculated amounts (of delay or packet loss) exceed these thresholds, the IP
connection is disallowed.
îš„ DNS resolution: When host name is used (instead of IP address) for the destination
route, it is resolved to an IP address by a DNS server. Connectivity and QoS are then
applied to the resolved IP address.
9.9.3 Relevant Parameters
The following parameters are used to configure the Alternative Routing mechanism:
îš„ AltRoutingTel2IPEnable
îš„ AltRoutingTel2IPMode
îš„ IPConnQoSMaxAllowedPL
îš„ IPConnQoSMaxAllowedDelay
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