3Com V6100, User Manual Manual, Page: 248 / 496

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V6100 and V7122 User Guide
Configuring the Gateway’s Alternative Routing (based on Connectivity and
QoS)
The Alternative Routing feature enables reliable routing of Tel to IP calls when Proxy isn’t
used. The V7122 gateway 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.
Note that if the alternative routing destination is the gateway itself, the call can be configured
to be routed back to one of the gateway’s trunk groups and thus back into the PSTN (PSTN
Fallback).
Alternative Routing Mechanism
When a Tel
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IP call is routed through the V7122 gateway, the call’s destination number is
compared to the list of prefixes defined in the Tel to IP Routing table (described in Tel to IP
Routing Table ). The Tel to IP Routing table is scanned for the destination number’s prefix
starting at the top of the table. When an appropriate entry (destination number matches one
of the prefixes) is found; the prefix’s corresponding destination IP address is checked. If the
destination IP address is disallowed, an alternative route is searched for in the following table
entries.
Destination IP address is disallowed if no ping to the destination is available (ping is
continuously initiated every 7 seconds), when an inappropriate level of QoS was detected, or
when DNS host name is not resolved. The QoS level is calculated according to delay or
packet loss of previously ended calls. If no call statistics are received for two minutes, the
QoS information is reset.
The V7122 gateway matches the rules starting at the top of the table. For this reason, enter
the main IP route above any alternative route.
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:
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Connectivity – The destination IP address is queried periodically (currently only by ping).
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QoS – The QoS of an IP connection is determined according to RTCP (Real-Time
Control Protocol) 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.
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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.