Configuring The Gateway's Alternative Routing (Based On Connectivity And Qos); Alternative Routing Mechanism; Determining The Availability Of Destination Ip Addresses; Pstn Fallback As A Special Case Of Alternative Routing - 3Com VCX V7122 User Manual

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 VCX 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 IP call is routed through the VCX V7122 gateway, the call's destination number
is compared to the list of prefixes defined in the Tel to IP Routing table (described in
Routing Table on page 52). 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 (delay or packet
loss, calculated according to previous calls) was detected, or when DNS host name is not
resolved.
The VCX 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:
Connectivity – The destination IP address is queried periodically (currently only by ping).
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.
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.

PSTN Fallback as a Special Case of Alternative Routing

The purpose of the PSTN Fallback feature is to enable the VCX V7122 gateway to redirect
PSTN originated calls back to the legacy PSTN network if a destination IP route is found
unsuitable (disallowed) for voice traffic at a specific time.
150 3Com VCX V7122 SIP VoIP Gateway User Manual
Tel to IP