User`s manual

Mediant 2000 & TP-1610 & TP-260
SIP User's Manual 216 Document #: LTRT-68805
8.7 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 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).
8.7.1 Alternative Routing Mechanism
When a TelÆIP call is routed through the gateway, the call’s destination number is
compared to the list of prefixes defined in the Tel to IP Routing table (described in Section
5.5.5.1 on page 70). 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 gateway matches the rules starting at the top of the table. For this reason, enter the
main IP route above any alternative route.
8.7.2 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:
 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.
8.7.3 PSTN Fallback as a Special Case of Alternative Routing
The purpose of the PSTN Fallback feature is to enable the 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.
To enable PSTN fallback, assign the IP address of the gateway itself as an alternative
route to the desired prefixes. Note that calls (now referred to as IP to Tel calls) can be re-
routed to a specific trunk group using the Routing parameters.