Nortel Networks TP-1610 SIP user manual Alternative Routing Mechanism

Mediant 2000 SIP

8.6Configuring 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 Mediant 2000 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).

8.6.1Alternative Routing Mechanism

When a Tel IP call is routed through the Mediant 2000 gateway, the call’s destination number is

compared to the list of prefixes defined in the Tel to IP Routing table (described in Section 5.8.4.1 on page 49). 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 Mediant 2000 gateway matches the rules starting at the top of the table. For this reason, enter the main IP route above any alternative route.

8.6.2Determining 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.

8.6.3PSTN Fallback as a Special Case of Alternative Routing

The purpose of the PSTN Fallback feature is to enable the Mediant 2000 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.

Mediant 2000 SIP User’s Manual 146 Document #: LTRT-72504