248 V6100 and V7122 User GuideConfiguring the Gateway’s Alternative Routing (based on Connectivity andQoS)The Alternative Routing feature enables reliable routing of Tel to IP calls when Proxy isn’tused. The V7122 gateway periodically checks the availability of connectivity and suitableQuality of Service (QoS) before routing. If the expected quality cannot be achieved, analternative IP route for the prefix (phone number) is selected.Note that if the alternative routing destination is the gateway itself, the call can be configuredto be routed back to one of the gateway’s trunk groups and thus back into the PSTN (PSTNFallback).Alternative Routing MechanismWhen a TelÆIP call is routed through the V7122 gateway, the call’s destination number iscompared to the list of prefixes defined in the Tel to IP Routing table (described in Tel to IPRouting Table). The Tel to IP Routing table is scanned for the destination number’s prefixstarting at the top of the table. When an appropriate entry (destination number matches oneof the prefixes) is found; the prefix’s corresponding destination IP address is checked. If thedestination IP address is disallowed, an alternative route is searched for in the following tableentries.Destination IP address is disallowed if no ping to the destination is available (ping iscontinuously initiated every 7 seconds), when an inappropriate level of QoS was detected, orwhen DNS host name is not resolved. The QoS level is calculated according to delay orpacket loss of previously ended calls. If no call statistics are received for two minutes, theQoS information is reset.The V7122 gateway matches the rules starting at the top of the table. For this reason, enterthe main IP route above any alternative route.Determining the Availability of Destination IP AddressesTo determine the availability of each destination IP address (or host name) in the routingtable, 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-TimeControl Protocol) statistics of previous calls. Network delay (in msec) and network packetloss (in percentage) are separately quantified and compared to a certain (configurable)threshold. If the calculated amounts (of delay or packet loss) exceed these thresholdsthe IP connection is disallowed. DNS resolution – When host name is used (instead of IP address) for the destinationroute, it is resolved to an IP address by a DNS server. Connectivity and QoS are thenapplied to the resolved IP address.
