3Com VCX V7122 SIP VoIP Gateway User Manual150Configuring 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 VCX V7122 gateway periodically checks the availability of connectivity andsuitable 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 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 VCX V7122 gateway, the call’s destination numberis compared to the list of prefixes defined in the Tel to IP Routing table (described in Tel to IPRouting Table on page 52). The Tel to IP Routing table is scanned for the destinationnumber’s prefix starting at the top of the table. When an appropriate entry (destinationnumber matches one of the prefixes) is found, the prefix’s corresponding destination IPaddress is checked. If the destination IP address is disallowed, an alternative route issearched for in the following table entries.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 (delay or packetloss, calculated according to previous calls) was detected, or when DNS host name is notresolved.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 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.PSTN Fallback as a Special Case of Alternative RoutingThe purpose of the PSTN Fallback feature is to enable the VCX V7122 gateway to redirectPSTN originated calls back to the legacy PSTN network if a destination IP route is foundunsuitable (disallowed) for voice traffic at a specific time.
