User`s manual

H.323 User's Manual 8. Telephony Capabilities
Version 5.0 267 December 2006
8.8 Configuring the Gateway’s Alternative Routing
(Based on Connectivity and QoS)
The Alternative Routing feature enables reliable routing of Tel to IP calls when a Proxy isn’t
used. The Mediant 1000 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: 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).
8.8.1 Alternative Routing Mechanism
When a TelÆIP call is routed through the Mediant 1000 gateway, the call’s destination
number is compared to the list of prefixes defined in the Tel to IP Routing table (described
in Section 5.5.4.2 on page 98). 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 seven 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 1000 gateway matches the rules starting at the top of the table. For this
reason, enter the main IP route above any alternative route.
8.8.2 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 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.
8.8.3 PSTN Fallback as a Special Case of Alternative Routing
The purpose of the PSTN Fallback feature is to enable the Mediant 1000 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.