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Version 5.6 335 November 2008

SIP User's Manual 7. IP Telephony Capabilities

condition occurs, the correction is performed by dropping several voice packets

simultaneously or by adding several BFI packets simultaneously, so that the Jitter Buffer

returns to its normal condition.

7.8 Configuring 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 device 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.

7.8.1 Alternative Routing Mechanism

When a Tel-to-IP call is routed through the device, the call’s destination number is

compared to the list of prefixes defined in the 'Tel to IP Routing' table (described in ''Tel to

IP Routing Table'' on page 160). The '

Tel to IP Routing' table is scanned for the destination

number’s prefix starting at the top of the table. For this reason, enter the main IP route

above any alternative route. When an appropriate entry (destination number matches one

of the prefixes) is found, the prefix’s corresponding destination IP address is verified. If the

destination IP address is disallowed (or if the original call fails and the device has made two

additional attempts to establish the call without success), an alternative route is searched in

the table. , after which an alternative route is used.

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 a 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.

7.8.2 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.

7.8.3 Relevant Parameters

The following parameters (described in ''Routing General Parameters'' on page 157) are

used to configure the Alternative Routing mechanism:

 AltRoutingTel2IPEnable

 AltRoutingTel2IPMode

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