Alternative routing

DUAL Softswitch offers a powerful mechanism for ensuring that the highest possible service availability and ASR are offered to your subscribers. Alternative routing allows certain failed calls to overflow to alternative carriers and for this process to continue until there are no further candidate carriers for the call. If a call fails, then the alternative carrier chosen will be the next least expensive in the list of candidate carriers that was selected during the routing process.

A failed call will only be considered for alternative routing if the carrier that has rejected the call has not yet negotiated a voice path and has not yet indicated that the destination is ringing using an Alerting message (H.323) or 180/183 status message (SIP). If a media path has already been established and a carrier fails a call, then that call will be cleared from the switch and the failure indication will be sent back to the caller.

A call will be sent to the next available carrier under the following circumstances:

•	For H.323 only, The TCP connection to the carrier cannot be established or fails before a media path has been negotiated.

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The carrier does not acknowledge the call within a predefined period of time. In H.323, this acknowledgement is a Call Proceeding message. With SIP, it is a 100 status message (Trying). The length of the timer is normally six seconds, but can be increased to fourteen seconds using the carrier's settings. The DUAL Softswitch support team can change these lengths if required. This timer is similar to the T303 timer used within ISDN switches.

•	The called carrier returns an invalid set of Fast Start elements when attempting to open an audio path (H.323 only).

•	The interconnection to the terminating carrier is an H.323 gatekeeper to gatekeeper and a registration with that gatekeeper has failed.

•	The interconnection to the terminating carrier is an H.323 gatekeeper to gatekeeper and that gatekeeper has rejected the call using a LocationReject message.

•	The carrier has been locked out using the Administrative Control Panel.

•	The carrier has been blocked for the tariff assigned to the caller.

•	For H.323, the carrier has been defined in DUAL Softswitch to use an H.245 tunnel, but, when the call was sent, the carrier refused the request to open a tunnel.

•	The maximum number of calls are in progress to the carrier. This is defined on a per carrier basis.

•	The call would result in a financial loss, and the subscriber's account and tariff settings do not allow this.

•	The carrier has been defined in DUAL Softswitch not to be sent calls where the only codecs offered by the caller are G.711 and the caller only offers G.711.

•	The routing would result in the call being sent back to the originating IP address, and the caller's account does not have the Allow Loop feature enabled.

•	The call would require an RTP proxy to be used, and the maximum number of allowed proxy calls has already been reached.

In addition, a call will overflow if it has been rejected by a carrier using a cause code that has been defined in DUAL Softswitch as an Overflow Cause Code. DUAL Softswitch is installed with a set of common cause codes that cause alternative routing to take place. These can be modified using the Overflow causes page of the Administration System. Not all failed calls should overflow. For example, if a carrier returns cause code 34 (congestion), it would be reasonable to attempt to continue with the call. But, if a carrier returns cause 17 (subscriber busy), then it would be more sensible to clear that call from the switch and send that busy indication back to the caller.