This Enhanced CAC-Restricted Subscriber Rerouting feature allows calls originated in the PSTN to be rerouted as well.
Correlation Framework Service
As the CAC-Rerouted call passes through the PSTN, the only possible correlating factor can be the called party number. Calling party numbers are not necessarily transparently transported by the central office and user-to-user ISDN services come at a higher cost. E.g. in ISDN, transporting the CLIP/CLIR is a subscribed service. If not subscribed, the CO routinely replaces any delivered CLIP/CLIR with a standard CLIP/CLIR for the whole trunk. Analog Central Offices may or may not transport Caller ID.
Using the Called Party Number requires the subscription to the Direct Inward Dialing (DID) service on the connection with the central office. This means that the numbers that will be used to correlate the CAC-Rerouted call legs must be directly addressable from the public network without passing through an attendant.
As it cannot be guaranteed that the number of the called party is actually a DID number (the called party may only have a private number, indicated in the Lotus Sametime Unified Telephony by not setting the External Directory Number flag), a special pool of DID numbers must be reserved in each survivable branch office to support this feature. Even in case the called party does have a valid DID number, a number from the DID pool should be used. The reason for this is that the correlation service uses only the DID number to correlate the call legs. So, on the terminating CAC-rerouted call leg, the correlation service blindly goes and looks for the originating CAC-Rerouted call leg. On a glare situation, where the Lotus Sametime Unified Telephony reroutes a call and at the same time a 'real' PSTN subscriber calls the called party, then the correlation service might correlate the wrong calls together and present the 'real' PSTN subscriber as the calling Lotus Sametime Unified Telephony subscriber to the B-party. The Lotus Sametime Unified Telephony's only choice in a glare situation would be to clear both calls. By using a number from the DID pool, the CAC-rerouted call can be thrown back to the originating CAC-rerouted call leg which can reestablish the call and the call from the PSTN can be safely cleared as the called number does not belong to any subscriber.
CAC-Restricted Subscriber Rerouting has a limitation that it is only allowed for called subscribers that have a valid E.164 number in the public network. With the use of the DID-pool (which of course are guaranteed to be valid E.164 numbers in the public network), Advanced CAC-Restricted Subscriber Rerouting lifts this restriction because it is not the called subscribers number anymore that is rerouted, but the DID number from the DID pool. This means that Subscriber Rerouting will also be activated for:
Calls to subscribers that only have a private number. Due to the correlation service the private number is linked to a public DID number from the DID pool.
Calls to DLS mobility subscribers. The Lotus Sametime Unified Telephony must allow calls to DLS mobility subscribers in the branch office to succeed. This means that the restriction that for rerouting to take place, the subscriber has to be registered from his/her home branch is lifted. Due to the correlation service, the call to the DLS Mobility subscriber is replaced with a call to a DID number from the DID pool of the branch office where the subscriber is registered.
Calls diverted to a Voice Mail Server. The Lotus Sametime Unified Telephony must allow calls to be forwarded to a Voice Mail System even in situations where the Voice Mail System is not accessible due to a CAC restriction. After all routes to the Voice Mail Server are exhausted, the CAC restricted Voice Mail Server can be contacted via the PSTN using a DID number from the DID pool of the office where the Voice Mail Server is located.
- The actual rerouting code in UCE AS is activated when it receives a release message with a reroute request from SIPSM. When the reroute request is because of a CAC-Restriction, then AS shall first check whether the destination of the call (B) belongs to a branch office with a DID pool and if so retrieve a DID number from that branch office's DID pool. If it is successful doing so, it starts the correlation service on the new outgoing call to the destination after using the DID number for creating the rerouting number and calling translation. The DID number is stored in the local UCE context and is available for the started correlation service. Creating the rerouting number is done by inserting the defined 'Subscriber Rerouting Prefix Access Code' before the DID number and translating the resulting number using the calling party's translation & routing information such as numbering plan, class of service, rate area, etc. Provisioning must be such that the resulting call goes out through the local gateway of the calling party.
- Once the decision is taken to 'subscriber reroute' the CAC-restricted call, the correlation service, retrieves the DID number from the local UCE context. In its service data the correlation service ties the selected DID number to the context of the CAC-Restricted call.
- The Lotus Sametime Unified Telephony now waits for an incoming call from the called party's branch office's gateway to the selected DID number. The Lotus Sametime Unified Telephony is provisioned to start the Correlation Service when this number is called (the DID number leads to a vertical service). The correlation service looks up the context of the originating CAC-Restricted call leg, connects it logically with the incoming terminating CAC-Restricted call leg and replaces the DID with the called party and any diversion information from the originating CAC-Restricted call leg.
- Finally, it releases the DID number which is then available again for the next CAC-Restricted call to the called party's survivable branch office.
If no DID number can be seized from the DID pool, the solution automatically falls back to the regular CAC-Restricted Subscriber Rerouting solution which uses the called party's number for setting up the rerouting through the PSTN.
If the Originating CAC-Rerouted call leg receives a release and the DID is still reserved, this would indicate a problem with the SSAL share between the cluster nodes and a fallback to basic CAC-Restricted Subscriber Rerouting must be attempted.
Parent topic: CAC (Call Admission Control) Rerouting
Rerouting a Call to a SIP Subscriber