24.2063GPPStage 3TSVoice call continuity between Circuit Switched (CS) and IP Multimedia Subsystem (IMS)
Figure A.5.8-1 shows the termination of a voice call that is capable of being subject to VCC, with the anchoring of the call in the IM CN subsystem being denied prior to its routeing. The voice call is then delivered in the CS domain according to standard procedures.
The anchor decision of such terminated calls is subject to operator policy.
As the terminated voice call is not anchored in the IM CN subsystem, domain transfer will not be supported for that call.
NOTE 1: For call termination, the use of CAMEL in the context of VCC is optional. In this example the GMSC support and will use terminating CAMEL service logic.
Figure A.5.8-1: call termination from CS domain with no anchoring
The details of the signalling flows are as follows:
Steps 1, 2, 3, 4 and 5 are identical to the example in subclause A.5.4.
NOTE 2: When processing terminating calls, the GMSC interrogates the HLR. In this example the GMSC indicates its support for CAMEL. The GMSC receives back from the HLR the terminating CAMEL subscriber information. As a result the gsmSSF of the GMSC triggers a CAMEL activity toward the gsmSCF.
6. Anchor decision
The CAMEL service function denies the anchoring of the terminating call according to the operator policy. In this example the CAMEL service function is unable to allocate a new IMRN for this call i.e. insufficient IMRN.
7. CAMEL CONTINUE (CAMEL service function to GMSC)
The CAMEL service function causes the gsmSCF to respond to the CAMEL IDP message with a CAMEL CONTINUE message. The CAMEL CONTINUE message contains no parameter.
8, 9 and 10. Further interrogation with the HLR (GMSC to HLR)
Following standard terminating call procedure for CAMEL subscribers, on the receipt of the CAMEL CONTINUE message, the GMSC interrogates the HLR again by including a parameter that indicates the suppression of CAMEL handling. On the receipt of the answer from the HLR, the GMSC resumes the processing and continues the call in the CS domain according to standard procedures.