03.933GPPStage 2Technical realization of Completion of Calls to Busy Subscriber (CCBS)TS
The CCBS service allows a calling subscriber A, encountering a NDUB destination B, to be notified when destination B becomes idle and to have the network automatically generate a CCBS call to destination B, if subscriber A desires. Subscriber A may make distinct CCBS requests for calls to the same destination B for different basic services.
Figure 4.2.1 is an architectural overview of the CCBS service when interworking between the originating and the destination networks involved. The originating network may be a mobile network or a fixed network and the destination network may also be a mobile network or a fixed network.
The call related signalling for CCBS is performed on ISUP links on the following interfaces:
VMSC A – GMSC B;
VMSC A – DLE;
OLE – GMSC B;
whereas the specific CCBS procedures are performed via the SSAP protocol, which is signalled on the following interfaces:
HLR A – HLR B;
HLR A – DLE;
OLE – HLR B.
Figure 4.2.1: Architectural overview showing common point of interworking
4.2.1 Architectural overview during roaming
Either the originating subscriber A or the destination subscriber B or both may be located outside of their HPLMNs during the CCBS service. When all the involved networks support CCBS service the normal handling described in this specification applies. When some network entities do not support CCBS service refer to clause 10 where the exceptions are described in more detail.
The signalling between different networks described in the subclause 4.2 applies also during roaming. HLR A and HLR B belongs always to the HPLMN of the subscriber whereas VLR A and MSC A and GMSC, VLR B and MSC B respectively may belong to a HPLMN or VPLMN. Refer to GSM 03.18 where call handling in the mobile network is described in more detail.