10 Interworking with other networks

23.0933GPPRelease 17Stage 2Technical realization of Completion of Calls to Busy Subscriber (CCBS)TS

The point of interworking shall be the interface between HLR A and HLR B. The flow of information across this interface shall be identical to that between exchanges for the ISDN version of CCBS.

Interworking with ISDNs which support the ISDN version of CCBS is therefore available without further modification.

Interworking with networks other than ISDNs, e.g. private networks, is not specified here, but is not precluded.

CCBS shall be able to interwork through networks which do not support CCBS. In this situation, the CCBS service may fail.

10.1 Interworking with network entities not supporting CCBS

CCBS requires support of the service by the following entities for it to operate as specified above:

– MS A;

– MSC A;

– VLR A;

– HLR A;


– HLR B;

– MSC B;

– VLR B.

The following shows the actions required to cover the situations where these entities do not support CCBS. Note that the situation is further complicated by mobility. For example, MSC A may support CCBS when the initial request is made, but if subscriber A has changed location, the new MSC A may not support CCBS.

It is assumed that the entities support the service at the points in the call where actions are initiated by them, and that an entity which receives an indication for the CCBS service but does not support it can indicate its lack of support to the sending entity.

10.1.1 CCBS not supported by MSC A

HLR A knows whether the MSC A supports CCBS or not through the transfer of subscriber data.

MS B becomes idle

When HLR A receives the "Remote User Free" indication for a particular CCBS Request and detects that the MSC A where MS A is currently registered does not support CCBS, then HLR A shall suspend the corresponding CCBS Request and shall send "CCBS Suspend" to HLR B. When MS A registers in a different MSC which supports CCBS, HLR A shall resume the corresponding CCBS Request and shall send "CCBS Resume" to HLR B, continuing as if MS A had become free.

10.1.2 CCBS not supported by HLR B

Initial call encounters busy

If an MSC B that supports CCBS recognises that HLR B does not support CCBS then "CCBS Not Possible" will be generated in the CCBS diagnostic.

10.1.3 CCBS not supported by MSC B

HLR B knows whether the MSC B supports CCBS or not through the transfer of subscriber data.

CCBS Request

If MSC B does not support CCBS, HLR B acknowledge the request and start T7. No processing of the destination B queue and subsequently no monitoring of destination B will be possible until destination B registers in a supporting MSC.

CCBS Call set-up

If HLR B knows that MSC B does not support CCBS the HLR B shall delete the corresponding CCBS Request and shall request a Roaming No without including the CCBS Call indicator in the request. Within Send Routeing Info Ack the HLR shall inform the GMSC to remove CCBS Call Indicator from the IAM message.

After a "Remote User Free" has been sent from MSC B but before the CCBS call set-up is completed, destination B may register in an MSC which does not support the service. MSC B shall accept the CCBS call, but MSC B will not be able to inform HLR B of the successful completion of the CCBS call, so the request will remain active in HLR B until the recall B timer (T9) expires and the request is cancelled.