F.3 Implementation options for prevention of circular looping of CS Terminations

23.2923GPPIP Multimedia Subsystem (IMS) centralized servicesRelease 17Stage 2TS

Several implementation techniques can be used to avoid circular looping, as detailed below:

1 Use of SCC AS for retrieving the CSRN

When the SCC AS decides that the call is to be terminated in the CS domain, the SCC AS retrieves the CSRN from the MSC/MSC-Server via the HSS using the following method:

a. MAP SRI Query: This option makes use of a MAP Interface between the SCC AS and the HSS to request allocation of a CSRN from the HSS for routing of the call to the MSC/MSC-Server currently serving the subscriber.

b. Sh UDR Query: This option makes use of the Sh interface between the SCC AS and the HSS. The SCC AS sends a Sh UDR query to the HSS to request allocation of a CSRN for routing of the call to the MSC/MSC-Server currently serving the subscriber.

2 Use of a specific CSRN to indicate the anchored call

When the SCC AS decides that the call is to be terminated in the CS domain, the SCC AS allocates a specific CSRN. The specific CSRN can be an MSISDN with added prefix digits. The SCC AS uses the specific CSRN to route the call to the CS network. The following options are available to resolve the MSISDN of original called party from the CSRN.

a. MGCF resolution: In this option, the SCC AS forwards the call to the MGCF (which has a GMSC function). The specific CSRN is used by the MGCF to recognise that the SCC AS has already anchored the call in IMS, and that the call will be terminated in the CS domain. The MGCF resolves the MSISDN from the CSRN and invokes GMSC functionality integrated in the MGCF for handling the CS call terminating procedure. In particular, the MGCF/GMSC includes the "Suppress T-CSI" parameter in the MAP SRI sent to the HSS.

b. GMSC resolution: In this option, the SCC AS forwards the call to the MGCF, which then forwards the call on to GSMC. The specific CSRN is used by the GSMC to recognise that the SCC AS has already anchored the call in IMS, and that the call will be terminated in the CS domain. The GMSC resolves the MSISDN from the CSRN and invokes the CS call terminating procedure. In particular, the GMSC includes the "Suppress T CSI" parameter in the MAP SRI sent to the HSS.

c HSS resolution: In this option, the SCC AS forwards the call to the GMSC. The GMSC treats the call as a normal terminating call and sends a MAP SRI request to the HSS for the specific CSRN. The specific CSRN is used by the HSS to recognise that the SCC AS has already anchored the call in IMS, and that the call will be terminated in the CS domain. The HSS resolves the MSISDN from the CSRN and uses CS call terminating procedures to obtain routing information from the MSC/MSC-Server.

3 Use of a dynamically allocated CSRN to route via the GMSC

This option uses a dynamically allocated CSRN to divert the call to the appropriate GMSC as described below:

– The SCC AS allocates a CSRN that maps to an appropriate GMSC. Standard CS termination procedures are then executed at the GMSC.

– The GMSC is configured with N-CSI and criteria corresponding to the CSRN. This causes the GMSC to issue an InitialDP containing the CSRN to the gsmSCF.

– The gsmSCF performs a function on the CSRN to obtain the MSISDN of the called party number and the MSISDN is returned to the GMSC in a CONNECT operation together with the CSRN as a Re-direction ID.

– The GMSC continues the normal CS call termination procedure by issuing an SRI to the HSS using the MSISDN, which causes T-CSI to be returned back to the GMSC.

– The GMSC issues another InitialDP containing both MSISDN and CSRN to the gsmSCF. The gsmSCF recognises that it has been triggered for the second time (due to the presence of the CSRN) and therefore returns a CONTINUE for the MSISDN, thus avoiding a circular loop.

– The GMSC sends a MAP SRI query to the HSS with the "Suppress T CSI" parameter, thus allowing the HSS to obtain the MSRN through standard call terminating procedures.

Annex G (normative):
Combined CS Access Authentication and IMS Registration Procedure for non-roaming UEs