F.2 Implementation options for call diversion from CS to the IMS

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

Several techniques available in the current CS networks can be used to implement the call diversion from CS to IMS, as detailed below:

1 Use of CAMEL for call diversion to IMS

This option applies to configurations requiring handling of incoming calls at the GMSC function. Upon receipt of an incoming call, the GMSC queries the HSS for routing information via the Send Routing Information (SRI) query. The user profile in the HSS is configured to return T-CSI including a gsmSCF address to the GMSC in response to the SRI query. When handling calls for a subscriber with a service provided by the IMS, the subsequent processing at the gsmSCF and the GMSC results in routing of the call to the IMS using the IMRN. The call is routed to the SCC AS according to standard IMS routing procedures. In order to determine the necessary information to complete the call, the SCC AS uses the IMRN or the ISUP information mapped to SIP headers.

2 HSS directed call diversion to IMS

This option also applies to configurations requiring handling of incoming calls at the GMSC function. Upon receipt of an incoming call, the GMSC queries the HSS for routing information using the MAP Send Routing Information (SRI) procedure (as defined in TS 29.002 [45]). Based on a non-standardized mechanism, the user profile in the HSS is configured to return an IP Multimedia Routing Number (IMRN) to the GMSC in response to the SRI query, when the call is directed to a subscriber with a service provided by the IMS. The subsequent processing at the GMSC results in routing of the call to IMS using the IMRN. Two methods can then be used to ensure correlation between the IMRN and the original called party.

a. Cooperative allocation/deallocation: In this method, the IMS is made aware of the assigned IMRN and when a call is received for that number, the original number is retrieved. This method is similar to the Provide Roaming Number procedure in MAP (see TS 29.002 [45]).

b. Algorithmic: In this method, a known algorithm is used to derive the IMRN at the CS, and to deduce the original called number from the IMRN at the IMS. One method of performing such an algorithm could be use of a prefix. In such a case, care is required in the network configuration to avoid call looping for the case when the call is subsequently routed from the IMS to the CS domain for call termination (see clause F.3 for possible solutions to prevent call looping).

3 Static diversion from GMSC with dedicated trunk groups

This option also applies to configurations requiring handling of incoming calls at the GMSC function. Dedicated trunk groups can be used at the GMSC to divert CS terminations to the MGCF.

NOTE 1: The handling of call types that an operator has not centralised the services into the IMS need to be taken into account if this solution is chosen.

4 Static diversion using Local Number Portability

This option can be used for routing of calls originating in PSTN networks to IMS. A Local Number Portability database dip can be used to reroute incoming calls to a subscriber with a service provided by the IMS with calls to the MGCF.

NOTE 2: The handling of calls other than calls needs to be taken into account if this solution is chosen.

5 Direct routing to IMS

Translations can be set up in the PSTN network to route the incoming call to a subscriber with a service provided by the IMS to the MGCF. This way the normal IMS routing technique specified in TS 23.228 [2] can be used.

NOTE 3: The handling of call types that an operator has not centralised the services into the IMS need to be taken into account if this solution is chosen.

NOTE 4: With some of the aforementioned options, care must be taken to avoid a circular loop which can occur when used in conjunction with certain techniques applied to route the call from IMS to the terminating user via CS network. Techniques for the prevention of circular looping are described in clause F.3.