C.4 Network Entities

23.2723GPPCircuit Switched (CS) fallback in Evolved Packet System (EPS)Release 17Stage 2TS

C.4.1 General

The following network entities specifically enable support of SMS in MME

C.4.2 MME

The MME needs to support the following features:

– UE-MME NAS procedures for SMS over SGs.

– S6a functionality as described in clause C.3.2.

– SMS procedures between UE and MME including support for the Short Message Control (SMC) and Short Message Relay (SM-RL) functions defined in TS 24.011 [28].

– Reporting UE’s availability for receiving MT SMS to the HSS.

– SMS functions including charging and Lawful Interception (LI).

– Support combined EPS/IMSI procedures for "SMS-only" and in these:

– provide a non-broadcasted LAI (not associated with any VLR); and

– if needed provide a reserved TMSI that cannot cause any ambiguities with other TMSIs (e.g. when the UE changes to an area where it needs to derive SMS services from a GERAN/UTRAN MSC); and

– indicate in the Attach/TAU Accept message that the IMSI attach is for "SMS-only"; and

– notify the HSS it is capable of SMS transfer without the need of establishing an SGs association with an MSC; and

– obtain SMS-related subscription information.

– Support the registration procedures for SMS in MME described in clause C.8.

– Support for Service Gap Control for MO SMS as specified in TS 23.401 [2], clause 4.3.17.9.

C.4.3 HSS

When an MME registers via the S6a including a registration for SMS Request, the HSS registers the MME as an MSC for MT SMS (i.e. in response to Send Routing Information (SRI) for SM the MME identity is passed back to the SMS‑GMSC/SMS router) when the HSS supports and decides to use SMS in MME. This registration as an MSC for MT SMS causes the HSS to cancel any old registered MSC. See clause C.8 for the detailed registration procedures. If the HSS receives a SRI for other CS Services than SMS it shall treat the UE as if it was detached from the CS domain. If the UE subsequently registers with an MSC for CS services, the HSS needs to replace the MME with the new MSC as the destination for SMS (but this is not a reason to cancel the MMEs registration for EPS Services).

NOTE 1: That an HSS that supports SMS in MME registers the MME as an MSC for MT SMS does not affect how the information is stored in the HSS, however the HSS reuses the MNRF flag intended for the controlling SM delivery via the MSC, for controlling the SM delivery via MME. It only affects what serving node identities are returned in response to SRI for SM. In such cases the MME identity is passed back to the SMS-GMSC/SMS router where usually the MSC identity is sent.

NOTE 2: For the PS and SMS only service to work also for outbound roamers, the 3GPP LTE-only operator may need to configure CS subscription data for SMS services in its HSS as a visited PLMN may deploy SMS over SGs. It may also use an IWF for MAP and Diameter translation. The 3GPP 2G/3G/LTE operator may need to upgrade its HSS to ensure that its outbound roamers get SMS services as a VPLMN may deploy only SMS in MME.

An HSS allows an operator to configure a subscription, which is limited to only PS domain services and to SMS service via the CS and the PS domain. This limitation is indicated in the PS subscription data as "PS and SMS only".

NOTE 3: The limitation of a subscription to only PS and SMS services can for example be based on an operator agreement with an MTC service provider, that only PS resources shall be used whenever possible.

When an HSS supports "SMS in MME", i.e. provides all HSS functionality described for "SMS in MME", and the operator configured the HSS to apply that functionality, the HSS indicates "SMS in MME Support" in the data provided to an MME.

NOTE 4: This separate handling of SMS subscriber data in the MME is also for aligning with the deployment option of an IWF for S6a that combines PS subscriber data with SMS subscriber data from CS subscriber data, where the data part that stems from CS subscriber data may be cancelled in MME independently from PS subscriber data. When the PS subscriber data are cancelled then any UE related subscriber data are cancelled in the MME.

If the HSS determines that an MME shall be deregistered for SMS (e.g. because of a removal of SMS subscription, CS location update, etc), the HSS shall indicate to the MME that it is not registered for SMS.