Content for TS 23.272 Word version: 19.0.0

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4.1 General Considerations 4.2 Reference Architecture 4.3 Functional entities 4.4 Control plane 4.5 Co-existence with IMS services 4.6 Emergency Calls 4.6a Interaction with IMS Emergency session 4.7 CSFB Priority Call Handling 4.8 CS services handling in specific cases
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4 Overall Description p. 11

4.1 General Considerations p. 11

The CS fallback in EPS enables the provisioning of voice and other CS-domain services (e.g. CS UDI video/ LCS/ USSD) by reuse of CS infrastructure when the UE is served by E-UTRAN. A CS fallback enabled terminal, connected to E-UTRAN may use GERAN or UTRAN to connect to the CS-domain. This function is only available in case E-UTRAN coverage is overlapped by either GERAN coverage or UTRAN coverage.

CS Fallback and IMS-based services shall be able to co-exist in the same operator's network.

The ICS architecture as defined in TS 23.292 shall be able to co-exist with utilising CS Fallback as the CS domain in the same operator's network.

This specification also specifies the architecture required for SMS over SGs. The MO SMS and MT SMS are signalled over SGs and do not cause any CS Fallback to GERAN/UTRAN RATs, and consequently does not require any overlapped GERAN/UTRAN coverage.

Additionally this specification specifies the SMS in MME architecture option as described in Annex C.

The support of SMS over SGs is mandatory for UE and MME and MSC supporting CS fallback, whereas UE and MME and MSC supporting SMS over SGs are not required to support CS fallback.

The support of CS fallback (and to a lesser extent SMS over SGs) can impose some operational constraints on Tracking Area boundary planning and the use of the "tracking area list concept" (see TS 23.401).

4.2 Reference Architecture p. 11

The CS fallback and SMS over SGs in EPS function is realized by using the SGs interface mechanism between the MSC Server and the MME.

The SGs interface functionality is based on the mechanisms specified for the Gs interface, TS 23.060.

Copy of original 3GPP image for 3GPP TS 23.272, Fig. 4.2-1: EPS architecture for CS fallback and SMS over SGs

Figure 4.2-1: EPS architecture for CS fallback and SMS over SGs
(⇒ copy of original 3GPP image)

4.2.1 Reference points p. 12

SGs:

It is the reference point between the MME and MSC server. The SGs reference point is used for the mobility management and paging procedures between EPS and CS domain, and is based on the Gs interface procedures. The SGs reference point is also used for the delivery of both mobile originating and mobile terminating SMS. Additional procedures for alignment with the Gs reference point are not precluded.

S3:

It is defined in TS 23.401 with the additional functionality to support ISR for CS fallback/SMS over SGs as defined in this specification.

4.3 Functional entities p. 12

4.3.1 UE p. 12

The CS fallback capable UE supports access to E-UTRAN/EPC as well as access to the CS domain over GERAN and/or UTRAN.

The SMS over SGs capable UE supports access to E-UTRAN/EPC and may support access to the CS domain over GERAN and/or UTRAN.

The support of SMS over SGs is mandatory for a UE that supports CS fallback, whereas a UE that supports SMS over SGs is not required to support CS fallback.

These UEs support the following additional functions:

Combined procedures specified in this document for EPS/IMSI attach, update and detach.
CS fallback and/or SMS over SGs procedures specified in this document for using CS domain services.

From the UE's perspective there is no difference whether the MME provides SMS via SGs or by "SMS in MME".

A UE that only supports NB-IoT (see TS 23.401) may request SMS service and use the EPS Attach, TA Update and EPS Detach procedures.

A UE using CS fallback and/or SMS over SGs supports ISR according to TS 23.401. In particular a UE deactivates ISR at reception of LAU accept or at reception of combined RAU/LAU accept response with no ISR indication.

The coexistence with IMS services for voice/SMS is defined in clause 4.5.

There are no other CS fallback/SMS over SGs ISR-specifics for the UE compared to ISR description in TS 23.401, i.e. if ISR is active the UE can change between all registered areas and RATs without performing update signalling. The UE listens for paging on the RAT it is currently camped on.

If the UE is a Service User according to TR 22.952 with subscription to CS domain priority service, the UE's USIM belongs to one of Access Class that indicates the priority is needed and the UE sets the RRC establishment cause to "HighPriorityAccess" as specified in TS 36.331.

4.3.2 MME p. 12

The CS fallback and/or SMS over SGs enabled MME supports the following additional functions:

Multiple PLMNs selection for the CS domain.
RAT selection for the CS domain.
Deriving a VLR number and LAI from the TAI of the current cell and based on the selected PLMN or the selected RAT for CS domain, or using a default VLR number and LAI.
Deliver the registered PLMN ID for CS domain (included in the LAI) to the eNodeB.
Includes the PLMN ID for the CS domain (included in the LAI provided to the UE) in the equivalent PLMNs list if it is different compared to the PLMN ID provided as part of the GUTI.
For CS fallback, generating a TAI list such that the UE has a low chance of "falling back" to a cell in a LA different to the derived LAI (e.g. the TAI list boundary should not cross the LA boundary).

NOTE 1:
Alignment of the TAI list boundary with a LA boundary can prevent the MME from making effective use of the "tracking area list" concept. To compensate for this, appropriate cell reselection hysteresis may need to be used within the E-UTRAN.
Maintaining of SGs association towards MSC/VLR for EPS/IMSI attached UE.
In order for the Serving MSC/VLR to always be up to date with the correct last used LTE PLMN, issuing a LOCATION-UPDATE-REQUEST over SGs when the selected LTE PLMN changes: the serving LTE PLMN is derived from the TAI included in that message.
Initiating IMSI or EPS detach.
Initiating paging procedure specified in this document towards eNodeB when MSC pages the UE for CS services.
Supporting SMS procedures defined in this document.
Rejecting CS Fallback call request (e.g. due to O&M reasons)
When configured to support the return to the last used PLMN after CSFB:
- Indicating to the MSC that the MO CS service is due to CSFB as specified in TS 29.118.
- Indicating to the SGSN that the mobility procedure is due to CSFB by adding an indication in Forward Relocation Request and Context Response as specified in TS 29.274.
  
  NOTE 2:
  This only applies when the S4 reference point is supported between the MME and the SGSN; no equivalent functionality exists on Gn.
- To be sure the Context Request is due to CSFB, a timer is set by the MME at CSFB execution and the indication in Context Response is included only if the timer is not expired.
If the network supports the CSFB procedures during ongoing IMS voice call with SRVCC capability (i.e. clause 8.3 and clause 8.4) and UE has SRVCC and EMM combined capability and CSFB is selected by the network (i.e. not SMS only), then:
- Selecting a MSC/VLR which is both SRVCC and CSFB capable during combined Attach/TAU procedure.
- If CSFB triggers SRVCC during ongoing IMS voice call, a PS to CS HO request is sent to the MSC/VLR that is registered during the combined Attach/TAU procedure, for which the MME selects an SRVCC and CSFB capable MSC/VLR.

An MME that supports "SMS in MME" shall support additional functions specified in Annex C, clause C.4.2.

Whether the MME provides SMS via SGs or by "SMS in MME" is not visible to the UE.

An MME that supports CS Fallback uses the LAI and the TMSI based NRI as provided by the UE or the LAI and a hash value from the IMSI to determine the VLR number as defined in TS 23.236 when multiple MSC/VLRs serve the same LAI. The NRI distribution or IMSI Hash table shall have a consistent configuration in all MMEs in the pool. The same NRI value/hash value/function is used by SGSN to determine the VLR number. An MME that supports SMS over SGs may use the same procedure as for CS Fallback. In some networks, the MME may be configured to select the MSC/VLR for UEs that send low access priority indication with a different load balance to that used for MSC/VLR selection for other UEs. In this case the MME maintains a separate hash/value function for UEs that send low access priority indication.

If the network supports CSFB priority call handling, the MME supports the following additional functions:

For paging message received on the SGs interface with an indication of the eMLPP priority level, the MME is configured with the priority level(s) for which it provides preferential treatment to the paging of the UE and also the subsequent CS fallback procedure compared to other normal procedures. If the UE needs to be paged, the MME sets the priority level in the S1AP paging request to eNodeB based on the value(s) configured in the MME for CSFB priority call handling. The MME also sets priority indication, i.e. "CSFB High Priority", in the S1AP UE Context Setup/Modification message to the eNodeB, so that the eNodeB can initiate the CSFB procedure with priority, as specified in TS 36.413.
For an MO CSFB request, the MME determines that the CSFB request needs CSFB priority call handling based on the "HighPriorityAccess" establishment cause received from the eNodeB and/or the MPS CS Priority stored in UE's EPS subscription. The MME provides preferential treatment to this request and also sets priority indication, i.e. "CSFB High Priority", in the S1AP UE Context Setup/Modification message to eNodeB to initiate CSFB procedure with priority, as specified in TS 36.413.

4.3.3 MSC p. 14

The CS fallback and/or SMS over SGs enabled MSC supports the following additional functions:

Maintaining SGs association towards MME for EPS/IMSI attached UE.
Supporting SMS procedures defined in this document.
When configured to support the return to the last used PLMN after CSFB:
- Indicating to GERAN/UTRAN during the release of an RR connection, that the RR connection was established as a result of CS Fallback, when this is the case.
- During the SGs location update procedure, obtaining the last used LTE PLMN ID via TAI in SGsAP-LOCATION-UPDATE-REQUEST as specified in TS 29.118.
- Indicating the last used LTE PLMN ID to GERAN/UTRAN (immediately following a successful RR connection setup or during the release of the UTRAN RR connection) to help GERAN/UTRAN re-route the UE back to the last used LTE PLMN by selecting the suitable dedicated target frequency list (for idle mode mobility) or target cell (for connected mode mobility in case of UTRAN).
- During a CSFB procedure, including the last used LTE PLMN ID into the Equivalent PLMN list provided to the UE in LAU Accept message.

If the network supports CSFB priority call handling, the MSC maps, depending on the deployed CS domain priority scheme, the priority level or another priority indication of the IAM message to the eMLPP priority level indication of the paging message that is sent over the SGs interface.

4.3.4 E-UTRAN p. 14

The CS fallback enabled E-UTRAN supports the following additional functions:

Forwarding paging request for CS domain to the UE.
Directing the UE to the target CS capable cell considering the registered PLMN ID and possibly the LAC for CS domain received from the MME.
The configuration of appropriate cell reselection hysteresis at Location Area boundaries (or across the whole E-UTRAN) to reduce Tracking Area Update traffic.
To facilitate the configuration of TA boundaries with LA boundaries, the E-UTRAN can gather statistics (from the inbound inter-RAT mobility events of all UEs) of the most common LAs indicated in the RRC signalling.
Configuration to permit the operator to choose the target 'fallback' RAT and frequency.

For SMS over SGs, no specific E-UTRAN functionality is required.

If the network supports CSFB priority call handling, the E-UTRAN supports the following additional functions:

For paging message received on S1AP with priority indication, the E-UTRAN should provide preferential treatment to this request compared to other normal paging requests.
For CS fallback S1AP UE Context Setup/Modification message with priority indication, i.e. "CSFB High Priority", the eNodeB should provide preferential treatment, e.g. in allocating E-UTRAN radio bearer resources compared to other normal resource requests. When CSFB based on PS handover is employed, the eNodeB forwards the priority indication to the target GERAN/UTRAN. When CSFB RRC connection release with redirection is used, the eNodeB sets the "CS Fallback High Priority" indication in the RRC Release message.

4.3.5 SGSN p. 15

If the SGSN supports ISR, SGSN shall follow the rules and procedures described in TS 23.401 and TS 23.060 with the following additions and clarifications:

The SGSN shall not send the ISR activated indication at combined RAU/LAU procedure if the UE indicates support of CSFB and/or SMS over SGs.

An SGSN that supports Gs uses LAI and the TMSI based NRI as provided by the UE or LAI and a hash value from the IMSI to determine the VLR number as defined in TS 23.236 when multiple MSC/VLRs serve the same LAI. The same NRI value/hash value/function is used by MME to determine the VLR number.

When configured to support the return to the last used PLMN after CSFB the SGSN shall, during a RAU procedure due to CSFB, include the last used LTE PLMN ID into the Equivalent PLMN list provided to the UE in RAU Accept message.

4.3.6 BSS p. 15

If the BSS receives indication from the MSC that the CS service (Call, CISS or LCS) was established as a result of CS fallback the BSS may e.g. select to use RR Connection Release with Redirect to send the UE back to E-UTRAN at release of the CS service.

When configured to support the return to the last used PLMN after CSFB and when a last used LTE PLMN ID is indicated after a successful RR connection setup, the BSS shall take the last used LTE PLMN ID into account when selecting the target cell in a PS handover to E-UTRAN or when selecting the dedicated target frequency list for idle mode mobility to E-UTRAN in RR Connection Release procedure. Furthermore, the BSS shall send the last used LTE PLMN ID to the target BSS/RNS during the handover preparation phase.

If the network supports ISR, the CS fallback enabled BSS exhibits the following behaviour:

Even if the network is operating in NMO II the BSS shall forward Gb interface paging messages onto the radio interface. The BSS in a network operating in NMO II shall not be configured to use PBCCH.

4.3.7 RNS |R10| p. 15

If the RNS receives indication from the MSC that the CS service (Call, CISS or LCS) was established as a result of CS fallback the RNS may e.g. select to use RRC Connection Release with Redirect to send the UE back to E-UTRAN at release of the CS service.

When configured to support the return to the last used PLMN after CSFB and when a last used LTE PLMN ID is indicated after a successful RR connection setup or during the UTRAN RR connection release, the RNS shall take the last used LTE PLMN ID into account when selecting the target cell in a PS handover to E-UTRAN or when selecting the dedicated target frequency list for idle mode mobility to E-UTRAN in RR Connection Release procedure. Furthermore, the RNS shall send the last used LTE PLMN ID to the target RNS/BSS during the handover preparation phase.

4.4 Control plane p. 16

4.4.1 MME - MSC Server p. 16

Copy of original 3GPP image for 3GPP TS 23.272, Fig. 4.4.1-1: SGs Interface

Figure 4.4.1-1: SGs Interface
(⇒ copy of original 3GPP image)

Legend:

SGsAP:

This protocol is used to connect an MME to an MSC Server based on the BSSAP+.

Stream Control Transmission Protocol (SCTP):

This protocol transfers signalling messages.

4.5 Co-existence with IMS services p. 16

A CS Fallback and IMS capable UE shall follow the procedures for domain selection for UE originating session/calls according to TS 23.221 'Domain selection for UE originating sessions / calls'.

An IMS capable UE which supports SMS over IP networks shall follow the procedures for domain selection for UE originating SMS according to TS 23.221 'Domain selection for UE originating SMS'.

4.6 Emergency Calls p. 16

4.6.1 General |R9| p. 16

When UE is performing CS fallback procedure for Mobile Originating Call for the purpose of emergency call, it shall indicate to the MME that this CS fallback request is for emergency purpose. MME also indicates to the E-UTRAN via the appropriate S1-AP message that this CS fallback procedure is for emergency purpose. If PS handover is initiated, E-UTRAN may indicate priority level of the CS fallback to the target RAT, as specified in TS 25.413, in order to prepare radio resource at target RAT in appropriate way, e.g. priority allocation of the RAB resource.

Based on local operator policy, if the MSC/VLR is required to handle emergeny call setup with IMSI, the network may perform the additional procedure defined in clause 4.6.2.

4.6.2 Procedures to handle emergency call setup with IMSI |R9| p. 16

This procedure is only invoked when permitted by the operator policy.

When MME detects that the CSFB request is due to emergency call and eNodeB indicates that the UE is not available for the PS service at the target RAT, it shall mark the SGs association as not valid (i.e., move to SGs-NULL state) and shall not send IMSI Detach Indication (IMSI) message to the MSC/VLR. After that, MME continues with the CSFB procedure. If ISR is active, MME shall initiate DETACH REQUEST message with the detach type set to "Local Detach" via S3 to the associated SGSN.

The SGSN in NMO I configuration with ISR capability shall support the "EMM combined UE Waiting Flag".

When MSC/VLR receives CS emergency call setup with IMSI due to CSFB and this IMSI is unknown, the MSC/VLR shall perform a Location Update procedure to the HLR on behalf of the UE after it successfully authenticated the UE. If required by configuration, MSC/VLR shall perform a TMSI reallocation towards the UE using its own non-broadcast LAI and, if the MSC Server is part of an MSC pool for that LA, with its own Network Resource Identifier (NRI).

4.6a Interaction with IMS Emergency session |R9| p. 17

If the MME receives a paging request which is not for SMS and detects that the UE currently has an ongoing IMS emergency session, the MME shall respond with a Paging Reject towards MSC to stop CS Paging procedure.

4.7 CSFB Priority Call Handling |R10| p. 17

CSFB Priority call handling ensures that, when the network supports eMLPP service (TS 22.067) or another CS domain priority scheme (e.g. TIA-917 [46] or TR 22.952), end-to-end priority handling is provided for both mobile originated CSFB calls by a user in E-UTRAN which is entitled for this by a CS domain priority scheme and for mobile terminated CSFB call that is marked for priority and destined to a user in E-UTRAN. An EPS subscription of an UE that is subscribed to a CS domain priority scheme may contain an indication of the users CS domain priority status, i.e. it may contain an MPS CS Priority indication. If the UE is subscribed to a CS domain priority scheme, the UE's USIM may belong to one of Access Class 11 to 15, e.g. when the UE is a Service User according to TR 22.952 the UE is assigned to an Access Class in the range of 11 - 15.

For mobile terminated CS fallback calls belonging to the eMLPP CS domain priority scheme, the MSC provides the call's eMLPP level to the MME along with the SGs paging message. Operator configuration in the MME defines which eMLPP level(s) indicated from MSC to MME require CSFB priority call handling. For CSFB priority call handling the MME is configured with the corresponding paging priority level(s) (i.e., mapped from eMLPP level received) that the MME shall set in the message to the eNodeB when requesting the eNodeB to page the UE if the UE is idle.

For mobile originated CS fallback calls, the MME determines that the CSFB request requires priority handling based on the "HighPriorityAccess" establishment cause received from the eNodeB and/or the UE's MPS CS Priority. For both mobile originated and mobile terminated CSFB priority call handling, the MME shall also provide priority indication to the eNodeB, i.e. "CSFB High Priority", when requesting the eNodeB to execute the CSFB priority procedure as specified in TS 36.413. The eNodeB should handle the S1AP paging message with the indicated paging priority level. And the eNodeB shall prioritize the CS fallback procedure to GERAN/UTRAN or 1xRTT according to the CSFB priority indicated in the S1AP UE context setup/modification message. If PS handover to GERAN or UTRAN is initiated, E-UTRAN may forward CS fallback priority indicator to the target RAT, as specified in TS 25.413, in order to prepare radio resource at target RAT in appropriate way, e.g. priority allocation of the RAB resource

4.8 CS services handling in specific cases |R10| p. 18

4.8.1 SGs paging without LAI p. 18

If the SGs Paging Request does not contain the LAI, and if the MME can retrieve the S-TMSI associated to the IMSI, the MME shall temporarily store this MSC/VLR number and force the UE to re-attach to the non-EPS services based on:

If the UE is in connected mode, the MME shall not send a SGs Service Request to the MSC/VLR as it normally would, but instead shall send a Detach Request (IMSI Detach) to the UE as described in clause 5.3.2;
If the UE is in idle mode, the MME shall send Detach Indication (Local Detach) message to the SGSN via S3 interface if ISR is active. If the mobile reachable timer is still running the MME shall page the UE with IMSI and the CS indicator (for CSFB) or with S-TMSI and the PS indicator (for SMS). At the reception of Extended Service Request (for CSFB) or Service Request (for SMS) from the UE, the MME shall not send a SGs Service Request to the MSC/VLR as it normally would, but instead shall send a Detach Request (IMSI Detach) to the UE as described in clause 5.3.2.

The MME shall not send IMSI Detach Indication message to the MSC/VLR in the above scenarios.

MME shall then wait for the UE to re-attach to non-EPS services via a Tracking Area Update Request; the MME shall IMSI attach the UE to the MSC/VLR using the stored MSC/VLR number. After successful IMSI attach, if the paging timer has not expired, the MSC/VLR shall page the UE again using SGs Paging Request with LAI.

4.8.2 Handling of SGs paging when MME cannot retrieve the S-TMSI p. 18

Upon receiving SGs Paging Request, if the MME cannot retrieve the S-TMSI associated with IMSI, the MME shall page the UE with IMSI and PS indicator to force the UE to re-attach to EPS and non-EPS services.