7.2 Mobile Terminating call in idle mode

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

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

The procedure for Mobile Terminating Call in idle mode is illustrated in figure 7.2-1, in the normal case. Clause 6.6 describes the procedure when the procedure is rejected by the MME.

Figure 7.2-1: Mobile Terminating Call in idle mode

1. G‑MSC receives IAM.

2. G‑MSC retrieves routing information of the terminating UE by Send Routing Info procedures as specified in TS 23.018 [5].

3. G‑MSC sends IAM to the MSC on the terminating side as specified in TS 23.018 [5].

4. The MME receives a Paging Request (IMSI, VLR TMSI, Location Information, eMLPP priority level indication) message from the MSC over a SGs interface. IMSI is used by the MME to find the S‑TMSI. If the TMSI and the LAI are received from the MSC/VLR, the S-TMSI is used as the paging address on the radio interface. If the TMSI is not received from the MSC/VLR, the IMSI shall be used as the paging address on the radio interface. If location information is reliably known by MME (i.e. MME stores the list of TAs), the MME shall page the UE in all the TAs. If the MME does not have a stored TA list for the UE, the MME may use the location information received from the MSC to page the UE.

NOTE 1: The case when LAI is not received from MSC/VLR and the case when S-TMSI is not available are further described in clause 4.8.

NOTE 2: This procedure takes place before step 3, immediately after MSC receives MAP_PRN from HSS, if pre-paging is deployed.

If the MME receives a Paging Request message for an UE which is considered as detach for EPS services, the MME sends the Paging reject message to the MSC with an appropriate cause value. This rejection triggers the MSC to page the UE over A or Iu-cs interface.

NOTE 3: In case of a CS fallback capable UE in NMO II, there is a case where, for example, the MME releases the SGs association due to the UE idle mode mobility while the VLR still maintains the SGs association.

If the MME holds stored Paging Restriction information (see TS 23.401 [2]) for the UE that restricts the SGs Paging Request from causing paging, the MME sends the SGs Paging Reject message to the MSC containing an appropriate cause. The rest of the steps in the procedure are skipped.

If the MME receives Paging Request with eMLPP priority level indication from the MSC, then the MME determines based on its configuration whether this priority level requires CSFB priority call handling.

5. If the MME did not return an "SMS-only" indication to the UE during Attach or Combined TA/LA Update procedures, the MME sends a Paging (as specified in TS 23.401 [2]) message to each eNodeB. The Paging message includes a suitable UE Identity (i.e. S‑TMSI or IMSI) and a CN Domain Indicator that indicates which domain (CS or PS) initiated the paging message. In this case it shall be set to "CS" by the MME.

If the MME returned the "SMS-only" indication to the UE during Attach or Combined TA/LA Update procedures, the MME shall not send the paging to the eNodeBs and sends Paging Reject towards MSC to stop CS Paging procedure and this CSFB procedure stops.

If the MME determined in step 4 that CSFB priority call handling shall apply, the MME sets the priority level in the S1AP Paging message based on the value(s) configured in the MME for CSFB priority call handling.

6. The radio resource part of the paging procedure takes place. The message contains a suitable UE Identity (i.e. S‑TMSI or IMSI) and a CN Domain indicator. The eNodeB performs the paging procedure according to the priority level received with the S1AP Paging Request.

7a. The UE establishes an RRC connection and sends an Extended Service Request for mobile terminating CS fallback to MME. The UE indicates its S-TMSI in the RRC signalling. The Extended Service Request message is encapsulated in RRC and S1‑AP messages. The MME sends the SGs Service Request message to the MSC containing an indication that the UE was in idle mode (and hence, for example, that the UE has not received any Calling Line Identification information). Receipt of the SGs Service Request message stops the MSC retransmitting the SGs interface Paging message.

NOTE 4: In order to avoid the calling party experiencing a potentially long period of silence, the MSC may use the SGs Service Request message containing the idle mode indication as a trigger to inform the calling party that the call is progressing.

If the UE, when in MUSIM mode, upon reception of paging indication in E-UTRAN access decides not to accept the paging, the UE attempts to send a Reject Paging Indication in the Extended Service Request message unless it is unable to do so e.g. due to UE implementation constraints. The MME sends the SGs Paging Reject message to the MSC containing an appropriate cause. The UE remains reachable for future paging attempts based on stored (if any) Paging Restriction Information in the MME. The rest of the steps in the procedure are skipped.

If the MME determined in step 4 that CSFB priority call handling shall apply and the MME receives subsequent Extended Service Request in Step 7a, it detects this message is the response to the priority CS Fallback procedure initiated in step5. In this case, the MME processes this message with priority and set the priority indication, i.e. "CSFB High Priority", in step7b as specified in TS 36.413 [35].

If UE has only LIPA PDN connection and the cell accessed by the UE does not link to the L-GW where the UE had the LIPA PDN Connection, the MME shall reject the Extended Service Request with a reason code which results in the UE selecting GERAN or UTRAN as specified in TS 24.301 [34].

If the UE has only SIPTO at the Local Network PDN connections:

(i) in a network with collocated L-GW deployment and the (H)eNB accessed by the UE is not collocated with the L-GW providing the SIPTO at the Local Network connectivity; or

(ii) in a network with stand-alone GW deployment and the (H)eNB accessed by the UE is not connected to the Local Network where the stand-alone GW (with S-GW and L-GW collocated) providing the SIPTO at the Local Network connectivity is residing,

the MME shall reject the Extended Service Request with an appropriate cause value which results in the UE selecting GERAN or UTRAN.

If the UE has only SIPTO at the Local Network PDN connections and LIPA PDN connections, where:

– the SIPTO at the Local Network PDN connections fulfil criterion (i) or (ii) above; and

– the cell accessed by the UE does not link to the L-GW where the UE had the LIPA PDN connection,

the MME shall also reject the Extended Service Request with an appropriate cause value which results in the UE selecting GERAN or UTRAN.

7b. MME sends S1‑AP: Initial UE Context Setup (UE capabilities, CS Fallback Indicator, LAI and other parameters specified in TS 23.401 [2]) to indicate the eNodeB to move the UE to UTRAN/GERAN. The registered PLMN for CS domain is identified by the PLMN ID included in the LAI, which is allocated by the MME.

7c. The eNodeB shall reply with S1-AP: Initial UE Context Setup Response message.

8a. If the eNodeB knows that both the UE and the network support PS handover: The information flow may continue as described in clause 7.3 "Mobile Terminating call in Active Mode – PS HO supported" from step 2, in clause 7.3, and onwards.

. If the eNodeB knows that either the UE or the network does not support PS handover: The information flow shall continue as described in clause 7.4 "Mobile Terminating call in Active Mode – No PS HO support" from step 2, in clause 7.4, and onwards.

NOTE 5: Even in case both the UE and the network support PS HO, the eNodeB may choose to use a different inter-RAT mobility procedure.