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Sep 24, 2013

Circuit Switched Fallback CSFB - Mobile Terminating call in idle mode

Previously I've covered the topic of CSFB when MT is in active mode. Today the below will cover situation when UE MT is in idle mode.

Fig. 1. CS Call with CSFB with MT UE in IDLE

1. G MSC receives IAM.
2. For the Send Routing Info (SRI) procedure please go to TS 23.018 3GPP.
3. G MSC sends IAM to the MSC on the terminating side.
4. The MME receives a Paging Request with MT side IMSI message from the MSC over a SGs interface. IMSI is used by the MME to find the S TMSI which is 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 should use the location information received from the MSC to page the UE.
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.
In case of a CS fallback capable UE in NMO II or III, 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.
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 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.
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.
7a. The UE establishes an RRC connection and sends an Extended Service Request (CS Fallback Indicator) 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 CS Fallback Indicator indicates to the MME that CS Fallback for this UE is required. 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.
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.
7b.MME sends S1 AP: Initial UE Context Setup 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. (not visable above) The eNodeB shall reply with S1-AP: Initial UE Context Setup Response message.

Further after completing the above procedure is as in the Active mode described previously in Circuit Switched Fallback CSFB - Mobile Terminating Call in Active mode, no PS HO.
Starting from Step 2 of the procedure presented below. This is the modified version (Step 1 is gone) of what you could find following the link above.

Fig. 2. Fig. 1. CS Paging over SGs in 4G, Call in 3G/2G 

TS 23.272 3GPP