CS Fallback

LTE is a packet only network and handles packet only data traffic. But if UE is registered on #LTE and service user make a CS call (circuit switched or voice call) or UE has to receive an MT CS call (mobile terminating call or incoming CS call) from the existing CS domain through EPC, what will happen??

CSFB (Circuit Switched (CS) fallback TS 23.272) which means that when the UE(phone) needs to make or receive a call it is moved to an older 3GPP technology (2G/3G). This mechanism is used if the IMS network is not available or the UE is not able to do Vo#LTE for some reason (for example registration to the IMS network failed)
#In order to be able to do CSFB one needs to first attach to the network in a special way. This special way is called Combined EPS/IMSI attach(in attach accept ). This special attach basically means that while the user is attached in 4G and residing in 4G, it is also attached to the CS network.To do this special attach the UE needs to set the attach type in Attach request message to Combined EPS/IMSI attach and indicate its capability for CSFB and/or SMS over SGs (old SMS). On receiving such a request, the MME will derive the VLR number and start an update procedure towards the MSC/VLR. The rest of the attach procedure is the same but the location of the UE will need to be updated in the CS network every time the UE moves to a new LA.
#When the UE wants to initiate a call (let’s assume for now that the UE is in connected mode) it will send a special message to the MME. This special message is called Extended Service Request and it contains an indication that the UE is asking the MME to start the handover to the CS network. The MME will ask the eNB to start the procedure for handover. The eNB may ask the UE to do some measurements and then initiate a normal handover procedure. The funny thing here is that it is possible for the eNB to decide to move the UE to a different LA (location area) than the one to which the UE is already connected. In this case along with the handover comes a LA Update or a Combined RA/LA Update. Once the UE is moved to the MSC it can initiate the call (this can happen before the release of the resources on the source network).

The EXTENEDED SERVICE REQUEST message is sent by the UE to the network to initiate a “CS fallback or 1xCS fallback call” or respond to an “MT CS fallback or 1xCS fallback” request from the network

• This message is also used if the UE wants to request the establishment of a NAS signaling connection (and of the radio and S1 bearers) for packet services and if the UE needs to provide additional information that cannot be provided via a SERVICE REQUEST message.
• The #UE shall send  EXTENEDED SERVICE REQUEST message when:
1. The UE in #EMM-IDLE mode receives a paging request with CN domain indicator set to “PS” from the network
2. The #UE, in EMM-IDLE mode, has pending user data to be sent
3. The #UE, in EMM-IDLE mode, has uplink signaling pending
4. The #UE in EMM-IDLE or #EMM-CONNECTED mode is configured to use CS fallback and has a mobile originating CS fallback request from the upper layer
5. The UE in EMM-IDLE mode is configured to use CS fallback and receives a paging request with CN domain indicator set to “CS”.                        or                                                                                                                                                                The UE in #EMM-CONNECTED mode is configured to use CS fallback and receives a CS SERVICE NOTIFICATION message.
6. The UE in EMM-IDLE or EMM-CONNECTED mode is configured to use 1xCS fallback and has a mobile originating 1xCS fallback request from the upper layer
7. The UE in EMM-CONNECTED mode is configured to use 1xCS fallback and accepts cdma2000®signalling messages containing a 1xCS paging request received over E-UTRAN.
8. The UE, in EMM-IDLE mode, has uplink cdma2000® signalling pending to be transmitted over E-UTRAN.
9. The #UE, in EMM-IDLE or EMM-CONNECTED mode, is configured to use 1xCS fallback, acceptscdma2000® signalling messages containing a 1xCS paging request received over cdma2000®1xRTT, and the network supports dual Rx #CSFB or provide CS fallback registration parameters
10. The #UE, in #EMM-IDLE or #EMM-CONNECTED mode, has uplink cdma2000® signalling pending to be transmitted over cdma2000® 1xRTT, and the network supports dual Rx #CSFB or provide CS fallback registration parameters
11. The #UE performs an inter-system change from S101 mode to S1 mode and has user data pending

NOTE:      For cases 1, 2, 3, 8 and 11 above, The #EXTENEDED SERVICE REQUEST message is used only if the UE is configured for NAS signalling low priority and the network supports EXTENEDED SERVICE REQUEST for packet services. Otherwise, SERVICEREQUEST is used

• #Upon receipt of the EXTENDED SERVICE REQUEST message, the MME may initiate the #EMM common procedures e.g. the authentication procedure and security mode control procedure
• #The Service type IE specifies the purpose of the service request procedure. This IE can indicate “mobile originating CS fallback or 1xCS fallback” or “mobile terminating CS fallback or 1xCS fallback” or “mobile originating CS fallback emergency call or 1xCS fallback emergency call”, or “packet services via S1”
• #The purpose of the #CSFB response IE is to indicate whether the UE accepts or rejects a paging for #CSFB. The IE’s values could be either “CS fallback rejected by the UE” or “CS fallback accepted by the UE” (see example2 below)
• #The IE EPS bearer context status shall be included if the UE wants to indicate the EPS bearer contexts that are active within the UE
• #The UE shall include the IE Device properties if the UE is configured for NAS signaling low priority. The network uses this IE for core-network congestion handling and for charging purposes

Note :  If PAGING is for PS domain then UE NAS layer triggers #SERVICE REQUEST otherwise, if the PAGING is for #CSFB then UE triggers Extended SERVICE REQUEST.

Circuit Switched Fallback #CSFB – Mobile Terminating call in idle mode

Circuit Switched Fallback #CSFB – Mobile Originating call in connected mode mode

Let discuss #STEP 1 to 5 of message sequence in detail as these are the #STEPs which constitutes #CSFB procedure:

#STEP 1: UE should be combined attached on #LTE cell. This is pre-requisite or you can say necessary condition for #CSFB to happen. When I say combined attached, means EPS attach type should be Combined EPS/IMSI attach. You can check this by decoding the attach accept message sent from the network during registration on #LTE.
Attach_Accept
…. .010 = Attach result: Combined EPS/IMSI attach
#STEP2: Now service user will initiate a voice call or say CS call. This will make UE to send #EMM_Extended_Service_Request (with #CSFB Indicator) to MME. When MME see #CSFB Indicator in Extended Service Request, it will perform CS Fallback. The UE only transmits this Extended Service Request with #CSFB indicator only if it is attached to CS domain (means combined attached as explained in previous #STEP).
#EMM_Extended_Service_Request <———-
Service type: Mobile originating CS fallback or 1xCS fallback (0)
#STEP3: After getting Extended Service Request, Network will trigger Rrc Connection Release with redirected carrier information which contains Target RAT and frequency as follows:
Rrc_Connection_Release ———-> 
  c1: rrcConnectionRelease-r8 (0)
                        rrcConnectionRelease-r8
                            releaseCause: other (1)
                            redirectedCarrierInfo: utra-FDD (2)
                            utra-FDD: 10700
#STEP4: After getting the Target RAT information, UE will find a suitable cell, read system information and establish radio connection on the Target RAT by initiating Routing Area Update Procedure.

#STEP5: UE will followup RAU procedure initiation by MO Call Setup by sending CM_Service_Request with following cause:
CM_Service_Request <———-
…. ..1. = CSMO: CS fallback mobile originating call
Signalling for CS call establishment procedure follows CM_Service_Request.

Overall Process in Call flow of CSFB :

Question : In #CSFB , what will happens after the CS voice call is terminated and if the UE is still in GERAN/UTRAN.

#After the CS voice call is terminated and if the UE is still in #GERAN and PS services are suspended, then the UE shall resume #PS services. A Gn/Gp- SGSN will resume the #PDP Context(s), which is done by the Routing Area Update procedure. During the Routing Area Update procedure in which the Gn/Gp SGSN sends a Update PDP Context Request message to the GGSN/P-GW, the #GGSN/P-GW resumes the PDP Context(s). An S4 SGSN will resume the bearers, and informs the S-GW and P-GW(s) to resume the suspended bearers. If the UE has returned to E-UTRAN after the CS voice call was terminated, then the UE shall resume PS service by sending TAU to MME. The MME will in addition inform S-GW and P-GW(s) to resume the suspended bearers. Resuming the suspended bearers in the S-GW and in the P-GW should be done by implicit resume using the Modify Bearer Request message if it is triggered by the procedure in operation e.g. RAU, #TAU or Service Request. The S-GW is aware of the suspend state of the bearers and shall forward the Modify Bearer request to the P-GW. Explicit resume using the Resume Notification message should be used in cases when Modify Bearer Request is not triggered by the procedure in operation.
If the #UE remains on #UTRAN/GERAN after the CS voice call is terminated the UE performs normal mobility management procedures.

======>#Click here For  #CSFB Full Call Flow while UE was in idle mode and receive paging for MT call.