CSFB Lte -> WCDMA

CSFB: 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??

Well, there are few options (VoIP, IMS, CSFB etc) to handle the CS call (Mobile Originating & Mobile Terminating) in LTE and one of them is CS fall back which uses existing technologies which support CS traffic like GSM, WCDMA to make voice call happen. CSFB makes UE to move to the CS supported RATs using redirect or handover and perform the CS call. Now you know why we need CSFB and what it does, we should be moving to nitty-gritties of this CSFB procedure.

In-order to perform CSFB, UE should support multiple radio access (Lte & Utran or Geran) and EPC should support CSFB. When I say EPC should support CSFB, it means co-ordination should be there at the network end between Lte and other CS enabled RAT (Utran or Geran). It's not very simple to make these two RATs with different protocol stack, work together !! specially at the network end.I will try to explain the network architecture for CSFB in short as the focus of this post is remain on the communication between UE and network.

The network architecture for CSFB is as follows:

EPC supporting CS Fallback means Mobile Switching Center (MSC) and Visited Location Register (VLR) in the CS domainconnectswith the Mobility Management Entity (MME), which provides EPC mobility management functionality. The interface connecting MSC/VLR and MME is called a SGs reference point. ThisSGsinterface is based on the concept of the Gs reference point that exchanges signaling with MSC, which connects to the Serving General Packet Radio Service Support Node (SGSN), a 3G packet switch. The SGs provides nearly all the functions provided by the existing Gs. You can also check linkunderstanding-cs-fallback-lteif you want more info regarding this.

Now lets have an example of CSFB and see the signalling sequence between UE & network !!

CSFB Lte -> WCDMA (Using Redirection):This procedure comes into picture for two kind of scenarios:

1. Mobile Originated Call: Message Sequence or Call flow diagram:

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 saynecessarycondition for CSFB to happen. When I say combined attached, means EPS attach type should beCombined 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 attachSTEP2: 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

c1: rrcConnectionRelease-r8 (0) rrcConnectionRelease-r8 releaseCause: other (1) redirectedCarrierInfo: utra-FDD (2) utra-FDD: 10700STEP4: 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