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LTE Call Flow - PS Redirection to UTRAN-V2015 0105-V1.0
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5/24/13
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Wi r el ess: CSFB Lte - > WC DM A
LTE
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CSFB Lte -> WCDMA
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CSFB: Lte 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 ( VoI VoIP, P, IMS, IMS, CSFB etc) to handle the CS call ( Mo Mobile bile Originating & Mobile 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 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 to perform CSFB, UE should support multiple radio rad io 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 other CS enabled RAT (Utran or Geran). It's not very simple to make these two RATs with different protocol stack, work together together !! specially at the network end. I will try to e xplain the network architecture for CSFB in short as the focus of this post pos t is remain on the communication between UE an d network. The network architecture for CSFB is as follows:
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EPC supporting CS Fallback means Mobile Switching Center (MSC) and Visited Location Register (VLR) in the CS domain connects with the Mobility Mobility Management Management Entity (MME), (MME), which provides EPC m obility management functionality. The interface connecting c onnecting MSC/VLR MSC/VLR and MME MME is called a SGs SGs reference reference point. This SGs SGs interface interface 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 switc h. The SGs provides nearly all the functions provided by the existing Gs. You c an also check link understanding-cs-fallback-lte if 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 This procedure comes into picture for two kind of scenarios: 1. Mobile Originated Or iginated Call: Message Sequence or Call flow diagram:
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5/24/13
Wireless: CSFB Lte -> WCDMA
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 (w ith CSFB Indicator) to MME. W hen 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 carr ier 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.
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Wireless: CSFB Lte -> WCDMA
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. That is all :) !! ! 2. Mobile Terminated Call: This will be sam e as above procedure except in step 2, UE will rec eive CS_Service_Notification with Paging Identity for incoming CS call and send EMM_Extended_Service_Request (with CSFB Indicator). CSFB Lte -> WCDMA ( Using handover ): Stay Tuned, Coming UP!! Reference: Docomo Technical Journal Labels: CSFB, IRAT
