SVCRC(Single Radio Voice Call Continuity) and Diff between CS Fallback and SRVCC

What is #SRVCC(Single Radio Voice Call Continuity)?
One important feature of Vo#LTE is implementation of Single Radio Voice Call Continuity (#SRVCC). The concept of #SRVCC requires that when a user moves outside the #LTE coverage area, it should experience a smooth handover to another technology such as 3G/UMTS or 2G/GSM.

What is ##CSFB (Circuit Switched (CS) fallback
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. 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 CSFC 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).

What is difference between CS Fallback and SRVCC..in which scenario both will come in picture?

• In case of CSFB, your voice call is never active in #LTE. When a voice call needs to be started,the UE initiates ESR(Extended Service Request) and the network moves it to a legacy network through redirection/handover.
• Whereas #SRVCC (Single Radio Voice Call continuity) is a process where your active voice call in #LTE (may be VOIP),is handed          over to the CS domain of the legacy network (MSC). This is useful when you are in an #LTE coverage area and have an active voice  call over IMS and you move out of the #LTE area into a Legacy only network coverage.

SRVCC Procedure
The general network procedure for #SRVCC from E-UTRAN to #UTRAN is as follows:

• The UE sends measurement reports to E-UTRAN.
• The E-UTRAN (based on measurement reports or some triggers) decides to initiate an #SRVCC handover to UTRAN and sends the    Handover Required message to the MME.

• The MME triggers the #SRVCC procedure for the voice bearer towards MSC Server.
• The MSC Server initiates the Session Transfer procedure to IMS and coordinates it with the CS handover procedure to the UTRAN, and standard IMS Service Continuity procedures are applied for execution of the Session Transfer.

• The MSC Server sends PS-CS handover response to MME, including CS handover command information for the UE to access the      UTRAN.
• The MME sends a Handover Command message to the E-UTRAN, which includes the information about the voice bearer.
• This message is encapsulated within a Mobility From E-UTRA Command and sent to the UE.
• The UE tunes to UTRAN and resumes its voice call.

How to Test #SRVCC
#Pre-Test Setup
Before the test, you need to make the following operations:
• #Run a scenario file on the simulator which includes appropriate configuration for the IMS bearer, specifically:
• #QoS Traffic Class being Conversational Class
• #Based on the QCI associated with the voice bearer (QCI 1) and the #SRVCC HO indication, the source MME splits the voice bearer     from the non voice bearers and initiates the PS-CS handover procedure for the voice bearer only towards MSC Server.
• #QoS When a voice call is established, an additional dedicated bearer with QCI 1 is set up to carry the actual voice traffic.
• #P-CSCF IP address being included and set to that of the IMS Server.
• The Attach Accept message(INITIAL CONTEXT SETUP REQUEST) containing the optional information #SRVCC Operation Possible  with ‘Possible’ being set to 1.
• #Establish the connection between the test set and the EPC emulator either via the test set or EPC interface
• #Launch and configure the IMS Server
• #Launch and configure the IMS Test Client. Register the client with the IMS Server
• #UE registers with combined EPS/IMSI attach on the test set and the UE IMS Client registers with the IMS Server.

• The source eNB shall include the SRVCC HO Indication IE in the HANDOVER REQUIRED message. The source eNB shall indicate to the MME in the SRVCC HO Indication IE if the handover shall be prepared for PS and CS domain or only for CS domain.

• SRVCC HO Indication depends on neighbor cell parameter which is VOIP_CAPABALE or       PS_HO_SUPPORTED.

• SRVCC HO Indication if UTRAN/GERAN Neighbors cell is defined as below

SRVCC from E-UTRAN to GERAN/UTRAN

E-UTRAN may determine the NCL, as well as the need to signal a #SRVCC indication, as follows:

• If the “SRVCC operation possible” indication is set to “true” (i.e. both EPC and UE are #SRVCC capable),then VoIP-incapable cells      may be included as candidate target cells in the NCL, regardless of the presence of an established #QCI=1 bearer for this UE. 

• If there is an established QCI=1 bearer for this UE and the selected target cell is VoIP-capable,then  EUTRAN does not include a SRVCC indication in the Handover Required message;

• If there is an established QCI=1 bearer for this UE and the selected target cell is VoIP-incapable, then EUTRAN includes a SRVCC   indication in the Handover Required message;

• If there is no established QCI=1 bearer for this UE, then E-UTRAN does not include a #SRVCC indication in the Handover Required  message;

• If the “SRVCC operation possible” indication is set to “false” (i.e.either EPC or UE is not #SRVCC capable), then E-UTRAN does not include a SRVCC indication in the Handover Required message.

• If there is an established QCI=1 bearer for this UE, then VoIP-incapable cell are not be included in the NCL;

• If there is no established QCI=1 bearer for this UE, then VoIP-incapable cells may be included in the NCL.