SRVCC - Single Radio Voice Call Continuity

SRVCC enables continuity of voice call anchored in IMS network, when UE changes its access network from packet based in eUTRAN to circuit based GERAN or UTRAN, without requiring the UE to have simultaneous access to both the radio network.

 

It accomplishes this by enhancing the MMEs, serving E-UTRAN and the 3GPP CS MSC serving GERAN/UTRAN such that when the UE is still anchored within the E-UTRAN network and triggers a measurement report indicating a need for handover (HO) to GERAN/UTRAN, the MSC can initiate IMS service continuity procedure towards the IMS domain in addition to preparing for a handover into its GERAN/UTRAN access, before the UE is directed to change its access network to GERAN/UTRAN. 

 

The SRVCC feature requires the new Sv interface between MSC and MME. Sv Interface runs over UDP. Following messages are exchanged over Sv Interface:

 

- PS to CS Request

- PS to CS Response

- PS to CS Complete Notification

- PS to CS Complete Acknowledge

- PS to CS Cancel Notification

- PS to CS Cancel Acknowledge

- Echo Request

- Echo Response

 

GTPv2-C Transports:

 

The GPRS Tunnelling Protocol (GTP) tunnels are used between two nodes communicating over a GTP based interface, to separate traffic into different communication flows.  A GTP tunnel is identified in each node with a TEID, an IP address, and a UDP port number.  The receiving end side of a GTP tunnel locally assigns the Tunnel Endpoint  Identifier (TEID) value the transmitting side has to use.  The TEID values are exchanged between tunnel endpoints using GTP-C message.  For SRVCC, GTP Version 2 Control Plane (GTPv2-C) based protocols are used across Sv.  There is one pair of TEID per UE on the Sv interface.  The same tunnel will be shared for control messages related to the same UE operation.   The TEID field in the SRVCC 'PS to CS Request' message header will be set to '0' because this is the 1st message the MME sends to the WCS to establish the tunnel for a UE. The destination UDP port of a response message is set to the source UDP port of corresponding message.   

 

SRVCC call Flow:

 

UE     eNB     MME     G-MSC     MSC      RNC/BSC      IMS

==========================================================

 

1. Measurement Report

------->

         2. HO Required

       --------->

                 3. PS to CS Req

                --------->

                                     4. Prepare HO Req

                                     ----------->
                                   5. Handover Request
                                   ------------>
                                                         6. HO ACK
                                   <------------
                                     7. Prepare HO Resp
                       <------------
                                     8. IAM
                       ------------->
                       9. ACM
                       <------------
                                     10.         INVITE (STN-SR)
                       --------------------------------->
                             11. PS to CS Response
                 <-------
    12. HO Command
<-------  <---------
<---------------- 13. HO Detection --------->
                                                        14. HO Complete
                     <-------------- <----------
                        15. ANM
                                  <-------------
                                    <--------- 200 OK ---
       16. PS to CS Complete Notification
       <-------------
     17. PS to CS Complete Ack
       ------------->

                       Update Location
                     -------------> HLR



1. UE sends measurement reports to Source UTRAN (HSPA).

2. If target is UTRAN, the source E UTRAN sends a Handover Required (Target ID, generic Source to Target Transparent Container, SRVCC HO indication) message to the source MME. SRVCC HO indication indicates to MME that this is for CS+PS HO.

3. Source MME initiates the PS-CS handover procedure for the voice bearer by sending a SRVCC PS to CS Request (IMSI, Target ID, STN-SR, C MSISDN, Source to Target Transparent Container, MM Context) message to the MSC Server. The message includes information relevant to the CS domain only. MME received STN-SR and C MSISDN from the HSS as part of the subscription profile downloaded during the E UTRAN attach procedure.

5. Source MME initiates the PS-CS handover procedure for the voice bearer by sending a SRVCC PS to CS Request (IMSI, Target ID, STN-SR, C MSISDN, Source to Target Transparent Container, MM Context) message to the MSC Server. The message includes information relevant to the CS domain only. MME received STN-SR and C MSISDN from the HSS as part of the subscription profile downloaded during the E UTRAN attach procedure.