What is SRVCC call flow in GSM? 

In the LTE network, SRVCC is a feature by which a UE can roam from EUtran to Utran (or vice versa) and other networks without disrupting an ongoing call.  The LTE  is packet-based while UTRAN or GERAN is a circuit-based network for voice calls.

In this tutorial, we will describe the SRVCC call flow from E-UTRAN to UTRAN.  All the details are as per 3GPP specification ETSI TS 123 21.

What is the SRVCC call flow trigger point?

The triggering point is, when the UE sends a measurement report to the eNodeB, that the cell in another RAT type is more suitable.  After that eNodeB instruct the MME for the handover is required. Next, MME instructs the other network components to initiate the SRVCC procedure.

Before SRVCC call flow from EUtran to Utran, UE attaches to the HSS over Diameter protocol after the call flow the UE attaches to the HLR over the ss7 network. For the data, the UE moves from PGW to GGSN.

SRVCC call flow procedure:

This section details all steps in the SRVCC call procedure.

  • UE sends a measurement report to the eNodeB.
  • eNodeB sends a Handover required to the MME for the UE.
  • MME sends PS to CS requests to the MSC server over the Sv interface.
  • MSC server sends prepare handover command to the target MSC.
  • The target MSC sends relocation on handover requests to the target RNC in UTRAN (BSS for GERAN).

After the above handover sequence, there will be a circuit setup between the MSC server and MSC. Now the existing voice can use this path.