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The provision of a handover of an existing VoLTE call to a legacy GERAN or UTRAN system is known as the SRVCC (refer to TS 23.216 [35]) because of the single RAT capability of a UE at a given time only. Interworking through the SRVCC scenario is a bit similar to the one described in Section 6.2.1. However, there is a difference from the E‐UTRAN UE capability point of view. SRVCC interworking scenario is suitable for an LTE UE having only one radio capability at a given time. Because of this, a VoLTE call over the IMS is required to be handed over to the legacy GERAN or UTRAN system for the continuation of the IMS voice call. The interworking of network elements, with IMS, for the SRVCC feature is illustrated in Figure 6.6.

Figure 6.6 Illustration: legacy and LTE network interworking for VoLTE call HO through SRVCC.

For the SRVCC capability to be realized, a new logical interface Sv, refer to TS 29.280 [71], is defined between the

 MME and the MSC server and

 MME and SGSN, as shown above.

Figure 6.7 further illustrates the SRVCC HO of an LTE/EPS IMS voice call made by the VoLTE user A to a PSTN user B.

In Figure 6.7, user A moved to a legacy 2G/3G coverage area. As the user moved away from the LTE coverage area and approaches a 2G/3G coverage area, the eNodeB detects a better signal level from the 2G/3G networks. UE sends the measurements reports to the eNodeB which in turn sends an SRVCC HO request to the MME. In Figure 6.7, the signaling and traffic paths are shown prior to and after the SRVCC HO. All the subsequent signaling and data communication takes place through the legacy network MSC server after the SRVCC HO is completed.

Figure 6.7 Illustration: VoLTE call HO through SRVCC feature.

 How does SRVCC work?

 Indication of Feature Support

An LTE UE indicates its SRVCC capability information through the class mark and network capability information sent along with the ATTACH Request message, refer to TS 24.301 [46], to the MME. MME indicates the SRVCC capability of a UE through the SRVCC operation possible IE in the INITIAL CONTEXT SETUP REQUEST MESSAGE, refer to TS 36.413 [97], to the eNodeB. MME may also request the SRVCC capability of a UE by sending the message UE Radio Capability Match Request (see Section 5.3.14 TS 23.401 [39]) to the UE through the eNodeB. Any subsequent changes in the SRVCC capability of a UE are also updated to the HSS by the MME.

 Initiation of Handover of LTE IMS Voice Call (VoLTE)

SRVCC works by performing the handover of the ongoing VoLTE call as well as data session over IMS to the legacy GERAN/UTRAN network. Initiation of a handover operation works the same way as the normal handover procedure based on the signal‐level measurement reports reported from the UE to the E‐UTRAN. The source eNodeB sends the HANDOVER REQUIRED message, refer to TS 36.413 [97], to the MME with the SRVCC HO Indication IE for a handover of the existing VoLTE call. SRVCC HO Indication may be PS or PS and CS type. MME, in turn, initiates the SRVCC request to the MSC server. The MSC server prepares and executes the handover of the IMS voice call successfully. For further illustrations of the VoLTE call handover through the SRVCC method, refer to Figures 4.2.2‐1 and 4.2.3‐1, TS 23.216 [35].

 Handover of PS Call/Data Session

The handover of an ongoing PS call during an SRVCC procedure is performed as usual as per the normal inter‐RAT handover procedures. For further illustrations of the inter‐RAT handover procedures, refer to Section 5.2.2 TS 23.401 [39].