O Required NAS: handoverToUTRAN HO Command SRVCC voice Connect to interruption UTRAN NAS: handoverToUTRANComplete Reloc Complete RRC: SecurityMode Voice call (CS) Activate Integrity can continue in Protection target network RRC: SecurityModeComplete Non-GBR bearers HO UE Context Release Command completion follows UE Context Release Complete Figure 14.36 QCI1 HO signalling flow from EUTRAN/UTRAN point view, excluding the non-GBR bearers' HO completion part (shown in Figure 14.37) SRVCC In this chapter the signalling flow of inter-system EUTRAN to GERAN or UTRAN handover for both voice and data bearers is discussed. Figure 14.36 shows the signalling between the UE and EUTRAN and after the handover command between the UE and UTRAN. As the voice bearer in EUTRAN is packet switched, it requires a conversion into CS domain and involves rather large number of network elements. The complete signalling flow in Figure 14.37 shows the signalling between core elements and radio access networks. Measurements and Decision to Initiate SRVCC Once the coverage is running thin and VoLTE call can no longer be maintained on LTE relocation/handover can be performed to 3G or 2G network. This procedure is triggered by the UE sending measurement reports at threshold configured in measurement configuration. In one of the measurement reports, the UE reports, for example, the RSRQ or RSRP of the serving LTE cell followed by the RSCP or ECNO of the underlying 3G cell. Based on the measurement reports the eNodeB makes the decision about starting the SRVCC procedure. It sends HO-required message to the MME and indicates there whether the target cell is both CS and PS capable or CS only capable and whether the request is for voice bearer only or for both voice and packet data bearers. Splitting the Voice and Data Bearers The minimum number of bearers required for a UE that has VoLTE call ongoing in LTE network is three bearers. They are QCI1 for VoLTE call, QCI5 for IMS signalling and, for example, QCI9 for the default bearer. LTE Small Cell O