ation and Ciphering procedures in order to reinitialize the keys on the PS domain and starts ciphering of the PS RABs by sending the Security Mode Command. The UE responds with Security mode complete message. The RRC sequence number is calculated using HFN and ciphering key. Routing area update accept and complete messages are exchanged. PSHO from UE point of view is now complete. Simultaneously the target UTRAN or GERAN sends Relocation Complete/Handover Com- plete message to target SGSN which sends a Forward Relocation Complete message to the source MME which acknowledges the information by sending a Forward Relocation Complete Acknowledge message to the target SGSN. Target SGSN updates the bearer with S-GW/P- GW/GGSN; the MME sends Delete Session Request to the S-GW and a Release Resources message to the Source eNodeB. The Source eNodeB releases its resources related to the UE. 14.5.2 Performance SRVCC - Voice Interruption LTE network stops delivering voice packets to the UE after Handover to UTRAN command is sent; UE immediately tunes on UTRAN target frequency and establishes connection. Once Security Mode message is delivered to the UE from UTRAN and UE has processed it, the UE can start decoding the voice information delivered to it. Lab testing was done in RF chamber and the results are shown in Figure 14.38. SRVCC handovers were made from EUTRAN to UTRAN while target network RF conditions were adjusted. The target network RSCP from last measurement report to EUTRAN was recorded and voice interruption time was measured. The number of samples is rather low, but it is y = -2.3609x + 3.7745 Target RSCP (dBm) Figure 14.38 Voice interruption time in milliseconds as a function of target network (WCDMA) RSCP value. Used AMR codec is WB-AMR 12.65 kbps in LTE and NB-AMR 12.2 kbps in WCDMA VoLTE Optimization already shown that interruption times vary between about 200 and 280 ms which is below the 3GPP target of 300 ms. It is also evident that the average voice interruption time increases as the target network RSCP 