ality exists in the HSPA evolution of 3G network. VoLTE and ViLTE Both handover modes are transparent to VoLTE and ViLTE services, the movement of the mobile being masked for the IMS network. Chapter 8 presents the roaming for which two routing methods of the RTP streams are described: - nominal routeing of the RTP stream that passes through the home network; - optimal routeing of the RTP stream that does not pass through the home network. Chapter 9 presents the centralization of services implemented by IMS centralized services (ICS) that enables the IMS network to offer VoLTE and ViLTE services regardless of the network where the mobile phone is connected. Chapter 9 also presents the continuity of services implemented by function enhanced single radio voice call continuity (e-SRVCC) which ensures that the communication is maintained in case of PS-CS (Circuit- Switched) inter-system handover (Figure 3). IP packet 2G / 3G Network IP packet CS mode Figure 3. PS-CS inter-system handover Preface Chapter 10 presents the two modes providing short message service (SMS). Short message service over SGsAP allows a mobile connected to the 4G network to send and receive SMS in the CS mode. Short message service over SIP is a supplementary telephone service provided by the IMS network. André PEREZ April 2016 List of Abbreviations Authorization-Authentication-Answer Authorization-Authentication-Request Accounting-Answer Address Complete Message Accounting-Request Application Function Authentication-Information-Answer Authentication-Information-Request Acknowledged Mode Aggregate Maximum Bit Rate Adaptive Multi-Rate AMR WB AMR Wide Band Answer Message Advice of Charge Application transport Mechanism Access Point Name Allocation and Retention Priority Automatic Repeat Request VoLTE and ViLTE Application Server Abort-Session-Answer Abort-Session-Request Access Transfer Control Function Access Transfer Gateway ATU-STI Access Transfer Update - Session Transfer Identifier Authentication Network B2BUA Back-to-Back User Agent Broadcas