This book presents the mechanisms used in the 4G evolved packet system (EPS) mobile network and in the IP Multimedia sub-system (IMS) for the supply of voice over long term evolution (VoLTE) and video over long term evolution (ViLTE) service (Figure 1).

The EPS network does not provide telephone service because it does not deal with telephone signaling.

The EPS network operates in packet-switched (PS) mode and acts as the transport of internet protocol (IP) packets through bearers.

The EPS network, therefore, transfers the IP packets containing voice or video real-time transport protocol (RTP) streams or telephone signaling session initiation protocol (SIP).

Telephone or videophone service is provided by the IMS network which provides the functions as follows:

1. – routing the call;
2. – supplementary telephone and videophone services;
3. – interconnection to the third-party networks.

Chapter 1 presents the architecture of EPS and IMS networks and these networks environment: databases, charging, policy and charging control (PCC), DIAMETER routing, ENUM system and internet protocol exchange (IPX).

Chapter 2 presents various signaling protocols:

1. – signaling of the EPS network, allowing the mobile to attach, to update its location, to establish sessions for the transport of IP packets and to change cells during a session (handover);
2. – signaling of the IMS network, allowing the mobile to register, to establish a session and to negotiate the media;
3. – DIAMETER signaling exchanged between, firstly, the EPS or IMS networks, and, secondly, the environment of these networks.

Chapter 3 presents the different basic procedures:

1. – the attachment and the detachment of the mobile with the EPS network and the establishment of the default bearer to transport SIP flows;
2. – the registration and the deregistration of the mobile with the IMS network;
3. – the establishment and the release of VoLTE and ViLTE session.

Chapter 4 presents the characteristics of the radio interface, for which the following features are described: data structure, transmission chain of the physical layer, frequency time and space multiplexing.

The same chapter also illustrates two procedures of the radio interface: access control of the mobile to network and data transfer.

Chapter 5 presents the supplementary telephone and videophone services offered by a particular entity of the IMS network, the telephony application server (TAS).

These services include call forwarding, identity presentation, message waiting indication, call hold, conference call, call waiting and call barring.

It also presents the characteristics of audio and video streams.

Chapter 6 presents the interconnection to the public switched telephone network (PSTN) or to the public land mobile network (PLMN) (Figure 2).

Chapter 6 also presents the interconnection of the IMS network with IMS third-party networks.

Chapter 7 presents the mechanisms of intra-system and PS-PS inter-system handover.

The intra-system handover is performed when the mobile changes cell but does not change the 4G network concerned.

The PS-PS inter-system handover is performed when the mobile changes cell and network but holds the PS mode. This type of handover is applied to VoLTE or ViLTE services if the same functionality exists in the HSPA evolution of 3G network.

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:

1. – nominal routeing of the RTP stream that passes through the home network;
2. – 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).

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