An Introduction to LTE: LTE, LTE-Advanced, SAE, VoLTE and 4G Mobile Communications, 2nd Edition

Chapter 22VoLTE and the IP Multimedia Subsystem

In this chapter, we discuss the likely long-term solution for LTE voice calls, namely the delivery of voice over IP streams that are controlled by the IP multimedia subsystem (IMS). The IMS is not part of LTE: instead it is a separate network whose relationship with LTE is the same as that of the Internet. Despite this, it is valuable to cover the IMS as part of this book, because LTE voice calls are important and because the IMS illustrates several aspects of LTE's operation. The delivery of voice calls over LTE and the IMS is often known as Voice over LTE (VoLTE).

In the course of the chapter, we review the history of the IMS and VoLTE, and discuss the architecture and protocols of the IMS and the procedures for registration and call setup. We then cover three aspects of interoperation between the IMS and the 2G/3G circuit switched domain, namely access domain selection, single radio voice call continuity and IMS centralized services, and conclude by discussing IMS emergency calls and SMS. There are some excellent accounts of the IP multimedia subsystem for those who need further detail, notably References [1–3].

22.1 Introduction

22.1.1 The IP Multimedia Subsystem

The IP multimedia subsystem (IMS) was originally designed for the management and delivery of real-time multimedia services over the 3G packet switched domain. It was first defined in 3GPP Release 5, which was frozen in 2002. The specifications attracted a great deal ...