6.1 Introduction

The main driver behind 2nd generation digital mobile networks, such as the global system for mobile communications (GSM) [1], was the need to provide a voice telephony service to mobile users. This has been achieved with incredible success. Moreover, GSM has established the starting point from which future mobile networks must evolve and an important benchmark for voice services that the 3rd generation of mobile networks must exceed in terms of functionality and quality.

The Universal Mobile Telecommunications System (UMTS), the 3rd generation network and systems standardised by the 3rd Generation Partnership Programme (3GPP ) [2], aims to provide voice services that will meet the needs of mobile users. This is being done in collaboration with the International Telecommunications Union (ITU) 'International Mobile Telecommunications 2000' project [3].

In the initial phase of UMTS, defined by the 3GPP Release 1999 standards, the voice telephony service is essentially an evolution of the GSM voice service that benefits from the 3rd generation technologies adopted for the UMTS Terrestrial Radio Access Network (UTRAN) (see Chapter 11 for more details).

However, the customer's needs for mobile voice telephony must also be considered in the light of the growing demand for mobile Internet multimedia services. In particular, voice will be a feature of many of these multimedia services, e.g. videoconferencing, mobile commerce (mCommerce), games and multimedia messaging. To enable such services, it is important that the voice service is as much part of the mobile Internet as the data and information...