In UMTS, the network elements communicate with each other by exchanging signalling messages, which are written using the signalling protocols that we introduced in Chapter 2. The signalling messages are organised into procedures, which define how the network elements interact with each other, and which ultimately control the operation of the system. These signalling procedures are the main theme of the next two chapters. In this chapter, we discuss the procedures which control the internal operation of the system, and which do not involve any communication with the outside world. In Chapter 5, we will discuss the procedures that are related to particular services, such as voice and GPRS.

Here, we start by reviewing the way in which the network manages its communications with the mobile, and the different internal states that a mobile can be in. We then describe the procedures that a mobile uses when it switches on, to discover the cells around it and establish communications with the network. This leads to a discussion of the techniques that are used to keep the system secure in the presence of intruders. The second half of the chapter describes the procedures that take place inside a mobile after it has switched on. These are covered in two sections, as the exact procedures depend on the internal state that the mobile is in. The chapter closes by describing how the mobile stops communicating with the network and switches off.