6781097070

Introduction 7

1.2.2.9 Equipment Identity Register

The theft of GSM mobile telephones seems attractive, sińce the iden-tities of subscribers and their mobile equipment are separate. Stolen eąuipment can be reused simply by using any valid SIM. Barring of a subscriber by the operator does not bar the mobile eąuipment. To prevent that kind of misuse, every GSM terminal eąuipment contains a uniąue identifier, the international mobile eąuipment identity (IMEI). It lies within the realm of responsibilities of a network operator to eąuip the PLNM with an additional database, the EIR, in which stolen eąuipment is registered and so can be used to bar fraudulent calls and even, theoretically, to track down a thief (by analyzing the related SIM data).

1.3 The Focus of This Book

This book describes briefly the GSM subsystems, their structure, and their tasks. However, the focus of this book lies not on the GSM network elements themselves but on the interfaces between them.

Among others, the following issues will be addressed:

•    What signaling standards and what protocols are used to serve connec-tion reąuests by mobile subscribers?

•    How are the various interfaces utilized?

•    What happens in case of errors?

•    Although GSM uses available signaling standards, where are the GSM specific adaptations?

One has to remember that most of the signaling is necessary to support the mobility of a subscriber. Ali messages of the area mobility management (MM) and radio resource management (RR), in particular, serve only that purpose. Only a fraction of the exchanged messages are used for the connec-tion setup as such, and those are all the messages that are related to cali control (CC).

A presentation of the Open System Interconnection (OSI) Reference Model is mandatory in a book in which the focus is on signaling.

Another focus of the text is on the application of the various protocols for error analysis. Which error indication is sent by the system and when? How is such an indication interpreted? What are the potential sources of errors?