GSM (Global System for Mobile Communication) is a member of the class of cellular mobile communication networks that use operating frequencies of around 900 MHz and 1800 MHz. The GSM network is hierarchically structured, as shown in the diagram below.
1.1.1 Mobile phones
A GSM mobile phone consists of two components, the mobile radio telephone itself and the SIM (Subscriber Identity Module). This enables a distinction to be made in the GSM network between user and mobile terminal.
The mobile radio telephone is characterised by its internationally unique serial number or International Mobile EquipmentIdentity ( IMEI). The user is identified by his customer number (International Mobile Subscriber Identity or IMSI), which is stored on the SIM card. This is assigned to the subscriber when he registers with the network provider and must be distinguished from the telephone number assigned to him, which is the Mobile Station ISDN Number ( MSISDN). This distinction enables a subscriber to use different mobile radio telephones with the same SIM card.
The subscriber-specific call number is also stored on the SIM card. The cryptographic algorithms for authentication and encryption of user data are also implemented on the SIM card. In addition, short text messages, call charge information and a personal telephone directory can be stored on the card too.
1.1.2 Base station
A GSM Base Transceiving Station ( BTS) houses the transmit and receive equipment for one or more cells. It constitutes the interface between the network provider and the mobile phone. The Base Station Controller ( BSC) administers the transmit and receive resources of the connected base stations. For example, the channels for signalling and for payload traffic are provided here and the data traffic between BTS and MSC is controlled here.
1.1.3 Switching nodes
The base station is controlled via the Mobile Switching Centre ( MSC). This switching node assumes all the technical functions of a landline network switching node, for example, path search, signal path switching and processing of supplementary services. If there is a requirement for a connection to a subscriber in the landline network, this is forwarded by the MSC to the landline network over a switching path.
In order that the network provider is in a position to provide all the services for which demand exists, it must store various items of data. For example, it must know which subscribers are using its network and which services they wish to use. This data, such as the name of the subscriber, his customer number and the services he requires, is stored in the Home LocationRegister ( HLR). If a connection is to be established, for example from a landline network connection to a mobile phone, the network provider needs to know where the subscriber is and whether his mobile phone is switched on. This information is held in the Visitor Location Register ( VLR) and the HLR.
To check whether a subscriber is entitled to use the mobile telecommunication network (i.e. he has taken out a card contract), the network provider maintains an Authentication Centre ( AUC). This holds algorithms and subscriber-related keys which amongst other things are required during authentication.
The network provider can also maintain the Equipment Identity Register ( EIR), which holds details of all the mobile transceivers permitted on the network, broken down into three groups known as the white, grey and black lists. The white list is a register of all the mobile phones which are functioning reliably, the grey list contains all the phones which may possibly be defective, while the black list holds details of all the phones which either have a fault or have been reported stolen. However, not all network providers maintain an equipment register.
1.1.4 Landline network
The public telephone network with its connecting paths is referred to as the landline network. As landline networks are also used in every mobile phone connection, the dangers entailed in the use of landline networks also apply to the use of mobile telecommunication networks
1.2 Connection set-Up
As soon as the owner switches on his mobile phone, it registers with the network provider via the nearest base station. At the network provider, data on the identity of the user, the serial number of the mobile phone and the identity of the base station over which registration has occurred is logged and stored. This is done even if no conversation takes place. Moreover, every time a number is dialled, this event is stored, irrespective of whether a connection is established or not.