n lifting the handset and dialing the number of the mobile subscriber. On receiving a number with the area code, the PSTN/ISDN network will route the call to the gateway switch of the mobile network and will also provide the telephone number of the mobile subscriber. The gateway switch then interrogates the mobile network's HLR to recover the subscriber's records. Once the call arrives at the MSC, the MS is paged to alert it to the presence of an incoming call. A paging call is then issued from each BS in the location area in which the subscriber is registered. On receiving a paging call the MS responds by initiating the access procedure. The access procedure commences with the MS sending a message to the BS requesting a channel. The BS replies by sending the MS details of a dedicated channel and the MS re-tunes to this channel. A certain degree of handshaking occurs to ensure that the identity of the subscriber is correct. Once the dedicated signalling channel is established, security procedures such as subscriber authenti- cation, take place over this channel. Following this, the network allocates a dedicated speech channel and both the BS and MS re-tune to this channel and establish a connection. It can be seen that until this point is reached all processes are carried out autonomously by the MS and no interaction is required from the subscriber. It is only once all these processes are completed that the MS begins to ring. The subscriber can now talk and the handover can take place between different BSs. Once the call has ended the call clear process initiates, which consists of a small exchange of signalling information ensuring that both the network and the MS know that the call has ended. The MS again returns to the idle mode and monitors the paging channel of its current cell. Several cryptographic algorithms are used for GSM security, which include the features link authentication, over-the-air voice privacy, etc. The security model of GSM, however, lacked some features such as authentication of the user 