LR of the new region. Authentication Centre (AUC) The AUC (or AC) is responsible for policing actions in the network. This has all the data required to protect the network against false subscribers and to protect the calls of regular subscribers. There are two major keys in the GSM standards: the encryption of communications between mobile users, and the authentication of the users. The encryption keys are held both in the mobile equipment and the AUC and the information is protected against unauthorised access. OVERVIEW OF MOBILE NETWORKS Equipment Identity Registers (EIR) Each item of mobile equipment has its own personal identification, which is denoted by a number - the International Mobile Equipment Identity (IMEI). The number is installed during the manufacture of the equipment and states its conformation to the GSM standards. Whenever a call is made, the network checks the identity number; if this number is not found on the approved list of authorised equipment, access is denied. The EIR contains this list of authorised numbers and allows the IMEI to be verified. 1.4.3 Network Management System (NMS) The main task of the network management system is to ensure that the running of the network is smooth. For this purpose, it has four major tasks to perform: network monitoring, network development, network measurements, and fault management. Once the network is up and running, the NMS starts to monitor its performance. If it sees a fault it generates the relevant alarm. Some faults may be corrected through the NMS itself (mostly software oriented), while for others site visits are be required. The NMS is also responsible for the collection of data and the analysis of its performance, thereby leading to accurate decisions related to the optimisation of the network. The capacity and the configuration of the NMS are dependent upon the size (both in terms of capacity and geographical area) and the technological needs of the network. 1.4.4 Interfaces and Signalling in GSM As can be seen from Figure 1.4, there are s