agement is a key tool in helping a network operator improve services and make them more competitive. 2.11.1 Introduction Traditionally, systems that take care of control and supervisory functions in a telecommunications network have been known as operation and maintenance (O&M) systems. Nowadays we prefer to use the term network management system because the functions performed by network management systems include much more than those supported by the conventional O&M systems. Operation functions cover subscriber management functions and enable the network operator, for example, to collect charging data and move and terminate subscriptions. Operation also includes traffic monitoring and controlling the network in such a way that the risk of overload is minimized, for example, by switching traffic from overloaded connections to other systems. Maintenance includes monitoring of the network and, when a fault occurs, corrective actions are performed. Bit error rates and other parameters are continuously measured for the early detection of faults. When a fault is detected, the operator's staff starts troubleshooting in order to localize the fault. This used to be quite a difficult task because it was done manually and many systems may detect a fault even when the actual fault may be in only one of them or even somewhere else. Maintenance, like other network man- agement functions, has become more and more computerized, making fault location easier and quicker with the help of centralized management systems that provide graphical information about the network's condition. 2.11.2 Who Manages Networks? Corporate networks are private networks containing LANs interconnected by circuits provided by a public telecommunications network operator. We can divide corporate networks into two main areas of network management responsibility: local networks in corporate sites and interconnections between sites implemented in a public network that provides interconnec- tions as shown in Figure 2.21. The corporate networks, LAN1 and L