tioning of the network. 2.2.1 Operations The first of the five areas of operational management is subscriber management. This includes connecting new subscribers and ensuring that services are provisioned and available. The second area of operational management is charging, which includes collecting billing data. This is the most obvious foundation of the operator's revenue-earning scheme and is carried out several times in any 24-hour period. The third and fourth aspects of operational management are traffic management and signalling. Traffic management distributes traffic in a way that improves grade of service and network utilisation, while minimising the possibility of network overload. Signalling is used for most traffic switching, and activation and discon- nection of services in public networks. Service management is about management of the other four areas of operational management but at the services level of abstraction. 2.2.2 Maintenance Preventing faults before they arise requires close supervision and accurate measure- ment of many network parameters. Systematic maintenance requires simple net- work monitoring methods and many measurements in order to prevent faults. The network operator must check that parameters, such as bit error ratio (BER) and probability of congestion, are within a tolerance range. This range is set to assure delivery of promised services at the agreed level of certainty. If a hardware fault is detected, the operator's staff first localise the fault and then allow software to pinpoint the fault. Next, corrective action is taken to eliminate the fault. The appropriate troubleshooting and fault localisation procedure depends on the nature of the fault and the type of equipment. Before modern OSS software, this process was manual, requiring many skilled, and therefore expensive, staff. Nowadays, software agents installed in network elements automate a large part of the process. Further, centralised network management software is equipped with fault correlation algorithms. These pr