monitoring and analysis network surveillance signalling management. In Figure 2.7, the OSS installed at the NOC has detected a problem in link 6. Combined with information such as traffic data, the cause of the fault may be deter- mined to be, say, a traffic overload in the case of excessive throughput or a cable break in the case of zero throughput. Traffic data are collected periodically from local exchanges, every 5-10 minutes, and made available to NOC staff. The data are analysed by software installed at the NOC. Traffic management functionality includes setting thresholds for alarms in exchanges and on paths. It also includes assisting network management staff in making rerouting and redistribution decisions such as restricting traffic along some OSS for Telecom Networks OSS at NOC OSS at OSS at OMC Exchange Exchange Optical fiber Wireless transmission box base station Figure 2.8 Network surveillance - the NOC has an integrated view of the entire network, unlike the OMCs. paths or through some exchanges. An OSS that is advanced in traffic management functionality would be able to make decisions on the basis of network data that have been collected and, on the basis of those decisions, issue rerouting commands to a local exchange. Network surveillance includes: network supervision furnishing network operations staff with an overall view of the NEs and network processes that comprise the network, including switching, transport and signalling. The OMC presiding over the part of a network in which an alarm originates sends any urgent alarms to the NOC for alarm filtering. The network surveillance function at the NOC compiles these alarms and often applies rule-based analysis determine action to be taken. This rule-based analysis is part of the expanding domain of policy-based management (PBM). 2.4 Management of Transport Networks The last two decades have seen great strides forward in operators' ability to manage and control the transport network efficiently. Manageability was improved by the introduction of th