urse, buying an OSS "off-the-shelf" has the downside of not being as customised as an in- house system can be. At the same time, this problem places an opportunity for prod- uct differentiation at the feet of OSS vendors. The challenge for OSS vendors is to develop software: 1. That is useful in its own right so that they are not effectively acting as contract software developers by having to write large amounts of custom code each time a telecom carrier wants to buy their product; and 2. That is customisable and extensible to a reasonable extent since no two carriers will have the same network or the same business needs. 2.2 Telecom network operations and maintenance Operations and maintenance are those business processes which enable the net- work infrastructure to provide customers with services that are profitable for the carrier. Thus, O&M is essentially the discipline of delivering value and extracting profit from a network infrastructure. network data corrective and preventive action Figure 2.1 Operation and maintenance. 2 Network Management OSS Overview Operations include subscriber management, charging, signalling, traffic manage- ment and service management. Of these five, OSS vendors have least control over signalling, because signalling functionality is generally built into the fabric of a tele- com network. On the other hand, maintenance involves (a) preventive action, through measurement and supervision, and (b) corrective action, such as finding the location of a fault ("fault localisation") and fault correction. See Figure 2.1. Effective operation and maintenance together require that the operator commu- nicate with the traffic-carrying network elements. These network elements must be able to inform the operator of alarm details, including the time and location of fault, and the status and performance of the network elements (NEs). The operator must be able to order network statistics and reroute traffic and must have as much flexi- bility as possible in implementing changes to ensure optimal func