terfaces and management functions that they are able to perform. The physical architec- ture of TMN (Figure 2.23) contains these elements: Operations system (OS) for centralized network management; Data communications network for management data transfer; Mediation devices (MD) to adapt proprietary management interfaces to Q3 interfaces under standardization; Management functions integrated in the network elements (NEs) of the telecommunications network. Management areas that TMN takes care of are called FCAPS func- tions, as listed next. The management system performs or is used to perform following actions: Fault management: collects alarm information and takes corrective action; detects a system malfunction and carries out measurements to locate the fault. To other operations systems os = operations system DCN = data communications network MD = mediation device NE = network element Figure 2.23 Physical architecture of the TMN. Introduction to Telecommunications Network Engineering Configuration management: changes the configuration of network elements, for example, disconnects a subscriber who has not paid a bill. Accounting: sets accounting functions in network elements. Performance: measures performance of the network to detect faults and bottlenecks in advance. Security: detects security threats, for example, collects data about users of a corporate network that frequently provide wrong security codes to detect hackers. The most important and most difficult standardization issue has been the specification of the highest layer of the management interface, Q3. Lower-level protocols, like the physical network that carries actual data and formats messages, are already standardized, but detailed information models are not. The specification work of information models is an endless task, because new systems require their own models and an update to a system often requires a revision of the information model. The information model defines the managed objects (manageable resources) of a system and their relationshi