quires careful planning of the DCN because a fault on a transmission link may disturb management messages that are necessary for fault localization. Therefore, the DCN should be designed to be as independent as possible from the network that transmits user data. Introduction to Telecommunications Network Engineering Operations Operations Operations system system system Data communications network (DCN) station Transmission Transmission Exchange Exchange Exchange systems systems Telecommunications network Figure 2.22 DCN. Sometimes a network operator can physically separate management data from user data by using another independent network for management links. For example, the packet-switched X.25 network may be used for tele- phone network management. The use of another network may also be feasi- ble to implement redundant routes to DCN, that is, the management data are sent via another connection when the one in use fails. 2.11.4 TMN The overall management concept that ITU-T has defined is known as TMN. The standardization of TMN is aimed at covering all aspects to make the centralized O&M of telecommunications networks possible in a multiven- dor environment. The complete standardization of TMN is designed to cover the follow- ing specifications: Physical architecture of TMN: what systems are needed in TMN and how they are interconnected; Interface protocols: how network elements and management systems exchange information (the structure and types of messages); Management functions: what functions in the network elements the network management system should be able to access; Information model: for each different system in the network, how each manageable function (in detail) is described in management messages. The Telecommunications Network: An Overview The recommendations for the TMN concept, approved by ITU-T, define the physical architecture of TMN as shown in Figure 2.23. TMN is understood to be separate from the actual telecommunications network, though network systems have to provide the management in