s in the gen- eral packet data service (GPRS) of the GSM, accounting based on the amount of transferred data is implemented. Public networks contain many different technologies and the opera- tor's organization is usually divided into different responsibility areas, such as transmission, telephone exchanges, leased-line data networks, and packet- switched data services. Today these organizations usually have their own dedicated and incompatible network management systems, probably with some kind of geographical hierarchy, and the integration of these is an important issue for the future. At least some level of integration is needed because, for example, all services usually use the same transmission network. To solve this problem, ITU-T has defined a common management concept that is known as the telecommunications management network (TMN). In the following section we describe the data communications network (DCN), which belongs to the TMN concept and is responsible for the transmission of management data. 2.11.3 DCN Not only different networks, but even network elements (equipment), may have their own O&M systems that may be incompatible today. As a conse- quence, if a fault occurs in the network, the network operator's personnel may have to use several different O&M systems for fault localization. ITU-T has worked a long time to define a vendor-independent network manage- ment concept. It is called TMN. In ITU-T's TMN concept, the transmission of management data between management workstations and network elements is separated from the transmission of user data as shown in Figure 2.22. The transportation network of management data is called the DCN. Even though DCN is supposed to be a logically separate network from the actual telecommunications network, the management messages often use the same network as the actual telecommunications services. Most transmis- sion systems, for example, synchronous digital hierarchy (SDH) as described in Chapter 4, provide data channels for network management purposes. This re