ayer', the this bandwidth with the normal data traffic. If, therefore, all man- functions which the management system needs to perform. agement instructions and operations have to be carried over these channels, the system can become quickly overloaded when a prob- lem arises. 16.3 Network management hierarchy The tree structure of Figure 16.3(b) provides a distributed man- agement hierarchy which overcomes this limitation. The element level (or plant level) manager carries out all the local servicing of A convenient hierarchical structuring of network management has alarms and disaster recovery, feeding back only major alerts to the been to consider the geographical location and span of control. For next higher level of manager. Data processing has been moved example the managers can be split into local, metropolitan, national closer to the network element that generates the data. This enables and international. At the local level, which is usually intrafacility, faster response to faults and also avoids the problem of a single speed of response and quality of service tend to be the most import- failure of the central manager bringing down the whole network. A ant considerations. Equipment being managed are limited in scope, distributed management structure, in which set functions are carried such as local area networks and modems. The greater the distances, out on separate machines, also makes it easier to design controllers the greater the likelihood that the service will be interfacility, and operating on the principles of knowledge based systems, which can now low cost and the importance of an uninterrupted flow of perform certain routine actions autonomously. information, become important. There is also a much greater variety The peer-to-peer structure shown in Figure 16.3(c) divides the of equipment involved, spanning the range from circuits, switches network into segments, which can then be controlled by separate but and services. interlinking managers. This structure is often used when resilience Figure 