work management varies according to the user counter reaches zero the message is either cleared or discarded. perspective. A network operator will have a different list of Life times are very similar. The message life time is set up when priorities to the network manager and to the managing director. In it enters the network and if it is not acknowledged within its lifetime OSI network management is being codified to give a common the originator sends another via a different route. platform SO that everyone plays by the same rules and uses the same terminology. Network management is described in Chapter 16. Hop counts and lifetimes can be dynamically adjusted to suit the traffic Quality of Service requirements and the real time network characteristics. 40.8.14 Different multiplexer types in a network 40.8.9 Configuration Figure 40.13 shows how time division multiplexers and packet switch exchanges may be intermixed in a network. The multiplexers Configuration of a network is normally proportional to its size, form a hierarchy from the LANs which are multiplexed on to although there are some aspects which seem to increase exponen- medium speed 64kbit/s circuits and then on to the high speed tially in complexity as a network grows. For large networks manual 2Mbit/s trunks. Not shown on the diagram is the distance factor, configuration becomes a major issue and in some cases out of the normally the higher speed trunks are geographically much further question. A widely distributed network can be a real headache in apart, up to hundreds of miles, than the lower speed circuits. terms of manpower to manage and support without the complication of configuring links. 40.8.15 Network expansion issues Configuration tends to fall into five categories: There are various reasons to expand a network: 1. Manual configuration using a locally attach terminal or push buttons. 1. Geographic changes. 2. Remote configuration via a virtual call. 2. More users. 3. Automatic configuration with remote configuration for fine 3. New faciliti