ontroller, DNPC, exists in all CSPs. A DNPC unit may communicate with TPCs according to a service access protocol. It communicates with other DNCPs according to a service distribution protocol. We have also seen one example of a management unit, the ICMP handler in IP networks. There are, of course, many more in a complete model, since OM func- tionality is a major part of the process control system in CSPs. Four types of server units have been identified: Diverse resource controllers: most frequently the switch controller (SWC) has appeared in our models, but there are many more. Resource controllers are a kind of agent, that act on behalf of other functions in the process control system to affect the state of units in the resource system. Routing Analyser, R:. every CSP that provides a switched service must include an RA. In CNL-CSP, RAs are used by the resource system. In all other types of CSPs, it is used by the process control system. Address Translator, AT: an AT is used by the switches of the resource system. ATs are needed only in CSPs that base their services on externally switched CSPs for transport (as, e.g. the IP-CSP). Route Manager, RM: an RM manages links in routes and channels in links. RMs are needed only in CSPs that base their services on multiple transport links between switches, or on transport links that contain multiple channels (as, e.g. the transport links in ISDN, and the VPs in ATM). The only composed unit we have encountered is the process control system, which we partitioned into TPCs and a network process controller (NPC). The NPC is a composed unit that we partitioned into DNCPs, resource controllers and RAs. Other composed units are indicated in the next section by the generic logical net- work structure that is presented (see Fig. 3.91). There are no abstract units to define for the Process Control System. 3.5.4 Logical Network Structures Let's repeat what we know about the logical networks of CSPs: basically, every logical system that exists in the DC-plane uses SAPs. It may als