 and provides temporary connectivity over SAPs. Relaying within a protocol network is (by definition) a permanent function, and does not require rout- ing analysis and switching to be performed. Basically, a relaying protocol element uses only two associations: whatever comes in over one of the associations is either terminated or relayed over the other. Two purposes for including relaying function- ality in a protocol network can be identified: As a result of introducing physical network levels in a sysnet, not only logical networks, but also protocol networks become fragmented according to physical network levels. Physical network level 0 and 1 are normally defined by stand- ards. The allocation of logical nodes, and thereby also of protocol elements, is not distribution transparent on these levels. Levels 2 and below are, however, 5 Protocol Network Modelling normally proprietary levels, which implies that they have architectures that are designed by the manufacturers of level 1 physical nodes. Protocol elements that are identified on level 1 are partitioned into level 2 protocol networks. In order to support flexible functional evolution within level 1 physical nodes, manufac- turers may create a distribution-transparent communication structure for level 2 protocol networks. One way is to implement a node-internal CSP (as a service of the node platform) that allows internal protocol elements to be addressed by names. for example, protocol elements could be designed as objects and run on a central or distributed CORBA platform. Another method, which is more flexible as regards functional modifications (and that does not require CSP functional- ity), is to allocate protocol elements of the same protocol network in a relaying chain. This techniques is described in Sect. 5.6.2. On physical network level 1, a number of standardized protocols exist. Some protocol elements on this level may not act on events, but just translate events between different protocols. This is a form of protocol conversion. An example fro