sage types must therefore be enhanced with an originating Fid (origFid). Since the uplink message is not aimed for the master, the uplink message format must be enhanced with a destination Fid (destFid) parameter as well. In the uplink message for the master, the destFid parameter has no value. If the master receives an uplink message with an origFid, it will just discard it (unless the master is also used for allocating service elements). Thus, a relaying chain can be used for allocating two types of service elements: such elements that are used by a master due to events in the external interface (like C, D, E and F in the previous models) and service elements that are unknown to the master. A relay chain often represents part of a (temporary) network transaction. In that case, each element in the chain occupies some resources that should be made avail- able to other transactions as soon as possible. Therefore it is important that only those parts of the chain that are needed for a particular transaction are established, and that the chain is released when the transaction is finished. The establishment of a relay chain is discussed through the scenario in Fig. 5.42. event initiating event Master Level 2 int. (d,SE23,evData} {d,SE23,evData} and relaying nd(SE11) send(SE12) ass.prot. ass.prot. Level 1 integrator {ai1,messData} Y SAPI (ai1,messData) Z SAPI Fig. 5.42 Association handling in a relay chain The model in Fig. 5.42 shows three logical nodes that comprise four discrimina- tion level 1 service elements (SE10 through SE12, SE14). SE10 through SE12 are parts of a master-slave relay chain in which SE10 is the master (there might exist more elements in the chain, connected downlink of SE12). At the transac- tion start, the master is the only element that exists on the chain. 5 Protocol Network Modelling The initiating event for the scenario is an external event that is detected by the master. The first internal event depends on what the external event is about. Here we have assumed that the master starts the t