te of an object interface as a resource model, and defining the mapping between resource models in different interfaces. If we assume that the master in Fig. 5.43 is an agent (i.e. a service object), the service elements C through F are also service objects. If we also assume that the external interface is a pure client-service interface, the internal structure can (on the abstract protocol level) be described as a substructure of a service object (a service network), in which the agent is the transaction-handling object, see Fig. 5.44. Objects that have only one interface defined are called virtual objects. The concepts agent and virtual object are influenced by the work of Booch (1983). Service networks and objects are described in Sect. 7.2. {io1,io2, } virtual objects {io3,. {eo1,eo2,.. } Agent {io4,io5, } Fig. 5.44 The abstraction of a protocol network on the service network level An obvious case for a protocol converter is when protocol elements on the same physical network level have to operate between two interfaces that define different protocols with similar purposes. Two forms of protocol conversion functions can 5 Protocol Network Modelling be identified: event relaying and message relaying. Figure 5.45 shows an example of event relaying. ANSI 41 event IS 136 events oldMSC chChanOrd(MSid) hoOrd(MIN) ET/EO ET/EO newMSC TCAP message IS 136 messages MT/MO MT/MOH association MT Message termination ET Event termination MO Message origination EO Event origination MSid MS identifier ER Event relaying Fig. 5.45 An event-relaying case The model in Fig. 5.45 contains three protocol elements that take part in the inter-MSC handover procedure in a TDMA/136 network: when the network decides that the MS shall select a traffic channel of a cell that belongs to another MSC (newMSC), the MS is still listening to the control channel of the oldMSC. The newMSC must therefore use the oldMSC to inform the MS which channel to turn to. Thus, a "change channel" order (chChanOrd(MSid)) is sent from newMSC to old- MSC, which tr