vice Service element element element Fig. 5.3 Interconnecting protocol networks Depending on the properties that are required by service elements, an integrator may comprise some or all of the following functions: Discrimination: a function that can address different service elements in a logi- cal node. Connection or association or handling: a function that can establish temporary relations (connections or associations) between service elements in different logical nodes by a negotiation procedure. Association handling manages service elements of different types, as well as service elements of the same type. Con- nection handling only manages service elements of the same type. Message handling: a function that can support different types of data transpar- ency between service elements in different logical nodes. Message handling may comprise all or some of the following functions: format handling (FOH), encoding (ENC) and transport service adaption handling (TAH). Depending on how many of these functions are performed, integrators can provide three levels of data transparency to the communication between ele- ments, represented by three types of protocols: abstract protocol, OSI applica- tion layer protocol and message protocols. Relaying: a function that can relay incoming messages or events that cannot be terminated in the actual logical node to other logical nodes in the same logical network. Relaying requires connection or association handling. Instantiation: a function that can instantiate and delete temporarily existing service elements and connect them to the integrator. How instantiation is imple- 5 Protocol Network Modelling mented in a logical node is specified in the P-dimension, i.e. is not visible in the DC-plane. There are basically two reasons why logical networks should be partitioned into protocol networks: Interfaces between logical nodes are SAPs. This makes logical nodes permanent configuration elements in a network. It also sets a limit to how much the func- tions of a physical network can b