communication services to all physical nodes in the core network that have process control functions. In ISDN this plat- form consists of a CCS system. Thus, the core network can be regarded as a physical node, where the node platform for process control is a CCS system. Common Object Request Broker Architecture, CORBA, is a reference architecture for plat- forms that support distributed object communication. The core of this architecture is the object bus, ORB, that allows objects to refer other objects, both locally and remote. CORBA is stan- dardized by the Object Management Group, OMG. 4.3 Physical Network Structures Another network concept, GPRS (packet network access over cellular networks, see Sect. 4.4), is moving towards a single node platform that supports GPRS nodes both in the access- and the core network. This node platform is an internal IP network, common for both the process control system and the resource sys- A single switching node can also be built by a number of internal physical nodes that use an internal wide area or local area network for communication inside the node. CORBA supports this type of architecture. On the lowest level of a physical network structure, node platforms first appear that are based on a single processor, or on shared multiprocessor systems. The model in Fig. 4.6 can be simplified to a physical substructure model, a sub- network, by defining internal network interfaces (ni1 through ni3 in Fig. 4.7) and by giving internal physical nodes local names (here x1 through x3). In this figure, a possible substructure solution to Y is also included. Note that the solution for Y dif- fers from the solution for X. {Q.931} {Q.931,X.25} {LAPB,IP} {1.411,LAPD,M} {1.411,LAPD,M} {LAPB,IP} {X.25} Fig. 4.7 Step 4: identify internal physical nodes and network interfaces An important conclusion from Fig. 4.7 is that there are two views on a network interface: on the external side of the physical node, the interface is defined by the discrimination structure of protocols, defined through t