hanisms are integrated in a single protocol (the GTP). To be able to separate user-data flows from control flows, one must delve into the GTP proto- col specification, where the flows are separated by message type identifications (e.g. message type 255 for user data). Thus, although it is a very ambitious concept, the GPRS network architecture, like many other architectures, is open for considerable improvements. Implementing Network Service Interfaces 4.5.1 The Network Service Interface Transform This transform is an example of interface refinement (see Sect. 2.3.3), applied on network service interfaces. In the early days of telecommunications, all functionality of physical nodes (resource system, process control system, management system- and node platform functions) was regarded as a single logical node, cooperating with logical nodes in other physical nodes over a single protocol, as depicted in Fig. 4.18. Complete integration: Physical node Logical Fig. 4.18 A physical node without any logical structure When switching systems became computer controlled, the functional complex- ity of these systems increased. With the increasing complexity of telecom systems in general and datacom systems in specific, the OSI RM was developed. The OSI 4 Physical Network Modelling RM introduced the layer concept (which roughly corresponds to the logical net- work concept that is defined in SM&AML) and the service interface (which roughly corresponds the network service interface in SM&AML), see Fig. 4.19. \Vertical integration: Physical node Logical network or layer A network service interface. It is either not specified at all, or specified by OSI service primitives, or (in SM&AML) as an abstract protocol. Fig. 4.19 Vertical integration A layered approach to describing functions in a structured way certainly was not invented by OSI, but the OSI RM was first to enforce modelling of network service interfaces by introducing service primitives, a service concept and a data-flow model (the IDU) for network service interfaces. Th