faces, as shown at the bottom of Fig. 3.92. This is an example of interface refinement. To refine a network service interface that is described by an abstract protocol to a message protocol requires that message-handling func- tions (MEH in Fig. 3.92) are added and that an extra CSP is made available for transporting messages. This subject is dealt with in Sect. 5.4. 3 Logical Network Modelling The importance of this discussion is that existing logical networks are not affected by whether the physical network will be vertically or horizontally inte- grated. It is therefore worthwhile to repeat what has been said before: logical network models are stable models of network functions, physical network mod- els are not. From a protocol handling point of view, the difference between vertical and hor- izontal integration is that, in the former case, physical nodes handle many proto- cols that represent functional distribution. In the latter case, they handle one or a few protocols of that kind, but must also handle protocols that represent service specialization. From a functional point of view, physical nodes will still be less complex, although they will have to handle about the same number of protocols. Since the distinction between network service interfaces and service distribution within a logical network is crucial for the network architecture, describing the abstract protocol part of message protocols is, also. Traditionally, a number of functions that could be handled in a common way for fixed-access and mobile-access networks have been defined differently. This emphasizes the importance of identifying differences, as well as similarities, between network types. That is why the only function (Mobility Process Control) that distinguishes cellular networks from fixed networks is included as a separate logical network in the generic model of Fig. 3.91. Call-connection separation is represented in Fig. 3.91 by the separation between the Connectivity System and the Teleservice System. This separation considers the 