e behavioural aspects of net- work service interfaces was, however, never well-covered by OSI RM. OSI stand- ards have been specified that way, but only recently has the telecom industry started to adopt network service interface modelling, more or less based on the OSI technique. SM&AML takes OSI a step further by introducing operations and prim- itive services, and by describing behaviour in network service interfaces in a for- mal way (as abstract protocols). The concept vertical integration was recently invented in order to emphasize the difference between OSI RM and the present trend towards horizontal integra- tion. The OSI RM never described how to model a network service interface, if it is implemented in a network interface. The OSI RM paradigm assumed that net- work service interfaces need not be standardized. Network service interfaces were assumed to be either integrated in the software in an inaccessible way, or they were implemented as proprietary software or hardware interface solutions inside physi- cal nodes. With horizontal integration, which basically means that network service interfaces exist as parts of network interfaces and thereby of standardized proto- cols, neither of these assumptions are valid any longer. Figure 4.20 shows the implications of the transition from vertical to horizontal integration. 4.5 Implementing Network Service Interfaces Vertical integration Horizontal integration Physical node Physical node in network Y in network X Abstract Message protocols protocol Physical node in network Z Physical network level 0 Physical network level 1 X.1 1 {PR2} {PR1,PR2} {PR1} Fig. 4.20 The transformation of a network service interface There are two important observations to be made regarding Fig. 4.20: 1. Assuming that the network service interface is specified in some way, it must be transformed into a message protocol (or part of a message protocol) by adding functions that can handle messages. These types of functions are called message handlers (MEH). If messages are designed as ca