considerable length of time, a sysnet model will be fragmented in some way into a large number of small information elements, which we will call (sysnet) model fragments. To keep the sysnet model consistent over time, with respect to all those fragments and their interrelations, is the primary goal for model management. Figure 1.12 summarizes fragment types and fragment relations (refines relations excluded) that may exist in a sysnet model. substructure / ogical Network integrates Logical Network Attr. comm. relation spec. Physical Network Physical Network Physical Network Protocol Network Protocol Network physical sysnet Protocol Network Attr. model Protocol Network fragment spec. network interface Abstract Network attribute Abstract Network Abstract Network Attr. Abstract Network Attr. spec. spec. Service Network integrates attribute Service Network Service Network Attr. Service Network spec. Service Network Service Network substructure Service Network Service Network comm. relation Fig. 1.12 Model fragments and relations in a sysnet model This structure is created by applying one of the major architectural principles of SM (see Sect. 2.3.7 for an explanation): integration hierarchy. By applying this principle, a sysnet model becomes fragmented in a large set of fragments: There are basically two classes of fragments: one class that shows a collabora- tion structure, and another that defines some property of some entity in a collab- oration structure. The former is defined as a kind of "network", the latter as an attribute specification. The five types of networks that are defined by the integration hierarchy identify five levels of system abstractions: service network (most abstracted), abstract 1.5 System Models network, protocol network, logical network, physical network (least abstracted). A sysnet model can comprise a number of model fragments on each level. There is normally one physical network fragment that acts as an entry point for all other model fragments of a sysnet model. Attribute specifications