ociation identifier ASH Association handler SAP symbol CNL-CSP substructure symbol Fig. 2.45 The generic model for association handling in logical nodes To specify a sysnet in terms of model fragments in the integration hierarchy implies that models on different levels can be developed and modified more or less independently. It also provides for a high degree of reusability of model frag- ments. For example, an abstract network may, in one logical network implementa- 2 Foundations of Sysnet Modelling tion, depend on a connection-oriented transport service with restricted message size. Later, IP is introduced as a general transport service. This affects the message protocol of the protocol network that implements the abstract network, and the association mechanism of the logical network that handles the protocol network. But, it does not necessarily affect any abstract network models. Thus, if the integra- tion hierarchy is also made visible in the implementation of the network, the inte- gration mechanism of protocol networks and the logical networks may have to be redesigned because of a new type of transport service, but the abstract network models can be reused without modifications. The integration hierarchy defines five model fragment types. Most of a sysnet's functionality is defined in terms of service networks. Integration mechanisms are specified as the functions of lower levels, and configuration structures are defined by integrates relations. A complex sysnet model may comprise hundreds of model fragments over the five levels. Most fragments will be of type service networks, a few fragments will be of type physical networks. Designers normally work only on one or a few integration levels, but they will depend on information provided by models on other levels, as context to their own problem, as background to their models and as requirement specifications. Therefore, it is important that each model type is identified through its graphical notation. There are two options: either the units or the communi