cation relations are given a unique graphical symbol on each level. For reasons discussed in Sect. 1.6.4, the latter is chosen. Figure 2.44 gives an overview of the notations that are used on different levels of the integration hierarchy. Figure 2.46 summarizes the names of the five model frag- ment types (generally regarded as "networks"), the names of units and of the com- munication relations. Integration Model fragment Communication Disintegration Integration levels Unit names type names relation names steps steps Service network Service object Service relation Abstract network Abstract machine Abstract protocol D: Message protocol Protocol network Protocol element C: Connection endpoint, association endpoint Logical network Logical node D: Message protocol C: Service access point D: Network interface Physical network Physical node C: Physical media connection endpoint Fig. 2.46 Concepts of the integration hierarchy Integration levels 4 (physical network) and 3 (logical network) define the static structure of a sysnet in the DC-plane of functional space. For physical networks, this structure is defined by network interfaces in the D-dimension and by physical media connection endpoints in the C-dimension. Logical networks are defined by 2.3 Architectural Principles message protocols in the D-dimension and by service access points (SAP) in the C- dimension. A sysnet structure on levels 4 and 3 is reasonably stable. Specifications of net- work interfaces and message protocols are, however, affected by functional modifi- cations of the sysnet, often caused by modified and new requirements on services (specified on levels 0-1) for the users of the sysnet. The service network level is regarded as integration level 0 to indicate that it is the only level that cannot be fur- ther disintegrated within the context of a sysnet. This implies that service objects are the smallest type of units in a sysnet model that can be mapped on components of implemented systems. Communication within a service network is distribution 