s it integrates. We could have designed a number of other abstract network structures just by taking other aggregation decisions. Integration from service network to abstract machine network is not just a mat- ter of aggregation, but also of adding object access-handling functions (the service object OBH). In this case, OBHs are needed in order to provide access to non-per- sistent objects (objects drawn with dotted lines). Also note that, contrary to com- munication relations in service networks, communicating abstract machines can both provide primitive services to each other, i.e. they can be in a peer relation. The rationale for separating abstract network modelling from service network modelling is to separate the specification of primitive services and their solution structure (i.e. substructures of service objects) from how service objects are to be packaged and accessed in the process control system. Thus abstract networks are used to specify solutions to packaging and access issues, while service networks specify service solutions, disregarding those issues. 7 Concepts and Notations for Process Control Systems Modelling Service networks aggregate disintegrate integrate Abstract machine Abstract {OS,S6} {S1,S2,S3} protocol collapse {OS,S5} Abstract network OBH Object handler os OBH primitive service S1-S7 Primitive service identifiers (PSid) Fig. 7.62 An example of integrating three service networks in one abstract network (integration step 1) To separate service network modelling from abstract network modelling is very important, especially in telecom process control systems. The reason is primarily that the specification of a complex, distributed process control system is as much a matter of creating an effective packaging and access structure, as specifying serv- ice networks. Often the latter also represents well-known services that are offered in a new network context, using new methods for aggregating and accessing serv- ice logic (e.g. as when the IN concept and CORBA were introduced). The detailed