ivity domains ATMU and AAL1U. The model in the middle of Fig. 3.85 abstracts the resources that are used below the AAL1 level to an ATM connection. This connection must be identified in the model by defining the type of CSP (here ATM) and, in simulations of a model, also by values of its VPI and VCI. The transport characteristics of the connec- tion are associated with the ATM-SAP, and are automatically inherited since it is identified as an ATM-connection. The model at the bottom of Fig. 3.85 abstracts the resources that are used below the teleservice level to an AAL1 connection. This connection is identified only by its type (AAL1). The transport characteristics of the connection are associ- ated with the AAL1-SAP and are automatically inherited by the connection, by identifying it as an AAL1 connection. The exemplified connections were both two-party connections. The CSP that is used may also support the connection structures point-to-multipoint connection and multiple access connection, see Fig. 3.86. Point-to-multipoint connection (YYY): XX-UDP XX-UDP Multiple access connection (ZZZ): XX-UDP XX-UDP XX-UDP Fig. 3.86 Connections with more than two endpoints In both cases, messages of the protocol XX-UDP must include a parameter that identifies the sending party. In the point-to-multipoint case, only the multipoint units must identify themselves in XX-PDUs. In the multiple access case, all units must identify themselves in XX-PDUs. 3.5 Logical Network Structures 3.5.3 Units in Process Control System Modelling, Summary The primitive units we have used SO far for modelling process control systems can be divided into protocol handling units and units that act as servers for other type of units. We have encountered three types of protocol handling units: Terminal process controller, TPC, exist in every terminal system. If the terminal system provides a CSI to service users, then the TPC unit communicates with a DNPC according to a protocol with the generic name service access protocol. Distributed Network Process C