e QoS properties of the high-level connection used by x11 and x21 are defined by the SAPs that are provided by x12 and x22. These properties depend, in turn, on the properties of the low-level connection that is used to interconnect x12 and x22. The interfaces i1 and i4 of the two systems can now be further transformed separately to create substructures for the units x11 through x22. 2.3.6 Functional Space The concept functional space is introduced as a means for modelling the complete structure of physical and logical networks of a sysnet. Functional space (also called the DCP-space) is defined by three logical dimensions: distribution dimension 2 Foundations of Sysnet Modelling (also called the D-dimension), communication dimension (the C-dimension) and processing dimension (the P-dimension). By applying message discrimination on the externally visible functionality of a physical node, a structure of logical nodes is identified inside each physical node. Interfaces between these logical nodes are called network service interfaces, and must include at least one service access point (a global SAP). Global SAPs can be referred to on the network level, which means that a logical node in one physical node can address a logical node in another physical node by referring to its global service access point identifier (SAPI). We say that a discrimination structure exists in the communication dimension, as is illustrated in Fig.2.29. Communication dimension (C) physical node messages (protocol) logical node network service interfaces (global SAPs) Fig. 2.29 A discrimination structure defines the communication dimension Each logical node communicates with logical nodes in other physical nodes. A collaboration structure of logical nodes that can communicate over the physical network is called a logical network. We say that such a structure exists in the distri- bution dimension (D), characterized by network interfaces, as shown in Fig. 2.30. logical network logical node physical node network network interface interface Dis