ther physical nodes, while pi1/A, SAPI=X and m1 belong to an internal network interface between two physical nodes in the substructure of this physical node. There is also nothing in the way the model is drawn to indicate what a protocol represents: A global SAP, a local SAP, an inter-processor protocol, an internal pro- tocol of a logical network or a protocol that supports partitioning of a logical nodes inside a physical node (as between A and D). Since both network designers and physical node designers have the right to place their view in focus, it is important to separate models drawn in the P-dimen- sion from models drawn in the D- and C-dimensions. The physical node designer's view of the original physical node would then appear as in the upper part of Fig. 2.40; the network designer's view is shown below. The logical dimensions are indexed after the physical network level: "0" is the higher level, where the node that is modelled is a single physical node. The lower level is "1", i.e. the substruc- ture of that physical node that is defined by the structure of platforms. The upper model shows applications that implement logical nodes and applica- tion protocols that, with one exception, implement network service interfaces. The exception is the message protocol between A and D in the upper model that imple- ments an abstract protocol between A and D in the lower model. Applications are parts (protocol elements) of logical nodes on physical network level 1 that use local SAPs of the different platforms in the physical node. 2.3 Architectural Principles Applications pi1/{pio/AD} pi1/{Y} pi1/{X} pi1/{Z} pi1/{W} pi1/A pi1/B local SAP, connecting a logical node on physical network level 1 Logical nodes global SAP, connecting a logical node on physical network level 0 pi0/F niO/A ni0/B SAPI=W ni0/C pi0/A pi0/AD piO/D SAPI=Z pi0/B SAPI=Y SAPI=X pi0/C pi0/E Fig. 2.40 The two main views of a physical node. A through E are names of logical nodes, X through W are names of global SAPs. {...} is a reference All extern