 form. The platforms are interconnected by (internal) physical media (m1 and m2) and communicate by inter-processor protocols (pi1/A and pi1/B). ni0/C pi0/F ni0/A ni0/B SAPI=W pi0/A piO/D SAPI=Z pi0/B pi1/A pi1/B SAPI=X SAPI=Y pi0/C pi0/E expand collapse P and C Pand C ni0/C pi0/F pi0/A pi0/B pi0/D ni0/A ni0/B pi0/C SAPI=X pi0/E SAPI=Y pi1/A pi1/B ni network interface PL platform pi0/A pr protocol A-F logical nodes function identifier m,M medium X-Z SAPIs physical network level protocol identifier Fig. 2.37 Global SAPs on a higher physical network level may exist in network interfaces on a lower physical network level As a result, the internal structure of the original physical node (which belongs to a higher physical network level) becomes a physical network on a lower level (a 2.3 Architectural Principles substructure). SAPs that were classified as local SAPs on the higher level will now appear as global SAPs of the internal physical network, while some of the global SAPs on the higher level (SAPI=X and Y) will appear in network interfaces of the lower level. Thus, local SAPs that are defined as part of a platform interface in a physical node become global SAPs of the (internal) C-dimension of the internal physical network. A general conclusion of this exercise is that each level of physical network has its own definition of the C-dimension (and the D- and P-dimensions, for that mat- ter). The principle for mapping logical dimensions between physical network levels is shown in Fig. 2.38. A physical node on network level 1 global A physical node local on network level 0 network interfaces Fig. 2.38 Transformation of logical dimensions between physical network levels Figure 2.38 shows a physical node on physical network level 0 and indicates the existence of an internal structure in that node, which is a physical network on level 1. The figure indicates that the local SAPs offered by the node platform to the logi- cal nodes of the NNF are parts of the P-dimension of network 0 and of the C- dimension of network 1.