 The global SAPs between logical nodes belong to the C- dimension of network 0 and to the D-dimension of network 1. Note that this trans- formation can be repeated recursively any number of times. Theoretically this means: An entire operator network, as well as the smallest physical component of a sys- tem, can be regarded as a physical node. A node platform can be a single processor platform, as well as an internal trans- port network in an operator system (a distributed CORBA-platform, a CCS-net- work, in intra-IP network, etc.). Requirements that are expressed as parts of a high-level network interface can be traced down to the smallest physical components of a system. Since global SAPs and local SAPs are completely independent (i.e. defined sep- arately), the only way to keep them separate in models is by carefully managing the levels of the physical network. The effect of not doing SO is demonstrated by Fig. 2 Foundations of Sysnet Modelling 2.39. This model shows the same physical node as in Fig. 2.37, but redrawn SO that the internal C-dimension (which corresponds to the external P-dimension) is shown vertically. As an effect of removing the boundary of the outer physical node, the external C-dimension is no longer explicitly visible. po0/A SAPI=W pi0/F pi0/B SAPI=Z SAPI=Y SAPI=X pi0/E pi0/C pi1/A pi1/B local SAP Fig. 2.39 A model of a physical node that is not shown correctly in functional space This way of viewing a network structure is processor centric and is quite com- mon in network modelling, probably since it represents an implementor's (of phys- ical nodes) view on the world. From a network modelling point of view, it is very deceptive and should be avoided. It is deceptive because two mutually independent C-dimensions are mixed in the same model. It is then too easy to think that, e.g. the protocols denoted pi1/A, SAPI=X, pi0/B, pi0/A and the medium m1 belong to the same network interface; which they do not. Protocols pi0/A and pi0/B belong to net- work interfaces between this physical node and o