al protocols and the external physical media connection endpoints are omitted from the upper model, since the platforms do not know what roles appli- cations play on physical network level 0. In order to support the mapping between physical network level 0 and 1, references to logical nodes, network serv- ice interfaces and abstract protocols that are defined on level 0 must be provided on level 1, as shown in Fig. 2.40. There are many network interfaces, platform interfaces and network service interfaces in a physical network. We can therefore conceptually visualize the three dimensions as a functional space where all units of a physical network exist, and where the relations between those units are defined, see Fig. 2.41. Functional space defines structural aspects that are reasonably independent of each other. These aspects have good orthogonal qualities. For example, for a logical network that is described in functional space, you can add or modify messages in a network inter- face without any consequences on the services of a platform service interface. In other words, modifying the view of the network in the D-dimension does not affect the view of the network in the P-dimension (and also not necessarily in the C- dimension). 2 Foundations of Sysnet Modelling network service interfaces platform service interfaces, including global SAPs including local SAPs network interfaces Fig. 2.41 The logical node structure of a physical network in functional space We could have chosen another set of dimensions as the basic architectural struc- ture for sysnet modelling, but it is difficult to find another set that has such good orthogonal qualities for distributed systems in general, and telecom networks and systems in specific. Functional space is defined to support the description of structures and behav- iour of the units of a physical network. However, functional space cannot be used for describing how properties of components that have inputs and outputs other than information and data can be described (as e.g. the