and isolate the data flow handling parts in physical nodes of their own. In addition, special physical nodes that take care of the relaying needs are introduced, as demonstrated in Fig. 2.11. 2 Foundations of Sysnet Modelling P(a2) P(b1) P(a1) P(TRt) P(TRt) P(TRr) P(TRt) P(b2) Fig. 2.11 A physical network with dedicated physical nodes The separation of logical nodes (A2->{a2, TRt}, etc.) means that interfaces between terminating physical nodes, P(TRt), and control information handling nodes, (P(a1), P(a2)), etc., logically become SAPs of the logical network that com- prises the TRt and TRr logical nodes. The network partition of P(TRt) and P(TRr) therefore constitute a physical solution to a communication service provider, which is an object that can be designed and operated as a separate physical network. It also defines the physical partitions P(a1)+P(a2) and P(b1)+P(b2) respectively as separate physical networks in the same sense. This is also what constitutes a a nat- ural sysnet. Network A Network B Transport network (TRN) Network A Network A Network B Network B Fig. 2.12 The sysnets Therefore we can say that the physical network of Fig. 2.11 comprises three dif- ferent sysnets: network A, network B and a transport network (TRN), as depicted in Fig. 2.12. Thus, we have three separate sysnets, although we had just one physical network. All sysnets are built by physical nodes, since they handle physical media connection endpoints (and may have internal physical media connections). If the SAP interfaces of the different sysnets are compatible (which normally implies 2.3 Architectural Principles standardized), the physical network can be built by interconnecting physical media connection endpoints of different sysnets to each other. However, that is part of the description of the concrete network, and therefore outside the context of sysnet modelling. Architectural Principles Sysnet modelling embraces a set of basic concepts and architectural principles used for modelling standardized and applied systems. Abstra