ts (switches, trans- mission equipment, etc.), together with instructions of how to interconnect them and how to configurate, operate and maintain the network. In that case, the sup- plier leaves to the operator to design the network, to connect it to other networks as well as to handle it in all respects. With the advent of an increasing number of operators not having the necessary skills in network design, traditional suppliers tend to also deliver "total solu- tions", i.e. they design the particular network, they build it, connect it to other networks and they train the operator's staff in network operation and mainte- nance. Suppliers of today may even take the network operation and maintenance role as a service to operators, who mainly deal with the business aspects of providing network services. This insight tells that we must discuss the needs for modeling languages and tools in the context of the overall network system development process. Figure 1.5 shows the main features of this process. Network services users Network operation Network design Common- The network system Developing the value added-value supplier view Model development development translation processes process Analyzing Developing the "market" needs standard view Fig. 1.5 A flow-chart model of the network system development process 1 Introduction to Network System Modeling Added-value development produces generic network solutions (such as PSTN/ ISDN, GSM, the Internet architecture) from which particular operator networks are designed, produced and operated. The development process has two outstanding features, symbolized by interfaces A and B in the model: The cooperative nature of networks and network products implies that the added-value development process is divided in two distinct parts: one process that provides a standard view of a network system (in interface A), and other processes that, for each supplier, provide a supplier view, i.e., a solution to the standard view. A supplier view includes parts of other systems, as well as 