e refined by defining a logical network struc- ture. Often, the whole process control system becomes a single logical network. Logical networks and nodes therefore tend to be functionally complex. To parti- tion logical networks into protocol networks creates network functions that are functionally simpler, that may execute more or less independently from other protocol networks and that have user-provide type of CSI interfaces to other protocol networks. Another common use of concurrently executing protocol networks in a logical network is to support multiple, identical network transactions. Provided that the logical network is functionally simple and that transactions are of some length, it might be worthwhile to instantiate individual service elements for each trans- action. This type of logical network normally exists only inside the physical nodes of a network, e.g. for handling individual calls. An Example: The GSM Process Control System 5.2.1 The Logical Network The rationale for functional partitioning of logical networks into protocol net- works can be exemplified by GSM. Figure 5.4 shows the logical network for the GSM process control system (GSM-PCS), as it is defined by the GSM standard (data communication services are not covered by this model, see the GPRS case study in Sect. 4.4 instead). The model also shows two typical physical node config- uration solutions. Note, however, that each physical node can also be confined to a single logical node. A reference point defines a protocol between logical nodes (annotated in the model in Fig. 5.4) and a route that is used in some transport network. There are a number of transport networks used for routes in the GSM-PCS. The only route that is explicitly shown in the model is the LAPDm route between BTS and MS, which is the route over the radio interface. This route (linked together with routes in the reference points BTS-BSC and BSC-MSC) is used to support the protocols that are defined for the reference points between BSC and MS, and between MSC and MS. M