is an unusual feature for a process control system. Some comments about the notation may be in place as well: if you compare the model in Fig. 3.87 with Fig. 3.39, you see that I have not drawn all network service 3.5 Logical Network Structures interfaces in the same direction. When you draw models "the OSI RM way", there are no symbols to rely on for network service interfaces and protocol relations. It therefore a convention, without exception, that network service interfaces are drawn vertically and protocol relations horizontally. This convention is a prohibit- ing factor for showing several physical network levels in the same model. If you recall the discussion in Sect. 2.3.6, you know that between adjacent physical net- work levels, the C-dimension on the upper level becomes mapped on the D-dimen- sion of the lower. Thus, to use the horizontal and vertical directions as indications of dimensions in functional space just does not hold. In AML, network service interfaces and protocol relations have unique symbols. Furthermore, symbols that denote a user-provider relation between parties are asymmetric, which means that you can rotate interface symbols any way you like, without risk of misinterpretation. This advantageous feature of the notation allows you to draw more compact models. We can extract other parts of the model of Fig. 3.39 in a similar way and define them as logical networks of the same CSP. The part of the resource system that pro- vides CN connections would be a natural logical network, see Fig. 3.88. Character- istic of a logical network that is part of a resource system is that it provides a number of SAPs to users, and a number of CSIs to its process control system. Q.920-921 -190 kb/s 64K CS DU D-channel user CNU B-channel user PCS process control system Fig. 3.88 The logical network model for the part of the ISDN resource system that provides CN connections and D-channels We could continue defining logical networks from Fig. 3.39 in the same way. We then get one logical network for the tel