eservice resource part of the ISDN, one for the LAPD parts, one for the CCS parts and one for transport links in the core network, in all six logical networks. Let's now turn to the structure of logical networks in a physical network. Any logical network can be collapsed to a single unit, representing an external view of the logical network. The effect of collapsing a logical network is that all protocols that are defined by the logical network become hidden. Only the SAPs and the CSIs of the C-dimension will be visible on the boundary of the collapsed logical network. From a notational point of view, the difference between a logical node 3 Logical Network Modelling and a collapsed logical network is that the former exists in the DC-plane (i.e. it handles protocols), the latter exists only in the C-dimension (i.e. it has no protocol relation to other logical networks). Figure 3.89 shows the collapsed logical net- work for two of the ISDN logical networks. 64K CS ISDN Process 64K CS 64K CS service Control System ISDN Connection switch logical network Resource System Signalling devices logical network SAPIO SAPI63 Teleservice Transport links resources Fig. 3.89 Collapsed logical networks of the ISDN model A collapsed logical network model only shows the type of network service interfaces that are supported. To define individual interfaces is part of the formal notation for logical networks, see Sect. 3.5.5. Collapsed logical networks allow you to describe the complete functionality of a CSP as a structure of logical networks and CSPs that in cooperation provide the connectivity service of the CSP. Figure 3.90 shows the logical network structure of ISDN, based on previous information in this chapter. The model should be further annotated by the type of service that is offered in each SAP and CSI, but this is left to the reader. Note that all SAPs and CSIs are types, not instances. The Transport CSP is drawn with dotted lines to indicate that this CSP is not specified within the ISDN concept. The CCS logical network 