is also drawn with dotted lines, which is a more speculative choice. When ISDN was specified, CCS was regarded as an integrated part of ISDN. It has, however, become obvious that the MTP and SCCP parts can be regarded as an independent packet service network that could be operated by separate network operators or could be substituted by another packet service CSP, such as TCP/IP. A logical network structure represents a distribution-free representation of a CSP, i.e. a pure C-model. The definition of logical networks reduces protocols to mechanisms for implementing service distribution, not for specifying services. Since a logical network structure defines no protocols (they are all hidden inside the logical networks), it is necessary that logical nodes outside the operator net- work of concern (i.e. logical nodes in terminal systems) are included in a logical network definition. Otherwise, access interfaces would not become part of any logical network definition. 3.5 Logical Network Structures other user-user services messages Process Control System logical network {RA,RM,AT,SWC} SAPIO SAPI63 logical network logical network Teleservice Resource System Connection Resource System logical network logical network Transport CSP (PDH, SDH, etc.) operator The ISDN-CSP Fig. 3.90 The logical network structure of the ISDN-CSP What are the benefits of a logical network structure? First, a logical network structure is valid for different solutions to how functions are distributed over phys- ical nodes. Let's take GSM as an example. In GSM, one can identify a part of the process control system as the mobility control system, handling functions for keep- ing a moving mobile connected to the network (locating), for keeping track of where the mobile is (registration) and for calling mobiles (paging). This logical network has logical nodes in three types of physical nodes: the Mobile Systems (MS), the Base Station Controllers (BSC) and the Mobile Switching Centres (MSC). All cellular network concepts must, in some way or another