, include the same functions, but may solve the distribution problem differently. For example, TDMA/136-based systems19 19 normally do not have a BSC. Suppose, for example that we in GSM decided to move the function in BSC that handles mobility control to the MSC, or to rearrange the distribution of functions between MSC, BSC and MS. This would certainly affect the structure of the logical network for mobility control (and the protocols), but the logical network structure for GSM would not be affected. The same could be said about TDMA/136 systems, TDMA/136 is the North American standard for cellular networks (corresponding to GSM), for- merly referred to as AMPS and D-AMPS. TDMA/136 is standardized by TIA/EIA. The access network interface (including among other things the service access protocol) is defined by the very extensive standard, TIA/EIA 136. The service distribution protocol is specified by the standard ANSI 41. 3 Logical Network Modelling if we decided to move mobility functions from the MSC to a TDMA/136-BSC (which is frequently discussed). The only kind of modification that can affect a logical network structure is when services are modified or added to the CSP. All kinds of changes in services of a sys- net will be reflected in specifications of network service interfaces in this model. Network service interface specifications should therefore be allocated the model of the logical network structure. A logical network structure is also a good starting point for understanding the network, and for allocating requirements for functional modifications of a sysnet. Figure 3.90 also indicates why SM&AML does not embrace the OSI RM layer- concept. A logical network structure defines a network-wide layered structure of logical networks. A logical network corresponds to the OSI RM layer concept, and the mapping of a logical network on a physical network defines to what extent the logical network is "network-wide". The ISDN logical network structure indicates (as do all other public network concepts, except O