ging and the traffic system (mpCP) and inside the managing system only or between cooperating management systems (msCP). The same symbol is used in all cases, however (see Chap. 5 about modeling management functions and systems). Note also that management control points are associated with layer models only. When we model management functionality, we reduce the management layer to its layer state machine (LSM) representation (as we do with all types of control layers). LSMs are modeled by the abstract-machine notation that is described in Muth (2001). A short version is given in Appendix D. The distinction between service points ( SP) and control points (...CP) is based on the observation that many layers are of the kind that they provide one or the other. There is, however, no rule that says that they must. The designer of a model may define a layer that offers a whole range of different services over a single layer interface (although we recommend avoiding that if possible). In such cases, symbols for SPs and CPs may be combined by the notation shown at the bottom of Fig. 2.2 for TCP. Figure 2.3 gives an example of the use of some of these symbols in an inter- layer structure (which is a model in the L dimension only). It also shows how multiplicity is denoted in a layer structures. The exclusive_or symbol denotes that a LAPD transport layer runs on either a BRA or a PRA layer. LAPD users layer Management control points (mrCP) management Asynchronous interfaces (aSP) 0..62 LAPD transport Symbol for multiple layers of the same type Service Synchronous providing interfaces (sSP) Exclusive_or symbol layers BRA PRA Fig. 2.3 An example of how to use symbols to define types of layer interface. L1,L2=layers 2 Layer Structures In the connection-oriented OSI RM, all layers except the "application layer" are regarded as layers that offer some kind of "connection" (the abstract primitives connectRequest and dataRequest are used on all the lower six layers). It is there- fore important to remember that the AMLn layer defin