the need for "sublayers," by separating services in two main classes: connectivity services and control services, the former offered over SPs, the latter over some type of control points (...CPs). To support this distinction in models, AMLn introduces different graphical notations for con- nectivity service points and control points, as discussed in the previous chapter. A result of the AMLn approach is that models will exhibit two types of layers as well: connectivity layers and control layers. In layer-structure models the type of a layer becomes very obvious when the point symbols for these layer-interface types are used. Figure 2.7 compares the OSI RM layer concept with those for AMLn. Note the differences in AMLn model structures when a layer supports SPs and CPs respectively: A connectivity layer has a symmetric appearance in models, similar to an OSI RM layer, since there are two or more users who together use a service. The dif- ference is that the SP layer interface only comprises send and receive opera- tions while the OSI RM layer interface may also define operations for making the service accessible (by connectRequest operations, etc.). The protocol is characterized as a connectivity protocol. A control layer has an asymmetric appearance in models. A service is used by a single user over an CP. Other layer elements may not have any control points because they represent remote resources or control functions of some kind. The 2 Layer Structures operations of a control point can be anything but send and receive. The proto- cols are characterized as control protocols. Interlayer structures Intralayer structures Using-layer Using-layer Using layer element element OSI layer interface Message protocol OSI RM OSI layer OSI-layer OSI-layer element element OSI layer interface Used layer Used-layer Used-layer element element Using-layer Using-layer Using layer element element Connectivity protocol connectivity connectivity connectivity layer layer element layer element Used layer Used-layer Used-layer element el