e exists, however, many other layers that define a mix of several types of services, such as TCP that defines both a control and a connectivity service, or the SCCP layer in the Signaling Sys- tem No. 7 (SS7) that, besides a control service, offers a whole range of different connectivity services, including several methods for addressing. The fact that a layer interface (from a service point of view) can comprise almost anything makes models of layered systems less valuable, unless layer inter- faces are not drawn with some kind of service-type indicating symbol. For exam- ple, an important distinction must be made between layer interfaces that offer We call such an expression not just "primitive" as in most languages but "abstract service prim- itive" in order to emphasize that, in implementation models, these primitives must be translated to the actual design language used. 4 The Domain Name System in the Internet architecture, which translates application system names to TCP/UDP "ports" and IP addresses. 2 Layer Structures connectivity services and control services (e.g., for distinguishing the parts that ISDN allocates to its user and control planes). In AMLn we deal with this problem as follows: since a layer interface can in reality comprise any type and mix of serv- ices, we define a small set of symbols for connectivity service points (CSP) and control points (CP). These symbols are used for drawing layer interfaces in mod- els. Each symbol/point characterizes a major class of services. Connectivity Service Points (CSP): Any type of connectivity, but physical media Synchronous connectivity (sSP) Specialized CSPs, Asynchronous connectivity (aSP) rarely used in this book Real-time medium connectivity (rSP) Non-real-time connectivity (nSP) Physical media interface Control Points (CP): service user's control point (uCP) (Network) resource control point (rCP) Management control point (mrCP, mpCP, msCP) Multi-purpose layer interfaces: This is a combination of point symbols that can denote e.g. a TCP layer inter