mewhat vague as regards the relation between services and layers. The basic view that OSI RM takes through its seven-layer structure is that a layer provides some kind of connectivity service, which in AMLn terms means that 2.1 Concepts Based on the OSI RM its layer interfaces represent different types of connectivity service points (...SP). A basic distinction has to be made between synchronous data streams (such as streams supported by circuit-switching layers) and asynchronous data streams (such as streams supported by packet-switching layers). More detailed distinctions are the transfer rate, length of packages, and the quality of service (QoS). In AMLn, a connectivity service always defines how to transfer bits in some type of structure: bitstream, octets (or "bytes"), frames, superframes, packages, etc. The OSI RM does, however, also define connectivity services that transfer things such as "abstract data types." In AMLn we handle these types of interfaces differently (see Sect. 2.3). Users who are connected to connectivity service points can in AMLn only send and receive data, while in the OSI RM they can do other things as well, such as requesting connections, resetting connections, etc. In the OSI RM-application layer there also exists many functions that do things other than offering connectivity services. For example, the ACSE function provides a user interface that offers the establishment of associations, the FTAM provides a user interface for accessing globally distributed files, and the ITU-T X.500 directory provides a user interface that offers address translations. Since these interfaces exist above layer 6, but are not intended for end-users, the OSI RM sometimes classifies them as "sublayers in layer 7." The OSI RM makes no distinction between these types of services and connectivity services in models, i.e., all layers and sublayers are modeled the same way. Only the names of layers give a hint of what they are doing (if you know what the name implies). AMLn corrects this, and also eliminates 