s or associations. The last two are depicted in Fig. 2.16 with their correct AML notations. protocol physical media connection endpoint physical media connection protocol logical connection endpoint logical connection association endpoint association Fig. 2.16 Connectivity relations If two units have a connectivity relation, then that if unit X1 creates an output at its endpoint, it becomes an input for X2 at its endpoint, and possibly the other way around. An endpoint is a means for the unit that owns it to refer to its partner. A connectivity relation can be a physical or a logical relation. It is regarded as a physical media connection if it terminates at physical media connection endpoints. Otherwise, it is a logical connection (or association), based on some means for identifying the receiver of an output. Logical connectivity relations may be connection-less (CNL) or connection-ori- ented (CON). If connection-oriented, the relation is called logical connection or association (the distinction between these two concepts is discussed in Sect. 5.3.4), and the terminating points are called logical connection endpoints (compare with the ports of ROOM). The QoS of a connection are those that are provided by the corresponding SAP. If connection-less, there are no connection endpoints and no connections, only a SAP that is used to communicate with all units of the same communication service 2 Foundations of Sysnet Modelling provider (CSP), unless the CSP constrains that possibility somehow (e.g. by "closed user groups"). Information exchange can start when the connectivity relation exists. The char- acteristics of information exchange depend on which abstraction of "interface" we are looking at. On lower abstraction levels, the information exchange is character- ized as message exchange (PDU exchange), described by a message protocol between the entities. Message types are information elements constructed as struc- tured data types, where each primitive data type is associated with an encoding method. A message prot