resentation layer element Connectivity layer element (any) Session layer Ism1: element Abstract protocol Connectivity Ism2: layer element (OSI layer 4) 1) Application layer element 2) Application protocol 3) Presentation connection endpoints 4) Session connection endpoints Fig. 6.1 Comparing AMLn layering with the OSI RM upper-layer architecture. AE=application entity; AP=application process; ASE=application service element 6 Applying AMLn The AMLn model to the left depicts a layer element that includes a common agent supporting actors of two actor layers: {a11,a12,..} and {a21,a22,...}. The LSMs (Ism1 and Ism2) of these layers are indicated below. An important difference is that an AMLn agent includes everything defined by the OSI RM upper-layer architecture, except actors, while the OSI application layer includes everything of the AMLn layer element, except encoding and dia- logue handling (which are functions placed in layers of their own). The corre- spondence in AMLn to a presentation layer function is the mapping performed inside an agent between invokes in actor-agent interfaces and messages between agents. The correspondence in AMLn to a session function is performed by actors and specified as a part of the abstract protocol (i.e., as a part of the element state machine, ESM, of an actor). Therefore, applying an AMLn layer architecture means that the complicated OSI presentation and session layer interfaces need not be implemented (and of course neither do the presentation and session protocols). In the OSI RM, these interfaces are defined (in analogy with real connectivity interfaces of lower OSI layers) by artificial "connect," "disconnect," and "data" primitives, used for establishing "presentation connections" and "session connec- tions," and for "sending" and "receiving" data over such connections. The way properties of such connections are defined also becomes complicated and rather strange, comprising the definition of things like "presentation context," "synchro- nization points," "tokens," etc., 