 to the layer above within the (N)-layer in a real system, and peer-entities are (N)-en- as the communication proceeds layer-by-layer. The physical media, tities within the same layer, but in different systems. SERVICE SERVICE (N+1) SERVICES PRIMITIVES SERVICES PRIMITIVES LAYER UPPER BOUNDARY PROTOCOL LAYER LAYER ENTITY ENTITY LAYER (SYSTEM A) (SYSTEM B) LOWER BOUNDARY (N-1) LAYER (N-1) SERVICE (N-1) SERVICE SERVICES PRIMITIVES SERVICES PRIMITIVES Figure 12.5 Concept of a layer 12/6 Open systems interconnection reference model SYSTEM A SYSTEM B CONFIRM is returned by the service provider to the origina- ting service user to acknowledge completion of a requested (N) SERVICE (N) SERVICE service. The primitives carry important parameter information between the layers. The nature of the parameters may vary for different services, but they can include address information, data to be trans- ferred, and invocation of service options. The specifics of the services and associated primitives for each layer are covered by the specific layer service definitions, which are part of the OSI family of standards. (N) SERVICE PROVIDER 12.3.4 Communication between peer entities Two modes of communication are defined in the OSI Reference Model: connection-mode and connectionless-mode. Connection- mode operation was the original basis for OSI. Later, the connec- Figure 12.6 Layer service model tionless-mode was defined in ISO 7498 AD1. The connection-mode functions through three phases of operation in support of an instance of communication. These are estab- The (N)-service relates the collective capability of the (N)-layer lishment, data transfer, and release. The initial definition of the OSI and underlying layers provided to support the (N+1)-entity. Associ- Reference Model included only the connection-mode for all the ated with each (N)-service are primitive interactions that pass layers. various parameters. The primitives will be described later in this An (N)-connection represents an association that has been estab- chapter. Th