e semantics of the services and primitive interactions are lished between (N+1)-entities and establishes the path for informa- defined in the family of OSI standards to facilitate an understanding tion flow that is maintained for the duration of the instance of of the dynamics of the OSIE but are not critical to OSI conformance communication, as shown in Figure 12.7. It is a result of the and interoperability between systems. (N)-layer and the underlying layers collectively. Each layer opera- Peer-entities communicate with each other using an (N)-protocol, ting in the connection-mode follows the procedure to establish the which is a set of rules and procedures to facilitate an orderly association, or connection, with its peer entity. Upon termination of communication in the performance of the associated functions. The the communication, the connection of each layer is released. peer protocols are the critical specifications of the OSI family of The establishment of a connection allows negotiation of par- standards that facilitate interoperability between systems and are ameter values and options between corresponding entities. It has a the basis for determining OSI conformance. distinguishable lifetime that can support interchange of multiple In the case of the uppermost layer, there is no (N+1)-layer, but this data units over a period of time. is the point at which the AP binds to the OSIE during an instance of In contrast, the connectionless-mode does not establish and main- a communication. There is also no (N-1)-layer associated with the tain a relationship between corresponding entities. Each transfer of lowest layer because that is the point of interface between the OSIE data in the connectionless-mode is a separate independent action for and interconnecting physical transmission media. that layer. It has often been called the "message mode" or "datagram mode" where each data unit is routed to the destination on an 12.3.3 Layer services individual basis with no regard to data units transferred before or aft