t establishing a presentation entities is needed. The common presentation layer structure for accommodating advances in technology and expan- service and protocol of ISO 8822 and ISO 8823 are used to establish ding operational requirements through continuing evolution. the transfer syntax that will be used to support the communication. The required functions to support successful communications The Basic Encoding Rules of ISO 8825 are now widely used in OSI between APs are distributed among seven architectural layers. The applications, but other transfer syntaxes can also be selected. layers, as shown in Figure 12.10, are defined below. In summary, the upper three layers provide the functions for a 12.5.3 Session layer meaningful communication SO that information reaching the desti- nation AP is processible and compatible with the local systems The session layer facilitates orderliness in a communication. A environment, The lower four layers provide the connectivity for session connection is established, and the specific functions that are interchange of data between systems and collectively represent the to be used are selected. Among the choices in typical use are: use of "bit pipe" for transparent data transfer. either two-way simultaneous or two-way alternate data transfer, expedited data transfer, synchronisation and recovery of data, activ- 12.5.1 Application layer ity management, and exception reporting. The session service and protocol are specified in ISO 8326 and ISO 8327, respectively. The The structure of the application layer has changed from that defined connectionless-mode in the upper three layers is under develop- in the OSI Reference Model of ISO 7498. The current concept ment. defines the application layer entity as consisting of two types of service elements. The Association Control Service Element (ACSE) 12.5.4 Transport Layer of ISO 8649 and ISO 8650 establishes the association between communicating APs and sets the context of the communication. The The transport layer provides for the trans