system is derivable from a machine independent working open systems. abstract syntax, such as Abstract Syntax Notation one (ASN.1). The transfer syntax is operated at the Presentation Layer according to Reliable Transfer Service Element (RTSE) standardised encoding rules, such as Basic Encoding Rules (BER). The Session Layer controls the exchange of information between These elements ensure reliable communications, protecting the Presentation Layers within the different systems, by establishing against failures and informing the sender when errors occur. connection, synchronisation, disconnection, flow control to prevent flooding, etc. The Session Layer (and the Transport Layer) has only one standard specified within it, but they have different classes or subsets which provide options to enable the lower layers to be 16.6.1.2 Specific Application Layer standards within OSI matched to the needs of the upper layers, depending on the applica- tion. File Transfer, Access and Management (FTAM) The Transport Layer ensures the correct quality of service, re- quested by the applications, and informs the Session Layer if it This Application Service Element provides functions for access- cannot maintain this service quality. There are five classes of ser- ing and modifying files on another open system and for transferring vice, characterised by parameters such as transmission delay, them between open systems. throughput, residual error rate, and connection set-up delay. Class 0 is used when the bearer is of good quality and Class 4 when it is not Message Handling System (MHS) as good. The Network Layer's prime aim is to provide a transparent path This comprises a set of standards for use in electronic mail, where for the transmission of data between the Transport Layers within the the message may consist of data, speech, text or graphics. MHS is a communicating systems. It handles addressing and routing of cir- CCITT standard, the ISO equivalent being Message Oriented Text cuits. The Data Link Layer provides the error free