ot be any slower than normal data but may be faster. Expedited data service is invoked as required by the upper layers. Expedited data may also be acknowledged by the destination (N)- PHYSICAL entity. Figure 12.10 Layers of the OSI model 12.4.2.2 Connectionless-mode Data transfer in the connectionless-mode involves minimal func- as the Application Service Elements (ASEs), which may be used tionality. Data units are sent, routed, and delivered with no knowl- individually or in multiples to satisfy specific operational contexts. edge being maintained by the (N)-layer of their status along the way. Example ASEs include: File Transfer, Access, and Management Some have referred to this mode as "send and pray" because there (ISO 8571); Virtual Terminal (ISO 9040 and ISO 9041); Transac- is no assurance that delivery has been successful. On the other hand, tion Processing (ISO 10025); Commitment, Concurrency, and Re- the shortcomings of this simple approach can be compensated for covery (ISO 9804 and ISO 9805); Remote Operations (ISO 9072); by use of appropriate functions at higher layers, which are operating Reliable Transfer (ISO 9066); Remote Database Access (ISO in the connection-mode. 9579); and others. There is a richness of functionality to select from in the application layer. 12.5 Specific OSI layers 12.5.2 Presentation layer As explained earlier in this chapter, the number and functional The application layer is defined in terms of its semantics and configuration of the specific layers for the OSIE is somewhat described in terms of an abstract syntax of the data types involved. arbitrary. A number of arrangements to satisfy the requirements are The Abstract Syntax Notation No. 1 (ASN.1) of ISO 8824 has been possible. However, the agreement has been made internationally for widely accepted for use in OSI specifications. When data are trans- the OSI configuration, which provides a solid generic basis to ferred through the OSIE, an encoding agreed upon by the peer characterise distributed information systems whils