Support Layers

The session layer, presentation layer and application layer are considered as support layers. All these layers explained in the following paragraphs.

SESSION LAYER. The session layer establishes a logical connection between the applications of two computers that are communicating with each other. The session layer concerns file management and overall networking functions. Access availability and system time allocations are included in this layer. The session layer can partition a transfer of a large number of messages by inserting synchronous points. Specific responsibilities of the session layers are authentication of user access, fault recovery if a break in service occurs, permitting multiple applications to share a virtual circuit, connection and disconnection of any node from the network.

PRESENTATION LAYER. The presentation layer receives information from the application layer and converts into ASCII or unicode or encrypts or decrypts. This layer is concerned with the syntax and semantics of the information exchanged between two systems. The three basic forms of protocols used in the presentation layer are

1. Virtual terminal protocol, which is used to allow different types of terminals to support different applications.

2. Virtual file protocol, which handles code coversions within files, file communication and file formating.

3. Job transfer and manipulating protocols which controls the structure of jobs and records.

A separate protocol function known as abstract syntax notation (ASN) specifies file data structure.

APPLICATION LAYER. This layer enables users to access the network with applications such as e-mail, FTP and Telnet. It provides user interfaces and support for various services. Specific services are (a) Network virtual terminal (b) File transfer access and management (FTAM), (c) Mail services (d) Directory services (e) Specific User Service Element (SUSE) — deals with actual user requirements for access and use of the network f) Common application service element (CASE) — This sets guidelines for the applications required quality of service and (g) Specific application service element (SASE) — deals with large amount of data, including database access.

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