and is presented to the systems at the presentation layer. The conversion of application specific system to presentation system happens both in the transmitting system and the receiving system. This common presentation syntax is called transfer syntax. There can be abstract forms to define data types called abstract data syntax and presentation forms for transfer and presentation of data called transfer syntax. Seven-Layer Communication Model In order to ensure that common syntax definitions are available for data recogni- tion, ISO defined a general abstract syntax for the definition of data types associ- ated with distributed applications. This abstract syntax is known as abstract syntax notation number one or ASN.1. The data types associated with ASN.1 are abstract data types. Along similar lines, transfer syntax was defined by OSI for use with ASN.1. This transfer syntax notation developed by OSI is known as basic encod- ing rules (BER). The functions performed by the presentation layer are: Translation for representing data in a common format: A network will be comprised of machines running on different OS like Windows, UNIX, Macintosh, and SO on. These machines differ in characteristics of data presen- tation. The presentation layer handles translation of data to a known format, thus hiding the differences between machines. This translation includes data transformation, formatting, and special purpose transformations. Request for session establishment, data transfer, and termination: Though session management is handled by the session layer, the request to perform session-related operations like the request for establishment of a session, the data transfer, and the final termination originates from the presenta- tion layer. Negotiation of syntax: The presentation layer does the job of negation of syntax for data transfer. Multiple negotiations are performed before a common format/syntax is agreed upon between systems that want to communicate. Compression and decompression: This function is not usually perfo