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Presentation Layer :

Before reaching this layer, the flow of DATA was treated simply like a succession of bits. That no preceding layer was charged to interpret. The presentation layer assembles the DATA in bytes and characters. It definite the rules of this process of conversion, has to know: quantity of bit which constitutes the characters, the direction of reading of the flow of bits and if a mechanism of encoding were used or not.

The example below clearly shows the various results obtained by dividing the flow of bits INTO 7 or 8 bits respectively.

1000100 1101010 1101001 0010101 0010111
10001001 10101011 01001001 01010010 11101101

The systems using of the numbers in floating point must build the DATA in three components :

  • Mantissa, exhibitor and bit of sign.

The individual systems can represent each component by a number of different bits. Several standards exist on the level of this layer, to represent the characters.

It is :

  • ASCII (American Standard Code for Information Interchange).
  • EBCDIC (Extended Binary Coded Decimal Interchange Code).
  • XDR (eXternal DATA Representation), a standard running for the numbers has floating point.

Easy : When the applications cooperate enter they, they exchange certain information. They must thus determine how to represent these data. In OSI model, this layer provides the routines of presentation of the standardized data. This function is often directly assured inside the application, although more and more of protocols TCP/IP such as XDR and MIME carry out this operation.



Author
Jean Théry

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