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OSI Networking Model Layer 6


6 - PRESENTATION - Data representation and encryption (Translation)


Function: Data representation conversions. This layer translates data, between what the network requires and what the computers or industrial devices at each end expect. This is mostly used by the computers or equipment that present the data in a graphical way.


Data unit: Packets.


Services (examples): MIDI, HTML, GIF, TIFF, JPEG, ASCII, EBCDIC.


Components: Gateway, Redirector


Topics & Methods Explained


Bit order:
When two different computer systems want to communicate. One using a 7-bit char. scheme and the other an 8-bit, they agree to use a generic 4-bit scheme where 0001=A & 1000=Z. They agree to read from right to left or reverse.


Byte order:
For example, the Intel microprocessors use the "little endian" (or reverse-byte ordering) method (LSB appears first) for representing complex numbers but Motorola use the "big endian" (MSB appears first).


Character code:
Through negotiation, peer Presentation layer processes can agree that only one will translate its data to the others code, or both will translate their native character code to a third mutually-agreeable code.


File syntax:
When local and network operating systems dictating different file syntax or format attempt to share data files, the file syntax must be translated to the required local formats.

Encryption (Transpos. Subst. Algebraic)

Public key:
Entities are provided a secret key and a known value. An entity creates a public key by manipulating the known value by the secret key value. The entity initiating the comm. sends its public key to the receiver.


Private key:
Only one key is used. Entities that have the key are able to encrypt and decrypt every message. Therefore the key has to be kept private. The key can be imbedded in hardware chipsets.



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