Presentation Layer
The presentation layer is the sixth layer of the OSI model. It responds to service requests from the application layer and issues service request to the session layer.
The layer performs certain functions that are requested sufficiently often to finding a general solution for users, rather than letting each user solve the problems. In particular, unlike all the lower layers, which are just interested in moving bits reliably from here to there, the presentation layer is concerned with the syntax and semantics of the information transmitted.
This layer sometimes called the syntax layer in the OPEN SYSTEMS INTERCONECTION MODEL (OSI) is basically used to address and resolve data communication across the 7 layers. For example, a sending computer after receiving data from the application layer will need to convert such data from the receiving format(eg ASCII) to a format understood and accepted by other layers of the OSI model to ensure smooth file transfer.
At the receiving computer is also responsible for the conversion of “the external format” with which data is received from the sending computer to one accepted by the other layers in the host computer. Data formats include postscript, ASCII, or BINARY such as EBCDIC (fully Extended Binary Coded Decimal Interchange Code).
An example of data formating functions of the presentation layer is the conversion of an EBCDIC-coded data to an ASCII style.
The Presentation Layer has two functions it carries out on behalf of PS users and that are negotiation of transfer syntaxes and transformation to and from transfer syntax. The function of transfer syntax negotiation is supported by presentation protocols. Transformation of syntax is a function contained within a presentation entity and has no impact on presentation protocol design. For connectionless mode transmission, the sending presentation entity selects the transfer syntaxes. No transfer syntax negotiation occurs. For connectionless mode transmission, the abstract syntaxes used are determined by the sending application entity. For successful communication to take place, these must be acceptable to the receiving application entity.
Typically, in the presentation layer, the layout/structure of data in transit is set, language to be used in transmission is agreed on, security features are put in place to guarantee privacy and authentication, and the volume of data to be transmitted is decided through compression.
Another function of the presentation layer is found in the encryption or protection of data offered to transmissions/sessions within the layers. This can be achieved by cryptography. Cryptography is a data security science achieved by hiding information/data in transit. Although, there are other forms of cryptography such as the use of microdots or merging words with images, today’s computing world is largely concerned with encryption- the process of scrambling plaintext-e.g. everyday sentences and messages into ciphertext -a language understood by the machine or translator at the receiving end which is then interpreted into plaintext again i.e. decryption. This function is however debated as one not exclusive to the presentation layer alone but also other layers of the OSI such as the session, network ,transport layer.
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