Presentation Layer - Data Security and Formatting

Over a network different types of information are transferred. Have you ever thought which layer deals with formatting and syntax checking of these information. As discussed in my earlier article about Application Layer (Layer 7 of OSI model) which deals with different applications (protocols).Thus the question arises that where and which layer of OSI model decides that a particular type of information has to be converted to a particular format by following a particular syntax. Answer is Presentation Layer (Layer 6 of OSI model). For example this layer decides that what will be http encoding and format (Remember http is a layer 7 protocol ). So, the layer deciding encoding must reside immediately below the layer 7. So, the Presentation Layer is the Layer 6 of OSI model. It converts a particular picture information received to gif format, a particular movie information received to mpeg format.

Question: Which Protocol or a set of protocol function(s) at Presentation Layer ?

Answer: No protocol functions at Presentation Layer.

Functions of Presentation Layer includes:

1. Data Formatting & Syntax Checking - Presentation Layer converts information received from Application Layer(Layer 8 of OSI model)to standard formats and syntaxes.

2. Support to all major formats & Services - Presentation Layer support all important formats like ASCII, EBCDIC,MPEG etc.

3. Data Security - Information received may be encrypted or decrypted by using Cryptographic principles. Thus this layer provides data and information security within OSI framework

4. Data Compression & Decompression - data can be compressed or decompressed as required by Presentation Layer

5. Communication with Application Layer and Session Layer- It receives information from Application Layer and syntactic, formatted, compressed and encrypted( whichever applicable) data is sent to the Session Layer (Layer 5 of OSI model).

Architecture of Presentation Layer-

Presentation Layer has two sublayers - Common Application Element Sublayer & Specific Application Element Sublayer.

Abbreviations used:

OSI - Open Systems Interconnection
HTTP - Hypertext Transfer Protocol
ASCII - American Standard Code for Information Interchange
EBCDIC - Extended Binary Coded Decimal Interchange Code
MPEG - Moving Picture Experts Group
GIF - Graphics Interchange Format

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