OSI Reference Model : Presentation Layer
OSI reference model is the standard model which is used in data communications between computing devices. OSI is a layered model having seven layers and each layer is responsible for specific task or services. The seven Layers in the OSI model are: Application Layer, Presentation Layer, Session Layer, Transport Layer, Network Layer,Data Link Layer and Physical Layer. Data travels through all the layer. Lower layers in the OSI reference models serve the higher layer in the model. Application Layer of the OSI reference model serves applications with desired the protocols like HTTP, SMTP etc.. In this layered approach Application Layer serves the Presentation Layer in the OSI model.
In the Application Layer of the OSI reference model, the sender decides what data to send over the network to the receiver. Application decides which protocol to use to send the data to the receiver. Once Application Layer has finished it's job, it transmits the data to Presentation Layer. Presentation Layer decides how to format the data, how data will be compressed and how the data can be encrypted or decrypted.
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Definition of Presentation Layer
Presentation Layer, also called syntax layer is the layer 6 in the OSI reference model and serves as data translator in the OSI model. Presentation Layer is responsible for delivering data to application layer. Presentation layer is also responsible for formatting data for Application layer so that it can be processed and displayed. Instead of sending data as datagram and packets, presentation layer considers data structure and presentation of data. Mainly, presentation layer is responsible for how the data will be presented. For example: If a host wants to send a picture to another host then presentation layer considers how the picture will be presented for communication, it may in the form of pixels or bits.
Presentation layer checks the compatibility issue with the remote host. Presentation layer ensures interoperability. Two hosts may be running on two different platforms, so it is upto the presentation layer how two hosts will communicate.
Functions and Services of Network Layer
The main function of the presentation layer is to take care of syntax and semantic of the data which is communicated between two communicating devices. It formats the data or information so that it can be transmitted over the network. It ensures that the data transmitted is understood by the receiver and can be used by the receiver.
1. Data Conversion
Before the data is transmitted, presentation layer translates all the data into bit streams.
Data conversion is one of the important functions of the presentation layer. Presentation layer considers the data format which can be transmitted over the network. Consider an audio file. The format of the file may be mp3 or mp4. But before transmitting data over the network, presentation layer may convert the file in binary bits so that it can be transmitted over the network. Generally pictures are stored as pixels. So when this picture is to be sent over the network, it has to be converted into bits. And this conversion is the responsibility of presentation layer.
Presentation layer provides character-code translation such as: ASCII and EBCDIC
Larger file may eat up larger bandwidth and larger files will take more time for transmission. Data compression and decompression is done at this layer. It improves data throughput over the network. Data compression reduces the bandwidth of the data and hence the throughput is improved.
3. Encryption and Decryption
In some cases data encryption and decryption is done at this layer. It ensures data security in the data transmission. So many encryption algorithms are there for encryption and decryption. Each algorithm has its own advantages and disadvantages. But practically encryption and decryption is not required at this level of data transmission and hence encryption and decryption is ignored by many applications.
Examples of encryptions at presentation layer are: SSL, TLS
Sub Layers of Presentation Layer
Presentation layer may be composed of two sub layers: CASE (Common Application Service Element) and SASE (Specific service Application Element)
CASE (Common Application Service Element) provides support for common application services, such as
- ACSE ( Association Control Service Element )
- ROSE ( Remote Operation Service Element )
- CCR ( Commitment concurrency and Recovery )
- RTSE ( Reliable Transfer Service Element )
SASE (Specific Application Service Element) provides application specific services, such as
- FTAM ( File Transfer, Access and Manager )
- VT ( Virtual Terminal )
- MOTIS ( Message Oriented Text Interchange Standard )
- CMIP ( Common Management Information Protocol)
- JTM ( Job Transfer and Manipulation )
- MMS ( Manufacturing Messaging Service )
- RDA ( Remote Database Access)
- DTP ( Distributed Transaction Processing )
Protocols at Presentation Layer
Different protocols used at presentation layer are as follows:
- AFP ( Apple Filing Protocol )
- ICA ( Independent Computing Architecture )
- LPP ( Lightweight Presentation Protocol )
- NCP ( NetWare Core Protocol )
- NDR ( Network Data Representation )
- XDR ( eXternal Data Representation )
- PAD ( Packet Assembler/Disassembler Protocol )
Although the function of the presentation layer is to format the data for transmission over the network, very often data encryption and decryption is ignored by many applications at this layer because, the same functions are done by other layers also, and it is not required at this level.