What is Presentation Layer? The Presentation Layer is the Sixth Layer of the OSI (Open Systems Interconnection) communications model. Basically, this layer ensures whatever information that passes through it is in the required form for the recipient application, i.e. it presents the data in a readable format from an application layer perspective.
Presentation Layer thus has the core functionality of conversion of Data Formats, in the form of packets, from one machine to another located on a network. It translates between Application and other layers, redirects, encrypts and compresses messages.
One common example is the sending of data from a machine that uses the ASCII format for characters to a machine that may use another format, such as EBCDIC format, for characters.
There may be letters, numbers, and symbols in one format which needs to be translated while communication between machines that use different formats. Thus, this is the responsibility of the Presentation Layer.
Purpose of the Presentation Layer
The lower layers in the OSI model might concern themselves with the task of reliably moving the bits from here to there. But, the presentation layer concerns itself with the syntax and the semantics of the information transmitted.
The layer is primarily concerned with the conversion of data formats, in the form of packets, from one machine to another. It is responsible for picking up differences such as these and translating those to compatible formats.
The typical example of this service is the encoding of data in an agreed standard way. Most user programs don’t exchange random binary strings; they rather share data as names, dates, amounts of money, invoices, addresses, etc. These items are represented in the programs as character strings, integers, floating-point numbers, data structures & classes which are made up of more fundamental units.
Different computers might have fundamentally different codes to represent the data character strings, integers, and other types of variables, such as in ASCII, Unicode, etc.
If the computers using different representations are to be able to communicate, then the data structures to be exchanged need to defined in an abstract way, along with some standard encoding to be used.
This layer manages such abstract data structures and helps in converting from the representation used inside a computer to the network standards and back.
To sum up the above-explained thing: This layer translates the more complex data objects into a storable and transportable format. This helps in rebuilding the object once it arrives at the other side of the communication stream.
Hence, this layer also deserializes (re-translate) the data and places it back into an object format that could be understood by the application or computer at the receiving or other end.
Roles, Functions, and Protocols
This layer is mainly responsible for managing two networking characteristics, that is, protocol and architecture. A standard set of guidelines under which a network works are defined by a Protocol, whereas the type of network architecture determines what protocol applies to it.
Much of the basic function has been discussed in above mentioned the purpose section. In addition to that, as a translator, this layer takes care of any issues which might occur when a transmitted data ought to be viewed in a disparate format as compared to the original format.
Performing its functional part, this layer performs a multitude of data conversion algorithms and character translation functions.
The various functions are:
- Character-Code Translation: It is where this layer translates the ASCII code to the EBCDIC. EBCDIC stands for Extended Binary Code Decimal Interchange Code.
- Data Conversion: This is where the presentation layer performs bit order reversal functions, converts CR (byte code for a carriage return) to CR/LF, (byte code for a carriage return with a line feed) and converts integer numbers to floating-point numbers.
- Data Compression, by reducing the number of bits requiring transmission, which improves the data throughput.
- Data Encryption and Decryption: Encryption is needed for security purposes when sending data across networks. During transmission, an encryption algorithm is used, whereas, on the receiving end, a decryption algorithm is used. Encryption and decryption typically involve the secure sockets layer (SSL) protocol, which has become more popular when used by the presentation layer. Here, encryption methods and keys are exchanged between the two communicating devices. Thus, only the sender and receiver can properly encode and decode data so it returns to a readable format.
- Data Translation: In a network, fundamentally different types of computers, such as servers and mainframes, might employ different character sets. The fixing of such irregularities and keeping the translations transparent between the networked systems is the responsibility of the Presentation Layer.
It is also known by other names such as Data Presentation Layer, Data Provisional Layer, and Syntax Layer, the Presentation Layer, as it is most famously known as is the sixth layer or Layer 6 of the Open Systems Interconnection (OSI) model. To know more about the Presentation Layer in the OSI Model, keep reading this article till the end.
The main function of the layer is to give the system-dependent presentation of data an independent form so the data can be exchanged between two entirely different systems.
To give it a simpler term, this layer works as a data translator for a network. Just like two persons speaking two different languages need a translator in order for them to interact with each other, the OSI model needs a presentation layer to keep its’ functionality while establishing a connection or session between two different systems on common ground.
The common example of this is the conversion of the Extended Binary Coded Decimal Interchange Code (EBCDIC) text computer file to the American Standard Code for Information Interchange (ASCII).
Working alongside Session Layer and Application Layer which are located above and below it, the session layer is included in the category of the upper layers in the OSI Model with both these other layers.
These three upper Layers are responsible for the communication of applications between hosts and have nothing to do with networking and networking addresses.
Functionality and the services:
In the process of translating, the codes received; their numbers and characters are converted into the bitstream, which simply means that the data is arranged in a binary form. Only after this conversion could there be initiated a process of data transmission.
Decoding and converting is not enough, reducing the bandwidth of the data is also important and it is done by the process of data compression. In this process, the bits of data is reduced before it can be made ready for the transmission. Although it may not be seen as a significant process in other kinds of media it is vital for multimedia such as audio, video, and texts.
ASCII and EBCDIC mentioned above are the formats used for the conversion of data for the transmission of the texts, Joint Photographic Experts Group (JPEG, named after the committee that created this standard), Graphics Interchange Format (GIF) and Tagged Image File Format (TIFF) for the images and Moving Pictures Expert Group (MPEG, origin of the name of this format is same as the JPEG format), Musical Instrument Digital Interface (MIDI) and QuickTime for the videos.
Some of the presentation layer protocols are the Apple Filing Protocol (AFP) and Lightweight Presentation Protocol (LPP).
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