What is the pre­sen­ta­tion layer in the OSI model?

The pre­sen­ta­tion layer is the sixth layer in the OSI model and is re­spon­si­ble for con­vert­ing different file formats. This allows two systems to com­mu­ni­cate. Other tasks carried out by the sixth layer include data com­pres­sion and en­cryp­tion.

The pre­sen­ta­tion layer is the sixth layer of the OSI model. It is primarily used to convert different file formats between the sender and the receiver. The OSI model is a reference model that is used to define com­mu­ni­ca­tion standards between two devices within a network. The de­vel­op­ment of this standard began in the 1970s and it was first published at the beginning of the following decade. This standard enables seamless in­ter­ac­tion between different technical systems.

The model is made up of a total of seven different layers, all having their own clearly defined tasks. While there are clear bound­aries between the layers, the layers interact with each other, with each layer building off the one below it. The different layers are as follows:

1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Pre­sen­ta­tion layer
7. Ap­pli­ca­tion layer

The pre­sen­ta­tion layer interacts closely with the ap­pli­ca­tion layer, which is located directly above it. The pre­sen­ta­tion layer’s main task is to present data in such a way that it can be un­der­stood and in­ter­pret­ed from both the system sending the data and the system receiving it. After this has been ac­com­plished, the ap­pli­ca­tion layer then de­ter­mines how the data should be struc­tured and what sort of data and values are per­mis­si­ble.

Using these entries, a command set, or an abstract transfer syntax, is then au­to­mat­i­cal­ly created. The pre­sen­ta­tion layer now has the task of trans­fer­ring the data in such a way that it is readable without changing the in­for­ma­tion contained within it.

The pre­sen­ta­tion layer is often also re­spon­si­ble for the en­cryp­tion and de­cryp­tion of data. The in­for­ma­tion is first encrypted on the sender’s side and then sent to the receiver in an encrypted state. Keys and en­cryp­tion methods are then exchanged in the pre­sen­ta­tion layer. The recipient is then able to decrypt the un­read­able data and convert it into a format that can be un­der­stood and in­ter­pret­ed.

If data is shown during a transfer, we often use the term transfer syntax. These are separated into the abstract transfer syntax, in which the trans­ferred values are written, and the concrete syntax, which contains a de­f­i­n­i­tion of the value coding.

The receiver can only process and un­der­stand the data they receive if they receive all of the in­for­ma­tion from the pre­sen­ta­tion layer. The most common de­f­i­n­i­tion language is Abstract Syntax Notation One (ASN.1), which is also rec­om­mend­ed by the ISO. The ISO is an or­ga­ni­za­tion that is re­spon­si­ble for de­vel­op­ing in­ter­na­tion­al standards in tech­nol­o­gy, man­age­ment and man­u­fac­tur­ing.

The pre­sen­ta­tion layer has many different formats. The most common text formats are the ASCII (American Standard Code for In­for­ma­tion In­ter­change) and EBCDIC (Extended Binary-Coded Decimal In­ter­change Code). The most common image formats are GIF, JPEG and TIFF. Widely used video formats include MIDI, MPEG and QuickTime.

There are many different pre­sen­ta­tion layer protocols as well as transfer and en­cryp­tion tech­nolo­gies in the pre­sen­ta­tion layer. These include:

• Telnet
• FTP
• HTTP
• SMTP
• NNTP
• NetBIOS

The tasks which are carried out by the pre­sen­ta­tion layer are not always necessary for com­mu­ni­ca­tion between two systems. In instances where both systems use the same formats, data con­ver­sion is not necessary. Ad­di­tion­al­ly, en­cryp­tion and com­pres­sion are not required for every in­ter­ac­tion and can also be carried out in another layer of the OSI model. If this is the case, the pre­sen­ta­tion layer can be skipped and the ap­pli­ca­tion layer (7) can com­mu­ni­cate directly with the session layer (5) instead.