The Presentation Layer is Layer 6 of the seven-layer OSI model of computer networking.
The Presentation Layer is responsible for the delivery and formatting of information to the application layer for further processing or display. It relieves the application layer of concern regarding syntactical differences in data representation within the end-user systems. Note: An example of a presentation service would be the conversion of an EBCDIC-coded text file to an ASCII-coded file.
The Presentation Layer is the lowest layer at which application programmers consider data structure and presentation, instead of simply sending data in form of datagrams or packets between hosts. This layer deals with issues of string representation - whether they use the Pascal method (an integer length field followed by the specified amount of bytes) or the C/C++ method (null-terminated strings, i.e. "
thisisastring\0"). The idea is that the application layer should be able to point at the data to be moved, and the Presentation Layer will deal with the rest.
Serialization of complex data structures into flat byte-strings (using mechanisms such as TLV or XML) can be thought of as the key functionality of the Presentation Layer.
Encryption is typically done at this level too, although it can be done on the Application, Session, Transport, or Network Layers; each having its own advantages and disadvantages. Another example is representing structure, which is normally standardized at this level, often by using XML. As well as simple pieces of data, like strings, more complicated things are standardized in this layer. Two common examples are 'objects' in object-oriented programming, and the exact way that streaming video is transmitted.
In many widely used applications and protocols, no distinction is made between the presentation and application layers. For example, HTTP, generally regarded as an application layer protocol, has Presentation Layer aspects such as the ability to identify character encoding for proper conversion, which is then done in the Application Layer.
Within the service layering semantics of the OSI network architecture, the Presentation Layer responds to service requests from the Application Layer and issues service requests to the Session Layer.
The Presentation Layer is composed of two sublayers:
- CASE (Common Application Service Element)
- SASE (Specific Application Service Element)
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