In the seven-layer OSI model of computer networking, the presentation layer is layer 6 and serves
as the data translator for the network.
It is sometimes called the syntax layer.
Description 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.
An example of a presentation service would be the conversion of an EBCDIC-coded text
computer 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 the form of datagrams
or packets between hosts.
This layer deals with issues of string representation - whether they use the Pascal method or the
C/C++ method.
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 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.
Decryption is also handled at the presentation layer.
For example, when logging on to bank account sites the presentation layer will decrypt
the data as it is received.
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, HyperText Transfer Protocol, 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.
In the OSI model: the presentation layer ensures the information that the application layer
of one system sends out is readable by the application layer of another system.
For example, a PC program communicates with another computer, one using extended binary
coded decimal interchange code and the other using ASCII to represent the same characters.
If necessary, the presentation layer might be able to translate between multiple data
formats by using a common format.
Services Data conversion
Character code translation Compression
Encryption and Decryption Sublayers
The presentation layer can be composed of two sublayers: common application service
element and specific application service element.
CASE The common application service element sublayer
provides services for the application layer and request services from the session layer.
It provides support for common application services, such as:
ACSE ROSE
CCR RTSE
SASE The specific application service element sublayer
provides application specific services, such as
FTAM VT
MOTIS CMIP
JTM a former OSI standard MMS
RDA DTP
Protocols Other protocols sometimes considered at this
level include: Apple Filing Protocol
Independent Computing Architecture, the Citrix system core protocol
Lightweight Presentation Protocol NetWare Core Protocol
Network Data Representation Telnet
Tox, The Tox protocol is sometimes regarded as part of both the presentation and application
layer eXternal Data Representation
X.25 Packet Assembler/Disassembler Protocol References