Editing Presentation layer

{{Use American English|date = March 2019}}
{{Short description|Sixth layer of the OSI model of telecommunications}}
{{Use mdy dates|date = March 2019}}
{{OSIModel}}
In the seven-layer [[OSI model]] of [[computer network]]ing, the '''presentation layer''' is '''layer 6''' and serves as the data translator for the [[computer network|network]].<ref name="Network+ Guide to Networks">{{cite book | last = Dean | first = Tamara | title = Network+ Guide to Networks | publisher = Delmar | year = 2010 | pages = 44–47 | isbn = 978-1423902454 | url = https://books.google.com/books?id=UD0h_GqgbHgC&q=network%2B+guide+to+networks}}</ref><ref name="Microsoft Technet">{{cite web |url=https://technet.microsoft.com/en-us/library/cc959885.aspx |title=Presentation Layer |website=[[Microsoft TechNet]] |publisher=[[Microsoft]] |access-date=2023-09-18}}</ref><ref name="Hodel 2023 pp. 133–174">{{cite book | last=Hodel | first=Brian | title=Beginning Microsoft Dataverse | chapter=Presentation Layer | publisher=Apress | publication-place=Berkeley, CA | date=2023 | isbn=978-1-4842-9333-1 | doi=10.1007/978-1-4842-9334-8_5 | page=133–174 | quote=The presentation layer consists of various methods of presenting and interacting with your data.}}</ref> It is sometimes called the syntax layer.<ref>{{cite book | last = Grigonis | first = Richard | title = Computer telephony encyclopedia | publisher = CMP | year = 2000 | pages = 331 | isbn = 9781578200450 | url = https://books.google.com/books?id=cUYk0ZhOxpEC&q=computer+telephony+encyclopedia}}</ref>

==Description==
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 through a unique presentation service access point (PSAP).<ref name="Hura">{{cite book | last = Hura | first = Gurdeep | title = Data and Computer Communications: Networking and Internetworking | publisher = CRC Press LLC | year = 2001 |chapter= Application Layer |pages = [https://archive.org/details/datacomputercomm0000hura/page/710 710]–712 | isbn = 9780849309281 | url = https://archive.org/details/datacomputercomm0000hura | url-access = registration }}</ref>

The presentation layer ensures the information that the application layer of one system sends out is readable by the application layer of another system. On the sending system it is responsible for conversion to standard, transmittable formats.<ref>{{cite book | last = Hura | first = Gurdeep | title = Data and Computer Communications: Networking and Internetworking | publisher = CRC Press LLC | year = 2001 |chapter= Presentation Layer |pages = [https://archive.org/details/datacomputercomm0000hura/page/671 671]–681 | isbn = 9780849309281 | url = https://archive.org/details/datacomputercomm0000hura | url-access = registration }}</ref> On the receiving system it is responsible for the translation, formatting, and delivery of information for processing or display. <ref>http://www.linfo.org/presentation_layer.html Linux Information Project</ref> In theory, it relieves application layer protocols of concern regarding syntactical differences in [[data]] representation within the [[User (computing)|end-user]] systems. An example of a presentation service would be the conversion of an extended binary coded decimal interchange code ([[EBCDIC]]-coded) text [[computer file]] to an [[ASCII]]-coded file. If necessary, the presentation layer might be able to translate between multiple data formats using a common format.

In many widely used applications and protocols no distinction is actually made between the presentation and application layers. For example, [[HyperText Transfer Protocol]] (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.

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 [[Network packet|datagrams]] or packets between hosts. This layer deals with issues of string representation - whether they use the [[Pascal (programming language)|Pascal]] method (an integer length field followed by the specified amount of bytes) or the [[C (programming language)|C]]/[[C++]] method (null-terminated strings, e.g. <code>"thisisastring\0"</code>). The idea is that the application layer should be able to point at the data to be moved, and the presentation layer will translate this to commands able to be understood by other applications and processes.

[[Serialization]] of complex data structures into flat byte-strings (using mechanisms such as [[Type–length–value|TLV]], [[XML]] or [[JSON]]) can be thought of as the key functionality of the presentation layer. Structure representation is normally standardized at this level, often by using [[XML]] or [[JSON]]. 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.

[[Encryption]] and [[Decryption]] are typically done at this level too,<ref name="Knipp Browne Weaver Baumrucker 2002 pp. 1–60">{{cite book | last=Knipp | first=Eric | last2=Browne | first2=Brian | last3=Weaver | first3=Woody | last4=Baumrucker | first4=C. Tate | last5=Chaffin | first5=Larry | last6=Caesar | first6=Jamie | last7=Osipov | first7=Vitaly | last8=Danielyan | first8=Edgar | title=Managing Cisco Network Security | chapter=Introduction to IP Network Security | publisher=Elsevier | date=2002 | isbn=978-1-931836-56-2 | doi=10.1016/b978-193183656-2/50005-2 | page=1–60 | quote=The presentation layer establishes the way in which information is presented, typically for display or printing. Data encryption and character set conversion (such as ASCII to EBCDIC) are usually associated with this layer.}}</ref> although it can be done on the [[application layer|application]], [[session layer|session]], [[transport layer|transport]], or [[network layer]]s, each having its own advantages and disadvantages. For example, when logging on to bank account sites the presentation layer will decrypt the data as it is received.<ref name="Network+ Guide to Networks"/>

==Services==
*[[Data conversion]]<ref name="Microsoft Technet"/>
*[[Character_encoding#Character_encoding_translation|Character code translation]]<ref name="Microsoft Technet"/>
*[[Data compression|Compression]]<ref name="Microsoft Technet"/>
*[[Encryption|Encryption and Decryption]]<ref name="Microsoft Technet"/>
*[[Serialization]]

==Protocols==
Protocols sometimes considered at this level (though perhaps not strictly adhering to the OSI model) include:
* [[Apple Filing Protocol]] (AFP)
* [[Independent Computing Architecture]] (ICA), the Citrix system core protocol
* [[Lightweight Presentation Protocol]] (LPP)
* [[NetWare Core Protocol]] (NCP)
* [[Network Data Representation]] (NDR)
* [[Tox (protocol)|Tox]], The Tox protocol is sometimes regarded as part of both the presentation and application layer
* [[External Data Representation|eXternal Data Representation]] (XDR)
* X.25 [[X.25|Packet Assembler/Disassembler Protocol]] (PAD)

==See also==
* [[ASN.1]]
* [[X.690]]

==References==
{{Reflist}}

[[Category:OSI model]]