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{{Short description|Model of communication of seven abstraction layers}}
{{Use British English|date=March 2020}}
{{Use dmy dates|date=April 2019}}
{{OSI model}}
The '''Open Systems Interconnection''' ('''OSI''') '''model''' is a [[reference model]] developed by the [[International Organization for Standardization]] (ISO) that "provides a common basis for the coordination of standards development for the purpose of systems interconnection."<ref name="ISO-7498-1">{{cite ISO standard |date=June 1999|title=ISO/IEC 7498-1:1994 Information technology — Open Systems Interconnection — Basic Reference Model: The Basic Model |csnumber=20269 |at=Introduction |access-date=26 August 2022}}</ref>
In the OSI reference model, the components of a communication system are distinguished in seven [[abstraction layer]]s: Physical, Data Link, Network, Transport, Session, Presentation, and Application.<ref>{{Cite web |date=2018-08-10 |title=What is the OSI Model? |url=https://web.archive.org/web/20220324014026/https://www.forcepoint.com/cyber-edu/osi-model |access-date=2022-05-20 |website=Forcepoint |language=en}}</ref>
The model describes communications from the physical implementation of transmitting [[bit]]s across a [[transmission medium]] to the highest-level representation of data of a [[distributed application]]. Each layer has well-defined functions and semantics and serves a class of functionality to the layer above it and is served by the layer below it. Established, well-known [[communication protocol]]s are decomposed in software development into the model's hierarchy of function calls.
The [[Internet protocol suite]] as defined in {{IETF RFC|1122}} and {{IETF RFC|1123}} is a model of networking developed contemporarily to the OSI model, and was funded primarily by the U.S. Department of Defense. It was the foundation for the development of the [[Internet]]. It assumed the presence of generic physical links and focused primarily on the software layers of communication, with a similar but much less rigorous structure than the OSI model.
In comparison, several networking models have sought to create an intellectual framework for clarifying networking concepts and activities,{{citation needed|date=April 2022}} but none have been as successful as the OSI reference model in becoming the standard model for discussing and teaching networking in the field of [[information technology]]. The model allows transparent communication through equivalent exchange of [[protocol data unit]]s (PDUs) between two parties, through what is known as [[peer-to-peer networking]] (also known as peer-to-peer communication). As a result, the OSI reference model has not only become an important piece among professionals and non-professionals alike, but also in all networking between one or many parties, due in large part to its commonly accepted user-friendly framework.<ref name="guide-to-networking-essentials">{{cite book|url=https://www.cengage.com/c/guide-to-networking-essentials-7e-tomsho/9781305105430PF/|title=Guide to Networking Essentials|edition=7th|first=Greg|last=Tomsho|publisher=Cengage|date=2016|access-date=2022-04-03}}</ref>
[[File:OSI-model-Communication.svg|thumb|400px|Communication in the OSI model (example with layers 3 to 5)]]
==History==
The development of the OSI model started in the late 1970s to support the emergence of the diverse computer networking methods that were competing for application in the large national networking efforts in the world (see [[OSI protocols]] and [[Protocol Wars]]). In the 1980s, the model became a working product of the Open Systems Interconnection group at the [[International Organization for Standardization]] (ISO). While attempting to provide a comprehensive description of networking, the model failed to garner reliance during the design of the [[Internet]], which is reflected in the less prescriptive [[Internet Protocol Suite]], principally sponsored under the auspices of the [[Internet Engineering Task Force]] (IETF).
In the early- and mid-1970s, networking was largely either government-sponsored ([[NPL network]] in the UK, [[ARPANET]] in the US, [[CYCLADES]] in France) or vendor-developed with proprietary standards, such as [[IBM]]'s [[Systems Network Architecture]] and [[Digital Equipment Corporation]]'s [[DECnet]]. [[Public data network]]s were only just beginning to emerge, and these began to use the [[X.25]] standard in the late 1970s.<ref name=":3" /><ref name=":2">{{cite journal|last1=Roberts|first1=Dr. Lawrence G.|date=November 1978|title=The Evolution of Packet Switching|url=http://www.ece.ucf.edu/~yuksem/teaching/nae/reading/1978-roberts.pdf|journal=IEEE Invited Paper|access-date=February 26, 2022}}</ref>
The [[Packet switching#EPSS|Experimental Packet Switched System]] in the UK {{circa|1973}}–1975 identified the need for defining higher-level protocols.<ref name=":3">{{Cite book|last1=Davies|first1=Howard|url=https://books.google.com/books?id=DN-t8MpZ0-wC&pg=PA3|title=A History of International Research Networking: The People who Made it Happen|last2=Bressan|first2=Beatrice|date=2010-04-26|publisher=John Wiley & Sons|isbn=978-3-527-32710-2|pages=2–3|language=en}}</ref> The UK [[National Computing Centre]] publication, ''Why Distributed Computing'', which came from considerable research into future configurations for computer systems,<ref>{{Cite book|url=https://books.google.com/books?id=fbm7AAAAIAAJ|title=Why distributed computing?: An NCC review of potential and experience in the UK|last1=Down|first1=Peter John|last2=Taylor|first2=Frank Edward|date=1976|publisher=NCC Publications|isbn=9780850121704|language=en}}</ref> resulted in the UK presenting the case for an international standards committee to cover this area at the ISO meeting in [[Sydney]] in March 1977.<ref>{{cite book |last=Radu |first=Roxana |doi=10.1093/oso/9780198833079.003.0003 |chapter=Revisiting the Origins: The Internet and its Early Governance |title=Negotiating Internet Governance |isbn=9780191871405 |date=2019 |pages=43–74 |publisher=Oxford University Press}}</ref><ref name="ieee201703">{{cite magazine|author=Andrew L. Russell|date=30 July 2013|title=OSI: The Internet That Wasn't|url=https://spectrum.ieee.org/osi-the-internet-that-wasnt|magazine=[[IEEE Spectrum]]|volume=50|issue=8}}</ref>
Beginning in 1977, the ISO initiated a program to develop general standards and methods of networking. A similar process evolved at the [[International Telegraph and Telephone Consultative Committee]] (CCITT, from French: ''Comité Consultatif International Téléphonique et Télégraphique''). Both bodies developed documents that defined similar networking models. The British [[Department of Trade and Industry (United Kingdom)|Department of Trade and Industry]] acted as the secretariat, and [[universities in the United Kingdom]] developed [[prototype]]s of the standards.<ref>{{Cite journal |last1=Campbell-Kelly |first1=Martin |last2=Garcia-Swartz |first2=Daniel D |date=2013 |title=The History of the Internet: The Missing Narratives |url=https://papers.ssrn.com/abstract=867087 |journal=Journal of Information Technology |language=en |volume=28 |issue=1 |pages=18–33 |doi=10.1057/jit.2013.4 |ssrn=867087 |s2cid=41013 |issn=0268-3962|url-access=subscription }}</ref>
The OSI model was first defined in raw form in [[Washington, D.C.]], in February 1978 by French software engineer [[Hubert Zimmermann]], and the refined but still draft standard was published by the ISO in 1980.<ref name="ieee201703" />
The drafters of the reference model had to contend with many competing priorities and interests. The rate of technological change made it necessary to define standards that new systems could converge to rather than standardizing procedures after the fact; the reverse of the traditional approach to developing standards.<ref>{{Cite book|last=Sunshine|first=Carl A.|url=https://books.google.com/books?id=Hzj2BwAAQBAJ&pg=PA35|title=Computer Network Architectures and Protocols|publisher=Springer Science & Business Media|year=1989|isbn=978-1-4613-0809-6|pages=35|language=en}}</ref> Although not a standard itself, it was a framework in which future standards could be defined.<ref>{{Cite book|last=Hasman|first=A.|url=https://books.google.com/books?id=u2KyQjiwwTUC&pg=PA251|title=Education and Training in Health Informatics in Europe: State of the Art, Guidelines, Applications|date=1995|publisher=IOS Press|isbn=978-90-5199-234-2|pages=251|language=en}}</ref>
In May 1983,<ref>{{Cite web |title=ISO/OSI (Open Systems Interconnection): 1982 - 1983 {{!}} History of Computer Communications |url=https://historyofcomputercommunications.info/section/11.10/ISO-OSI-(Open-Systems-Interconnection)-1982-1983/ |access-date=2024-07-12 |website=historyofcomputercommunications.info}}</ref> the CCITT and ISO documents were merged to form ''The Basic Reference Model for Open Systems Interconnection'', usually referred to as the ''Open Systems Interconnection Reference Model'', ''OSI Reference Model'', or simply ''OSI model''. It was published in 1984 by both the ISO, as standard ISO 7498, and the renamed CCITT (now called the Telecommunications Standardization Sector of the [[International Telecommunication Union]] or [[ITU-T]]) as standard X.200.
OSI had two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a [[OSI protocols|set of specific protocols]]. The OSI reference model was a major advance in the [[standardisation]] of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software.
The concept of a seven-layer model was provided by the work of [[Charles Bachman]] at [[Honeywell Information Systems]].<ref>{{cite web|url=https://history.computer.org/pioneers/bachman.html|author=J. A. N. Lee|title=Computer Pioneers by J. A. N. Lee|publisher=IEEE Computer Society}}</ref> Various aspects of OSI design evolved from experiences with the NPL network, ARPANET, CYCLADES, [[Packet switching#EIN|EIN]], and the [[International Network Working Group]] ([[IFIP]] WG6.1). In this model, a networking system was divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacted directly only with the layer immediately beneath it and provided facilities for use by the layer above it.
The OSI standards documents are available from the ITU-T as the X.200 series of recommendations.<ref>
{{cite web| url = http://www.itu.int/rec/T-REC-X/en| title = ITU-T X-Series Recommendations}}</ref> Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO/IEC standards for the OSI model were available from ISO. Not all are free of charge.<ref>{{cite web|url=http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html |title=Publicly Available Standards |publisher=Standards.iso.org |date=30 July 2010 |access-date=11 September 2010}}</ref>
OSI was an industry effort, attempting to get industry participants to agree on common network standards to provide multi-vendor interoperability.<ref>{{Cite book|url=https://books.google.com/books?id=OVpzAwAAQBAJ&q=Open+Standards+and+the+Digital+Age:+History,+Ideology,+and+Networks|title=Open Standards and the Digital Age: History, Ideology, and Networks|last=Russell|first=Andrew L.|date=2014-04-28|publisher=Cambridge University Press|isbn=978-1-139-91661-5|language=en}}</ref> It was common for large networks to support multiple network protocol suites, with many devices unable to interoperate with other devices because of a lack of common protocols. For a period in the late 1980s and early 1990s, engineers, organizations and nations became [[Protocol Wars|polarized over the issue of which standard]], the OSI model or the [[Internet protocol suite]], would result in the best and most robust computer networks.<ref name="ieee201703" /><ref>{{Cite journal|url=https://courses.cs.duke.edu//common/compsci092/papers/govern/consensus.pdf|title=Rough Consensus and Running Code' and the Internet-OSI Standards War|last=Russell|first=Andrew L.|journal=IEEE Annals of the History of Computing|volume=28|issue=3|date=July–September 2006|pages=48–61|doi=10.1109/MAHC.2006.42}}</ref><ref>{{Cite web|date=2006|title=Standards Wars|url=https://courses.cs.washington.edu/courses/csep590a/06au/projects/standards-wars.pdf}}</ref> However, while OSI developed its networking standards in the late 1980s,<ref>{{Cite book|url=https://books.google.com/books?id=CRMEAAAAMBAJ|title=Network World|date=1988-02-15|publisher=IDG Network World Inc|language=en}}</ref>{{page needed|date=July 2024}}<ref>{{Cite book|url=https://books.google.com/books?id=dBMEAAAAMBAJ&pg=PA50|title=Network World|date=1988-10-10|publisher=IDG Network World Inc|language=en}}</ref>{{page needed|date=July 2024}} [[TCP/IP]] came into widespread use on multi-vendor networks for [[internetworking]].
The OSI model is still used as a reference for teaching and documentation;<ref>{{Cite web|title=The OSI model explained: How to understand (and remember) the 7 layer network model|url=https://www.networkworld.com/article/3239677/the-osi-model-explained-how-to-understand-and-remember-the-7-layer-network-model.html|last=Shaw|first=Keith|date=2018-10-22|website=Network World|language=en|access-date=2020-05-16|archive-date=4 October 2020|archive-url=https://web.archive.org/web/20201004172259/https://www.networkworld.com/article/3239677/the-osi-model-explained-how-to-understand-and-remember-the-7-layer-network-model.html|url-status=dead}}</ref> however, the [[OSI protocols]] originally conceived for the model did not gain popularity. Some engineers argue the OSI reference model is still relevant to [[cloud computing]].<ref>{{Cite web|title=An OSI Model for Cloud|url=https://blogs.cisco.com/cloud/an-osi-model-for-cloud|date=2017-02-24|website=Cisco Blogs|language=en-US|access-date=2020-05-16}}</ref> Others say the original OSI model does not fit today's networking protocols and have suggested instead a simplified approach.<ref>{{Cite web|title=Why it's time to let the OSI model die|url=https://www.networkworld.com/article/2276158/why-it-s-time-to-let-the-osi-model-die.html|last1=Taylor|first1=Steve|last2=Metzler|first2=Jim|date=2008-09-23|website=Network World|language=en|access-date=2020-05-16}}</ref><ref name="Crawford"/>
==Definitions==
{{More citations needed section|date=November 2019}}
[[Communication protocol]]s enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions, like the OSI model, abstractly describe the functionality provided to a layer ''N'' by a layer ''N−1'', where ''N'' is one of the seven layers of protocols operating in the local host.
At each level ''N'', two entities at the communicating devices (layer N ''peers'') exchange [[protocol data unit]]s (PDUs) by means of a layer N ''protocol''. Each PDU contains a payload, called the [[service data unit]] (SDU), along with protocol-related headers or footers.
Data processing by two communicating OSI-compatible devices proceeds as follows:
# The data to be transmitted is composed at the topmost layer of the transmitting device (layer ''N'') into a ''[[protocol data unit]]'' (''PDU'').
# The ''PDU'' is passed to layer ''N−1'', where it is known as the ''[[service data unit]]'' (''SDU'').
# At layer ''N−1'' the ''SDU'' is [[concatenated]] with a header, a footer, or both, producing a ''layer N−1 PDU''. It is then passed to layer ''N−2''.
# The process continues until reaching the lowermost level, from which the data is transmitted to the receiving device.
# At the receiving device the data is passed from the lowest to the highest layer as a series of ''SDU''s while being successively stripped from each layer's header or footer until reaching the topmost layer, where the last of the data is consumed.
===
The OSI model was defined in ISO/IEC 7498 which consists of the following parts:
*ISO/IEC 7498-1 The Basic Model
*ISO/IEC 7498-2 Security Architecture
*ISO/IEC 7498-3 Naming and addressing
*ISO/IEC 7498-4 Management framework
ISO/IEC 7498-1 is also published as ITU-T Recommendation X.200.
==Layer architecture==
The recommendation X.200 describes seven layers, labelled 1 to 7. Layer 1 is the lowest layer in this model.
{{OSI model table}}
===Layer
{{main|Physical layer}}
The physical layer is responsible for the transmission and reception of unstructured raw data between a device, such as a [[network interface controller]], [[Ethernet hub]], or [[network switch]], and a physical [[transmission medium]]. It converts the digital bits into electrical, radio, or optical signals (analogue signals). Layer specifications define characteristics such as voltage levels, the timing of voltage changes, physical data rates, maximum transmission distances, modulation scheme, channel access method and physical connectors. This includes the layout of [[Lead (electronics)|pins]], [[voltage]]s, line [[Characteristic impedance|impedance]], cable specifications, signal timing and frequency for wireless devices. Bit rate control is done at the physical layer and may define transmission mode as [[Simplex communication|simplex]], [[half duplex]], and [[full duplex]]. The components of a physical layer can be described in terms of the [[network topology]]. Physical layer specifications are included in the specifications for the ubiquitous [[Bluetooth]], [[Ethernet physical layer|Ethernet]], and [[USB]] standards. An example of a less well-known physical layer specification would be for the [[CAN bus|CAN]] standard.
The physical layer also specifies how encoding occurs over a physical signal, such as electrical voltage or a light pulse. For example, a 1 bit might be represented on a copper wire by the transition from a 0-volt to a 5-volt signal, whereas a 0 bit might be represented by the transition from a 5-volt to a 0-volt signal. As a result, common problems occurring at the physical layer are often related to the incorrect media termination, EMI or noise scrambling, and NICs and hubs that are misconfigured or do not work correctly.
===Layer 2: Data link layer===
{{main|Data link layer}}
The data link layer provides [[node-to-node data transfer]]—a link between two directly connected nodes. It detects and possibly corrects errors that may occur in the physical layer. It defines the protocol to establish and terminate a connection between two physically connected devices. It also defines the protocol for [[flow control (data)|flow control]] between them.
[[IEEE 802]] divides the data link layer into two sublayers:<ref>{{cite book|title=IEEE Std 802-2014, ''IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture''|publisher=ieee|section=5.2 RM description for end stations|doi=10.1109/IEEESTD.2014.6847097|isbn=978-0-7381-9219-2}}</ref>
* [[Medium access control]] (MAC) layer – responsible for controlling how devices in a network gain access to a medium and permission to transmit data.
* [[Logical link control]] (LLC) layer – responsible for identifying and encapsulating network layer protocols, and controls error checking and frame synchronization.
The MAC and LLC layers of IEEE 802 networks such as [[802.3]] [[Ethernet]], [[802.11]] [[Wi-Fi]], and [[802.15.4]] [[Zigbee]] operate at the data link layer.
The [[Point-to-Point Protocol]] (PPP) is a data link layer protocol that can operate over several different physical layers, such as [[synchronous serial communication|synchronous]] and [[asynchronous serial communication|asynchronous]] serial lines.
The [[ITU-T]] [[G.hn]] standard, which provides high-speed local area networking over existing wires (power lines, phone lines and coaxial cables), includes a complete data link layer that provides both [[error correction]] and flow control by means of a [[selective repeat|selective-repeat]] [[sliding window protocol|sliding-window protocol]].
Security, specifically (authenticated) encryption, at this layer can be applied with [[MACsec]].
===Layer 3: Network layer===
{{main|Network layer}}
The network layer provides the functional and procedural means of transferring [[network packet|packets]] from one node to another connected in "different networks". A network is a medium to which many nodes can be connected, on which every node has an ''address'' and which permits nodes connected to it to transfer messages to other nodes connected to it by merely providing the content of a message and the address of the destination node and letting the network find the way to deliver the message to the destination node, possibly [[routing]] it through intermediate nodes. If the message is too large to be transmitted from one node to another on the data link layer between those nodes, the network may implement message delivery by splitting the message into several fragments at one node, sending the fragments independently, and reassembling the fragments at another node. It may, but does not need to, report delivery errors.
Message delivery at the network layer is not necessarily guaranteed to be reliable; a network layer protocol may provide reliable message delivery, but it does not need to do so.
A number of layer-management protocols, a function defined in the ''management annex'', ISO 7498/4, belong to the network layer. These include routing protocols, multicast group management, network-layer information and error, and network-layer address assignment. It is the function of the payload that makes these belong to the network layer, not the protocol that carries them.<ref>{{cite ISO standard|csnumber = 14258|title = ISO/IEC 7498-4:1989 – Information technology – Open Systems Interconnection – Basic Reference Model: Management framework|date = 15 November 1989|access-date = 16 November 2024|website = ISO Standards Maintenance Portal|publisher = ISO Central Secretariat.|author = International Organization for Standardization|author-link = International Organization for Standardization}}</ref>
===Layer 4: Transport layer===
{{main|Transport layer}}
The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source host to a destination host from one application to another across a network while maintaining the quality-of-service functions. Transport protocols may be connection-oriented or connectionless.
This may require breaking large protocol data units or long data streams into smaller chunks called "segments", since the network layer imposes a maximum packet size called the [[maximum transmission unit]] (MTU), which depends on the maximum packet size imposed by all data link layers on the network path between the two hosts. The amount of data in a data segment must be small enough to allow for a network-layer header and a transport-layer header. For example, for data being transferred across [[Ethernet]], the MTU is 1500 bytes, the minimum size of a TCP header is 20 bytes, and the minimum size of an IPv4 header is 20 bytes, so the maximum segment size is 1500−(20+20) bytes, or 1460 bytes. The process of dividing data into segments is called [[packet segmentation|segmentation]]; it is an optional function of the transport layer. Some connection-oriented transport protocols, such as [[Transmission Control Protocol|TCP]] and the OSI connection-oriented transport protocol (COTP), perform segmentation and reassembly of segments on the receiving side; connectionless transport protocols, such as [[User Datagram Protocol|UDP]] and the OSI connectionless transport protocol (CLTP), usually do not.
The transport layer also controls the reliability of a given link between a source and destination host through flow control, error control, and acknowledgments of sequence and existence. Some protocols are state- and [[connection-oriented]]. This means that the transport layer can keep track of the segments and retransmit those that fail delivery through the acknowledgment hand-shake system. The transport layer will also provide the acknowledgement of the successful data transmission and sends the next data if no errors occurred.
Reliability, however, is not a strict requirement within the transport layer. Protocols like UDP, for example, are used in applications that are willing to accept some packet loss, reordering, errors or duplication. [[Streaming media]], real-time multiplayer games and [[voice over IP]] (VoIP) are examples of applications in which loss of packets is not usually a fatal problem.
The OSI connection-oriented transport protocol defines five classes of connection-mode transport protocols, ranging from class 0 (which is also known as TP0 and provides the fewest features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the session layer. Also, all OSI TP connection-mode [[Communication protocol|protocol]] classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0–4 classes are shown in the following table:<ref>{{cite web |title=ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073, ''Open Systems Interconnection – Protocol for providing the connection-mode transport service'' |publisher=ITU|url=http://www.itu.int/rec/T-REC-X.224-199511-I/en/}}</ref>
{| class="wikitable" style="margin:0 auto;"
! Feature name
! TP0 !! TP1 !! TP2 !! TP3 !! TP4
|-
| Connection-oriented network
| {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Connectionless network
| {{No}} || {{No}} || {{No}} || {{No}} || {{Yes}}
|-
| Concatenation and separation
| {{No}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Segmentation and reassembly
| {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Error recovery
| {{No}} || {{Yes}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Reinitiate connection
| {{No}} || {{Yes}} || {{No}} || {{Yes}} || {{No}}
|-
|style="padding-right:1.0em;"| Multiplexing
| {{No}} || {{No}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Explicit flow control
| {{No}} || {{No}} || {{Yes}} || {{Yes}} || {{Yes}}
|-
| Retransmission on timeout
| {{No}} || {{No}} || {{No}} || {{No}} || {{Yes}}
|-
| Reliable transport service
| {{No}} || {{Yes}} || {{No}} || {{Yes}} || {{Yes}}
|-
|colspan="6" style="font-size:90%;"| {{sup|a}} If an excessive number of [[Protocol data unit|PDUs]] are unacknowledged.
|}
An easy way to visualize the transport layer is to compare it with a post office, which deals with the dispatch and classification of mail and parcels sent.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the transport layer, the [[Transmission Control Protocol]] (TCP) and the [[User Datagram Protocol]] (UDP) of the Internet Protocol Suite are commonly categorized as layer
[[Transport Layer Security]] (TLS) does not strictly fit inside the model either. It contains characteristics of the transport and presentation layers.<ref name="ccnp">{{cite book
|last1 = Hooper
|first1 = Howard
|title = CCNP Security VPN 642-648 Official Cert Guide
|date = 2012
|publisher = Cisco Press
|isbn = 9780132966382
|page = 22
|edition = 2
|url = https://books.google.com/books?id=5PJisOKJ0k8C&pg=PA22
}}</ref><ref name="stackex_layer">{{cite web|url=https://security.stackexchange.com/a/93338|title=What layer is TLS?|website=Information Security Stack Exchange|first1=Andrew|last1=Spott|first2=Tom |last2=Leek|display-authors=etal}}</ref>
===Layer
{{main|Session layer}}
The session layer creates the setup, controls the connections, and ends the [[teardown (communications)|teardown]], between two or more computers, which is called a "session". Common functions of the session layer include user logon (establishment) and user logoff (termination) functions. Including this matter, authentication methods are also built into most client software, such as FTP Client and NFS Client for Microsoft Networks. Therefore, the session layer establishes, manages and terminates the connections between the local and remote applications. The session layer also provides for [[full-duplex]], [[half-duplex]], or [[Simplex communication|simplex]] operation,{{citation needed|date=April 2024}} and establishes procedures for checkpointing, suspending, restarting, and terminating a session between two related streams of data, such as an audio and a video stream in a web-conferencing application. Therefore, the session layer is commonly implemented explicitly in application environments that use [[remote procedure call]]s.
===Layer 6: Presentation layer===
{{main|Presentation layer}}
The presentation layer establishes data formatting and data translation into a format specified by the application layer during the encapsulation of outgoing messages while being passed down the [[protocol stack]], and possibly reversed during the deencapsulation of incoming messages when being passed up the protocol stack. For this very reason, outgoing messages during encapsulation are converted into a format specified by the application layer, while the conversion for incoming messages during deencapsulation are reversed.
The
===Layer 7:
{{main|Application layer}}
The application layer is the layer of the OSI model that is closest to the end user, which means both the OSI application layer and the user interact directly with a software application that implements a component of communication between the client and server, such as [[File Explorer]] and [[Microsoft Word]]. Such application programs fall outside the scope of the OSI model unless they are directly integrated into the application layer through the functions of communication, as is the case with applications such as [[web browser]]s and [[email program]]s. Other examples of software are Microsoft Network Software for File and Printer Sharing and Unix/Linux Network File System Client for access to shared file resources.
Application-layer functions typically include file sharing, message handling, and database access, through the most common protocols at the application layer, known as HTTP, FTP, SMB/CIFS, TFTP, and SMTP. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. The most important distinction in the application layer is the distinction between the application entity and the application. For example, a reservation website might have two application entities: one using [[HTTP]] to communicate with its users, and one for a remote database protocol to record reservations. Neither of these protocols have anything to do with reservations. That logic is in the application itself. The application layer has no means to determine the availability of resources in the network.<ref name="guide-to-networking-essentials" />
==Cross-layer functions==
{{Further|Cross-layer optimization}}
<!--Portions of this section have been duplicated in an external source after their placement here. Please see note towards the bottom of the top of the talk page.-->
Cross-layer functions are services that are not tied to a given layer, but may affect more than one layer.<ref name="Mao 2009">{{cite book |last1=Mao |first1=Stephen |editor-first1=Alexander |chapter=Chapter 8: Fundamentals of communication networks |editor-last1=Wyglinski |editor-first2=Maziar |editor-last2=Nekovee |editor-first3=Thomas |editor-last3=Hou |title=Cognitive Radio Communications and Networks: Principles and Practice |year=2009 |publisher=Elsevier |page=[{{GBurl |id=d3HBCmFEmicC |pg=PA201}} 201] |isbn=978-0-08-087932-1 |id={{OCLC|635292718|528550718}}}} [{{GBurl |id=d3HBCmFEmicC}} Partial preview] at [[Google Books]].</ref> Some orthogonal aspects, such as management and [[Security service (telecommunication)|security]], involve all of the layers (See [[ITU-T]] X.800 Recommendation<ref name="x800"/>). These services are aimed at improving the [[Information security|CIA triad]]—[[confidentiality]], [[data integrity|integrity]], and [[availability]]—of the transmitted data.
Cross-layer functions are the norm, in practice, because the availability of a communication service is determined by the interaction between [[network design]] and [[network management]] protocols.
Specific examples of cross-layer functions include the following:
* [[Security service (telecommunication)]]<ref name=x800>{{cite web|url=http://www.itu.int/rec/T-REC-X.800-199103-I/e|title=ITU-T Recommendation X.800 (03/91), ''Security architecture for Open Systems Interconnection for CCITT applications''|publisher=ITU|access-date=14 August 2015}}</ref> as defined by [[ITU-T]] X.800 recommendation.
* Management functions, i.e. functions that permit to configure, instantiate, monitor, terminate the communications of two or more entities: there is a specific application-layer protocol, [[Common Management Information Protocol]] (CMIP) and its corresponding service, [[Common Management Information Service]] (CMIS), they need to interact with every layer in order to deal with their instances.
* [[Multiprotocol Label Switching]] (MPLS), ATM, and X.25 are 3a protocols. OSI subdivides the Network Layer into three sublayers: 3a) Subnetwork Access, 3b) Subnetwork Dependent Convergence and 3c) Subnetwork Independent Convergence.<ref name="Hegering, Abeck, & Neumair 1999">{{cite book |last1=Hegering |first1=Heinz-Gerd |last2=Abeck |first2=Sebastian |last3=Neumair |first3=Bernhard |chapter=Fundamental Structures of Networked Systems |chapter-url=https://archive.org/details/integratedmanage0000hege/page/12/mode/2up |chapter-url-access=registration |title=Integrated management of networked systems: concepts, architectures, and their operational application |url=https://archive.org/details/integratedmanage0000hege/page/12/mode/2up |url-access=registration |publisher=Morgan Kaufmann |publication-place=San Francisco, Calif. |year=1999 |isbn=978-1-55860-571-8 |oclc=1341886747 |page=[https://archive.org/details/integratedmanage0000hege/page/54/mode/2up 54] |via=Internet Archive}}</ref> It was designed to provide a unified data-carrying service for both circuit-based clients and packet-switching clients which provide a [[datagram]]-based service model. It can be used to carry many different kinds of traffic, including IP packets, as well as native ATM, SONET, and Ethernet frames. Sometimes one sees reference to a Layer 2.5.
* Cross MAC and PHY Scheduling is essential in wireless networks because of the time-varying nature of wireless channels. By scheduling packet transmission only in favourable channel conditions, which requires the MAC layer to obtain channel state information from the PHY layer, network throughput can be significantly improved and energy waste can be avoided.<ref name="Miao & Song 2014">{{cite book |last1=Miao |first1=Guowang |author1-link=Guowang Miao |last2=Song |first2=Guocong |title=Energy and spectrum efficient wireless network design |url=https://archive.org/details/energyspectrumef0000mia |url-access=registration |publisher=Cambridge University Press |publication-place=New York |year=2014 |isbn=978-1-139-62677-4 |oclc=898138775 |page= |via=Internet Archive}}</ref>{{page needed |date=July 2024}}
==Programming interfaces==
Neither the OSI Reference Model, nor any [[OSI protocols|OSI protocol]] specifications, outline any programming interfaces, other than deliberately abstract service descriptions. Protocol specifications define a methodology for communication between peers, but the software interfaces are implementation-specific.
For example, the [[Network Driver Interface Specification]] (NDIS) and [[Open Data-Link Interface]] (ODI) are interfaces between the media (layer 2) and the network protocol (layer 3).
==Comparison to other networking suites==
The table below presents a list of OSI layers, the original OSI protocols, and some ''approximate'' modern matches. This correspondence is rough: the OSI model contains idiosyncrasies not found in later systems such as the IP stack in modern Internet.<ref name="Crawford">{{cite web |last1=Crawford |first1=JB |title=The actual OSI model |url=https://computer.rip/2021-03-27-the-actual-osi-model.html |date=2021-03-27}}</ref>
{| class="wikitable nowraplinks" style="line-height:1.3em;"
!colspan="2"| Layer
!rowspan="2" style="width:16.0em;"| [[Open Systems Interconnection|OSI]] protocols
!rowspan="2"| [[Internet Protocol Suite|TCP/IP protocols]]
!rowspan="2" style="width:9.0em;"| [[Signaling System 7|Signaling<br />System 7]]<ref>
!rowspan="2"| [[AppleTalk]]
!rowspan="2"| [[IPX]]
!rowspan="2"| [[Systems Network Architecture|SNA]]
!rowspan="2"| [[UMTS]]
!rowspan="2"|
!rowspan="2"| Miscellaneous examples
|-
! No. !! Name
|-
|style="text-align:center;"| 7
| Application
| {{hlist |[[FTAM]]
|date=1995-04-10 |url=https://www.itu.int/itu-t/recommendations/rec.aspx?rec=X.227 |access-date=2024-07-12}}</ref><ref name="ITU-T X.217">{{cite web |title=ITU-T X.217 |website=Open Systems Interconnection |date=1995-04-10 |url=https://www.itu.int/rec/T-REC-X.217-199504-I |access-date=2024-07-12}}</ref> |[[Common management information protocol|CMIP]]<ref name="X.700">{{cite web |title=X.700: Management framework for Open Systems Interconnection (OSI) for CCITT applications |website=ITU |date=1992-09-10 |url=https://www.itu.int/rec/T-REC-X.700-199209-I |access-date=2024-07-12}}</ref><ref name="X.711">{{cite web |title=X.711 |website=Open Systems Interconnection |date=2014-05-15 |url=https://www.itu.int/rec/T-REC-X.711/en |access-date=2024-07-12}}</ref><ref name="9596-1:1998(en)">{{cite web |title=ISO/IEC 9596-1:1998(en) |website=ISO |url=https://www.iso.org/obp/ui/#iso:std:iso-iec:9596:-1:ed-3:v1:en |access-date=2024-07-12}}</ref><ref name="9596-2:1993(en)">{{cite web |title=ISO/IEC 9596-2:1993(en) |website=ISO |url=https://www.iso.org/obp/ui/#iso:std:iso-iec:9596:-2:ed-1:v1:en |access-date=2024-07-12}}</ref>}}
| {{hlist |[[HTTP]]| [[HTTPS]] |[[FTP]] |[[SMTP]]}}
| {{hlist |[[INAP]] |[[Mobile Application Part|MAP]] |[[Transaction Capabilities Application Part|TCAP]] |[[ISDN User Part|ISUP]] |[[Telephone User Part|TUP]]}}
| {{hlist |[[Apple Filing Protocol|AFP]] |[[Zone Information Protocol|ZIP]] |[[Routing Table Maintenance Protocol|RTMP]] |[[AppleTalk#Name Binding Protocol|NBP]]}}
| {{hlist |[[Service Advertising Protocol|SAP]]}}
| {{hlist |Transaction Services |Presentation Services}}
|
| {{hlist |[[Web Browser]] |[[WebDAV]] |[[SOAP]] |[[OpenID]] |[[REST API]] |[[OAuth]] |[[GraphQL]] |[[ActivityPub]] |[[Git]] |[[DNS over HTTP]]}}
| {{hlist |[[HL7]] |[[Modbus]]|[[WebSocket]]|[[CoAP]]}}
|-
|style="text-align:center;"| 6
| Presentation
| {{hlist |ISO/IEC 8823
| {{hlist |[[MIME]]
|
| {{hlist |[[Apple Filing Protocol|AFP]]}}
|
| Presentation Services
|
| {{hlist |[[XML]] |[[JSON]] |[[MIME]] |[[Percent-encoding]] |[[base64]] |[[gzip]] |[[brotli]]}}
| {{hlist |[[Tabbed Document Interface|TDI]]
|-
|style="text-align:center;"| 5
| Session
| {{hlist |ISO/IEC 8327
| [[Network socket|Sockets
|
| {{hlist |[[AppleTalk#AppleTalk Session Protocol|ASP]]
| {{hlist |[[NWLink]]}}
|
|
| {{hlist |HTTP [[List of HTTP header fields|headers]], including [[HTTP cookie|cookies]] |[[CSRF token]]s |[[WebSocket]]}}
| {{hlist |[[Named pipes]] |[[NetBIOS]] |[[Session Announcement Protocol|SAP]] |[[Remote Procedure Call|RPC]] |[[SOCKS]]}}
|-
|style="text-align:center;"| 4
| Transport
| {{hlist |ISO/IEC 8073
| {{hlist |[[Transmission Control Protocol|TCP]]
|
| {{hlist |[[Datagram Delivery Protocol|DDP]]}}
| {{hlist |[[Sequenced packet exchange|SPX]]}}
| {{hlist|Data Flow Control |Transmission Control}}
|
| {{hlist |[[HTTP]]/[[HTTP/2]]/[[HTTP/3]] |[[WebSocket]] |[[DTLS]]}} Port number can be specified.
| {{hlist |[[NetBIOS Frames|NBF]]}}
|-
|style="text-align:center;"| 3
| Network
| {{hlist |ISO/IEC 8208
| {{hlist |[[Internet Protocol|IP]]
| {{hlist |[[Signalling Connection Control Part|SCCP]]
| [[AppleTalk#AppleTalk Transaction Protocol|ATP]] {{small|([[TokenTalk]]
| {{hlist |[[IPX]]}}
| {{hlist |Transmission Control |Path Control}}
| {{hlist |[[Radio Resource Control|RRC]]
| Out of scope. [[IP address]]es can be used instead of [[___domain names]] in [[URL]]s.
| {{hlist |[[NetBIOS Frames|NBF]] |[[Q.931]] }}
|-
|style="text-align:center;"| 2
| Data link
| {{hlist |ISO/IEC 7666 |[[X.25]] ([[LAPB]]) |<br />[[Token Bus]] |X.222 |ISO/IEC 8802-2 |[[IEEE 802.2|LLC]] (type 1{{\}}2)<ref name=Cisco>{{cite web|url=http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html#wp1020669|title=Internetworking Technology Handbook – Internetworking Basics [Internetworking]|date=15 January 2014|work=Cisco|access-date=14 August 2015}}</ref>}}
| {{hlist |[[Point-to-Point Protocol|PPP]] |[[SBTV]] |[[Serial Line Internet Protocol|SLIP]]}}
| {{hlist |[[Message Transfer Part|MTP]] |[[Message Transfer Part|Q.710]]}}
| {{hlist |[[LocalTalk]] |[[AppleTalk Remote Access|ARA]] |[[Point-to-Point Protocol|PPP]]}}
| [[IEEE 802.3|IEEE 802.3 framing]]<br />[[Ethernet II framing]]
| {{hlist |Path Control | [[Data Link Control]]}}
| {{hlist |[[PDCP]]<ref name="TS36300">{{cite web|url=http://www.3gpp.org/ftp/Specs/html-info/36300.htm|title=3GPP specification: 36.300|work=3gpp.org|access-date=14 August 2015}}</ref> |[[Logical Link Control|LLC]] |[[Medium access control|MAC]]}}
| Out of scope.
| {{hlist |[[Address resolution protocol|ARP]] | [[Neighbor Discovery Protocol|NDP (Neighbor Discovery Protocol)]] | [[Automatic repeat-request|ARQ]] | [[Asynchronous Transfer Mode|ATM]] | [[Bit stuffing]] | [[Cisco Discovery Protocol|CDP]] | [[DOCSIS]] | [[FDDI]]| [[Foundry Discovery Protocol|FDP]] | [[Fibre Channel]] | [[Frame Relay]] | [[HDLC]] | [[Ethernet|IEEE 802.3]] [[Medium access control|MAC]] | [[IEEE 802.11]] [[Medium access control|MAC]] |[[IEEE 802.1Q]] | [[Cisco Inter-Switch Link|ISL]] | [[G.hn|ITU-T G.hn DLL]] | [[Link aggregation#Linux bonding driver|Linux interface bonding]] | [[Point-to-Point Protocol|PPP]] |Q.921 |[[Token Ring]]|[[Nortel Discovery Protocol|NDP (Nortel Discovery Protocol)]] |[[IS-IS]]}}
|-
|style="text-align:center;"| 1
| Physical
| {{hlist |[[X.25]] ([[X.21bis]]
| <small>TCP/IP stack does not care about the physical medium, as long as it provides a way to communicate octets</small>
| {{hlist |[[Message Transfer Part|MTP]]
| {{hlist |[[RS-232]]
|
| {{hlist |Physical}}
| [[Universal Mobile Telecommunications System#Air interfaces|UMTS
| Out of scope.
| {{hlist |[[RS-232]] |[[8P8C|RJ45 (8P8C)]] |[[ITU-T V.35|V.35]] |[[ITU-T V.34|V.34]] |[[I.430]] |[[I.431]] |[[T-carrier|T1]] |[[E-carrier|E1]] |[[IEEE 802.3 PHY]] ([[10BASE-T]] |[[100BASE-TX]] |[[1000BASE-T]]) |[[Plain old telephone service|POTS]] |[[SONET]] |[[Bluetooth]] |[[Synchronous digital hierarchy|SDH]] |[[DSL]] |[[IEEE 802.11|IEEE 802.11 PHY]] |[[G.hn|ITU-T G.hn PHY]] |[[DOCSIS]] |[[DWDM]] |[[Optical Transport Network|OTN]]}}
|}
===Comparison with TCP/IP model===
{{See also|Internet protocol suite#Comparison of TCP/IP and OSI layering}}
The design of protocols in the [[TCP/IP model]] of the Internet does not concern itself with strict hierarchical encapsulation and layering. {{IETF RFC|3439}} contains a section entitled "Layering [[considered harmful]]".<ref>{{cite IETF|rfc=3439|title=Some Internet Architectural Guidelines and Philosophy|section=3|sectionname=Layering Considered Harmful|date=December 2002|access-date=25 April 2022}}</ref> TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols: the scope of the software application; the host-to-host transport path; the internetworking range; and the scope of the direct links to other nodes on the local network.<ref>
{{cite book|title=The Illustrated Network: How TCP/IP Works in a Modern Network|author=Walter Goralski|year=2009|page=26|isbn=978-0123745415|publisher=[[Morgan Kaufmann]]|url=http://www.exa.unicen.edu.ar/catedras/comdat1/material/TP1-Ejercicio5-ingles.pdf}}</ref>
* The Internet [[application layer]]
*
* The
* The [[link layer]]
These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in
The [[OSI protocols|OSI protocol suite]] that was specified as part of the OSI project was considered by many as too complicated and inefficient, and to a large extent unimplementable.<ref name="Tanenbaum 2023">{{cite book |last=Tanenbaum |first=Andrew S. |author-link=Andrew S. Tanenbaum |title=Computer networks |publisher=Prentice Hall PTR |publication-place=Upper Saddle River, NJ |year=2003 |isbn=978-0-13-066102-9 |oclc=50166590 |page=}}</ref>{{page needed|date=July 2024}} Taking the "forklift upgrade" approach to networking, it specified eliminating all existing networking protocols and replacing them at all layers of the stack. This made implementation difficult and was resisted by many vendors and users with significant investments in other network technologies. In addition, the protocols included so many optional features that many vendors' implementations were not interoperable.<ref name="Tanenbaum 2023"/>{{page needed|date=July 2024}}
Although the OSI model is often still referenced, the [[Internet protocol suite]] has become the standard for networking. TCP/IP's pragmatic approach to computer networking and to independent implementations of simplified protocols made it a practical methodology.<ref name="Tanenbaum 2023"/>{{page needed|date=July 2024}} Some protocols and specifications in the OSI stack remain in use, one example being [[IS-IS]], which was specified for OSI as ISO/IEC 10589:2002 and adapted for Internet use with TCP/IP as RFC 1142.<ref name="RFC 1142">{{cite IETF |rfc=1142 |title=OSI IS-IS Intra-___domain Routing Protocol |website=IETF Datatracker |access-date=2024-07-12}}</ref>
==See also==
{{div col}}
* [[Government Open Systems Interconnection Profile]]
* [[Hierarchical internetworking model]]
* [[
* [[Layer 8]]
* [[List of information technology initialisms]]
* [[Management plane]]
* [[Recursive Internetwork Architecture]]
* [[Service layer]]
* [[
{{div col end}}
==References==
{{Reflist
==Further reading==
*{{cite book |last=Day |first=John D. |author-link=John Day (computer scientist) |title=Patterns in Network Architecture: A Return to Fundamentals |publisher=Pearson Education |publication-place=Upper Saddle River, N.J. |year=2008 |isbn=978-0-13-225242-3 |oclc=213482801}}
*{{cite book |last=Dickson |first=Gary |last2=Lloyd |first2=Alan |title=Open Systems Interconnection |publisher=Prentice Hall |publication-place=New York |year=1992 |isbn=978-0-13-640111-7 |oclc=1245634475 |url=https://archive.org/details/opensystemsinter0000dick |url-access=registration |via=Internet Archive}}
*{{cite book |last=Piscitello |first=David M. |last2=Chapin |first2=A. Lyman |title=Open systems networking : TCP/IP and OSI |publisher=Addison-Wesley Pub. Co. |publication-place=Reading, Mass. |year=1993 |isbn=978-0-201-56334-4 |oclc=624431223 |url=https://archive.org/details/opensystemsnetwo0000pisc |url-access=registration |via=Internet Archive}}
*{{cite book |last=Rose |first=Marshall T. |author-link=Marshall Rose |title=The Open Book: A Practical Perspective on OSI |publisher=Prentice Hall |publication-place=Englewood Cliffs, N.J. |year=1990 |isbn=978-0-13-643016-2 |oclc=1415988401 |url=https://archive.org/details/openbookpractica00rose |url-access=registration |via=Internet Archive}}
*{{cite book |last=Russell |first=Andrew L. |title=Open Standards and the Digital Age: History, Ideology, and Networks |date=2014 |publisher=Cambridge University Press |isbn=978-1-139-91661-5 |oclc=881237495}} [{{GBurl |id=OVpzAwAAQBAJ}} Partial preview] at [[Google Books]].
* {{cite journal |last=Zimmermann |first=Hubert |title=OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection |journal=IEEE Transactions on Communications |volume=28 |issue=4 | date=April 1980 |pages=425–432 |doi=10.1109/TCOM.1980.1094702 |citeseerx=10.1.1.136.9497 |s2cid=16013989 |issn=0090-6778 |oclc=5858668034}}
==External links==
{{Sister project links |wikt= |c= |n=no |q=no |s=no |b= |v=}}
*{{cite web |title=Windows network architecture and the OSI model |website=Microsoft Learn |date=2024-02-02 |url=https://learn.microsoft.com/en-us/windows-hardware/drivers/network/windows-network-architecture-and-the-osi-model |access-date=2024-07-12}}
*{{cite ISO standard |csnumber=20269 |title=ISO/IEC standard 7498-1:1994 - Service definition for the association control service element |website=ISO Standards Maintenance Portal |access-date=2024-07-12}} ([[Portable Document Format|PDF document]] inside [[ZIP archive]]) (requires [[HTTP cookies]] in order to accept licence agreement)
*{{cite web |url=http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.200-199407-I!!PDF-E&type=items |title=ITU Recommendation X.200 |website=International Telecommunication Union |date=1998-06-02 |access-date=2024-07-12}}
*{{cite web |url=https://infchg.appspot.com/usr?at=1263939371 |url-status=dead |archive-url=https://web.archive.org/web/20120526075133/http://infchg.appspot.com/usr?at=1263939371 |archive-date=26 May 2012 |title=INFormation CHanGe Architectures and Flow Charts powered by Google App Engine |website=infchg.appspot.com}}
*{{cite web |title=Internetworking Technology Handbook |website=docwiki.cisco.com |date=2015-07-10 |url=http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook |archive-url=https://web.archive.org/web/20150906053851/http://docwiki.cisco.com/wiki/Internetworking_Technology_Handbook |archive-date=2015-09-06 |url-status=dead}}
*{{cite web |last=EdXD |last2=Saikot |first2=Mahmud Hasan |title=7 Layers of OSI Model Explained |website=ByteXD |date=2021-11-25 |url=https://bytexd.com/osi-model/ |access-date=2024-07-12}}
{{ISO standards}}
{{Authority control}}
{{DEFAULTSORT:Osi Model}}
[[Category:OSI model| ]]
[[Category:Computer-related introductions in 1977]]
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