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OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that characterizes and standardizes the communication functions of a telecommunication or computing system without regard to their underlying internal structure and technology. Its goal is the interoperability of diverse communication systems with standard protocols. The model partitions a communication system into abstraction layers. The original version of the model defined seven layers. Systems Interconnection. The standard is usually referred to as the Open Systems Interconnection Reference Model, the OSI Reference Model, or simply the 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 set of specific protocols. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides errorfree communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that comprise the contents of that path. Two instances at the same layer are visualized as connected by a horizontal connection in that layer. The concept of a seven-layer model was provided by the work of Charles Bachman at Honeywell Information Services. Various aspects of OSI design evolved from experiences with the ARPANET, NPLNET, EIN, CYCLADES network and the work in IFIP WG6.1. The new design was documented in ISO 7498 and its various The model is a product of the Open Systems Inter- addenda. In this model, a networking system was divided connection project at the International Organization for into layers. Within each layer, one or more entities imStandardization (ISO), maintained by the identification plement its functionality. Each entity interacted directly only with the layer immediately beneath it, and provided ISO/IEC 7498-1. facilities for use by the layer above it. Protocols enable an entity in one host to interact with a corresponding entity at the same layer in another host. Service definitions abstractly described the functionality provided to an (N)-layer by an (N-1) layer, where N was one of the seven layers of protocols operating in the local host. The OSI standards documents are available from the ITUT as the X.200-series of recommendations.[1] Some of the protocol specifications were also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model were available from ISO, but only some of them without fees.[2] Communication in the OSI-Model (example with layers 3 to 5) 1 2 Description of OSI layers History The recommendation X.200 describes seven layers, labeled 1 to 7. Layer 1 is the lowest layer in this model. In the late 1970s, one project was administered by the International Organization for Standardization (ISO), while another was undertaken by the International Telegraph and Telephone Consultative Committee, or CCITT (the abbreviation is from the French version of the name). These two international standards bodies each developed a document that defined similar networking models. At each level N, two entities at the communicating devices (layer N peers) exchange protocol data units (PDUs) by means of a layer N protocol. Each PDU contains a payload, called the service data unit (SDU), along with protocol-related headers and/or footers. In 1983, these two documents were merged to form a Data processing by two communicating OSI-compatible standard called The Basic Reference Model for Open devices is done as such: 1 2 2 DESCRIPTION OF OSI LAYERS 1. The data to be transmitted is composed at the top- 2.2 Layer 2: Data Link Layer most layer of the transmitting device (layer N) into The data link layer provides node-to-node data transfer— a protocol data unit (PDU). a link between two directly connected nodes. It detects 2. The PDU is passed to layer N-1, where it is known and possibly corrects errors that may occur in the physias the service data unit (SDU). cal layer. It, among other things, defines the protocol to establish and terminate a connection between two phys3. At layer N-1 the SDU is concatenated with a header, ically connected devices. It also defines the protocol for a footer, or both, producing a layer N-1 PDU. It is flow control between them. then passed to layer N-2. IEEE 802 divides the data link layer into two sublayers:[6] 4. The process continues until reaching the lowermost level, from which the data is transmitted to the re• Media Access Control (MAC) layer - responsible for ceiving device. controlling how devices in a network gain access to medium and permission to transmit it. 5. At the receiving device the data is passed from • Logical Link Control (LLC) layer - responsible for the lowest to the highest layer as a series of SDUs identifying Network layer protocols and then encapwhile being successively stripped from each layer’s sulating them and controls error checking and frame header and/or footer, until reaching the topmost synchronization. layer, where the last of the data is consumed. Some orthogonal aspects, such as management and security, involve all of the layers (See ITU-T X.800 Recommendation[5] ). These services are aimed at improving the CIA triad - confidentiality, integrity, and availability - of the transmitted data. In practice, the availability of a communication service is determined by the interaction between network design and network management protocols. Appropriate choices for both of these are needed to protect against denial of service. 2.1 Layer 1: Physical Layer The physical layer defines the electrical and physical specifications of the data connection. It defines the relationship between a device and a physical transmission medium (e.g., a copper or fiber optical cable, radio frequency). This includes the layout of pins, voltages, line impedance, cable specifications, signal timing and similar characteristics for connected devices and frequency (5 GHz or 2.4 GHz etc.) for wireless devices. It is responsible for transmission and reception of unstructured raw data in a physical medium. It may define transmission mode as simplex, half duplex, and full duplex. It defines the network topology as bus, mesh, or ring being some of the most common. The physical layer of Parallel SCSI operates in this layer, as do the physical layers of Ethernet and other local-area networks, such as Token Ring, FDDI, ITU-T G.hn, and IEEE 802.11 (Wi-Fi), as well as personal area networks such as Bluetooth and IEEE 802.15.4. 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 that can operate over several different physical layers, such as synchronous and 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 sliding-window protocol. 2.3 Layer 3: Network Layer The network layer provides the functional and procedural means of transferring variable length data sequences (called datagrams) from one node to another connected to the same network. It translates logical network address into physical machine address. 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 need not, report delivery errors. The physical layer is the layer of low-level networking equipment, such as some hubs, cabling, and repeaters. The physical layer is never concerned with protocols or other such higher-layer items. Examples of hardware in Message delivery at the network layer is not necessarily this layer are network adapters, repeaters, network hubs, guaranteed to be reliable; a network layer protocol may modems, and fiber media converters. provide reliable message delivery, but it need not do so. 2.5 Layer 5: Session Layer 3 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.[7] transport packet. 2.4 2.5 Layer 5: Session Layer Layer 4: Transport Layer The transport layer provides the functional and procedural means of transferring variable-length data sequences from a source to a destination host via one or more networks, while maintaining the quality of service functions. An example of a transport-layer protocol in the standard Internet stack is Transmission Control Protocol (TCP), usually built on top of the Internet Protocol (IP). The transport layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connectionoriented. This means that the transport layer can keep track of the segments and retransmit those that fail. The transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred. The transport layer creates packets out of the message received from the application layer. Packetizing is a process of dividing the long message into smaller messages. OSI 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 protocol classes provide expedited data and preservation of record boundaries. Detailed characteristics of TP0-4 classes are shown in the following table:[8] 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-4 protocols within OSI. The session layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The session layer is commonly implemented explicitly in application environments that use remote procedure calls. 2.6 Layer 6: Presentation Layer The presentation layer establishes context between application-layer entities, in which the application-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the protocol stack. This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.[9] The original presentation structure used the Basic Encoding Rules of Abstract Syntax Notation One (ASN.1), with An easy way to visualize the transport layer is to com- capabilities such as converting an EBCDIC-coded text pare it with a post office, which deals with the dispatch file to an ASCII-coded file, or serialization of objects and and classification of mail and parcels sent. Do remember, other data structures from and to XML. however, that a post office manages the outer envelope of mail. Higher layers may have the equivalent of double envelopes, such as cryptographic presentation services 2.7 Layer 7: Application Layer that can be read by the addressee only. Roughly speaking, tunneling protocols operate at the transport layer, The application layer is the OSI layer closest to the end such as carrying non-IP protocols such as IBM's SNA or user, which means both the OSI application layer and the Novell's IPX over an IP network, or end-to-end encryp- user interact directly with the software application. This tion with IPsec. While Generic Routing Encapsulation layer interacts with software applications that implement (GRE) might seem to be a network-layer protocol, if the a communicating component. Such application programs encapsulation of the payload takes place only at endpoint, fall outside the scope of the OSI model. ApplicationGRE becomes closer to a transport protocol that uses IP layer functions typically include identifying communicaheaders but contains complete frames or packets to de- tion partners, determining resource availability, and synliver to an endpoint. L2TP carries PPP frames inside chronizing communication. When identifying commu- 4 6 nication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network resources for the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. This layer supports application and end-user processes. Communication partners are identified, quality of service is identified, user authentication and privacy are considered, and any constraints on data syntax are identified. Everything at this layer is application-specific. COMPARISON WITH TCP/IP MODEL state information from the PHY layer, network throughput can be significantly improved and energy waste can be avoided.[10] 4 Interfaces Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers, known as network sockets are implementation-specific. For example, Microsoft Windows' Winsock, and Unix's Berkeley sockets and System V Transport Layer Inter3 Cross-layer functions face, are interfaces between applications (layer 5 and above) and the transport (layer 4). NDIS and ODI are Cross-layer functions are services that are not tied to a interfaces between the media (layer 2) and the network given layer, but may affect more than one layer. Examples protocol (layer 3). include the following: Interface standards, except for the physical layer to media, are approximate implementations of OSI service • Security service (telecommunication)[5] as defined specifications. by ITU-T X.800 recommendation. • Management functions, i.e. functions that permit to configure, instantiate, monitor, terminate the com- 5 Examples munications of two or more entities: there is a specific application-layer protocol, common management information protocol (CMIP) and its corre- 6 Comparison with TCP/IP model sponding service, common management information service (CMIS), they need to interact with every The design of protocols in the TCP/IP model of the Inlayer in order to deal with their instances. ternet does not concern itself with strict hierarchical encapsulation and layering.[16] RFC 3439 contains a sec• Multiprotocol Label Switching (MPLS) operates at tion entitled “Layering considered harmful".[17] TCP/IP an OSI-model layer that is generally considered to lie does recognize four broad layers of functionality which between traditional definitions of layer 2 (data link are derived from the operating scope of their contained layer) and layer 3 (network layer), and thus is often protocols: the scope of the software application; the endreferred to as a “layer-2.5” protocol. It was designed to-end transport connection; the internetworking range; to provide a unified data-carrying service for both and the scope of the direct links to other nodes on the circuit-based clients and packet-switching clients local network.[18] which provide a datagram-based service model. It can be used to carry many different kinds of traf- Despite using a different concept for layering than the fic, including IP packets, as well as native ATM, OSI model, these layers are often compared with the OSI layering scheme in the following way: SONET, and Ethernet frames. • ARP is used to translate IPv4 addresses (OSI layer 3) into Ethernet MAC addresses (OSI layer 2). • Domain Name Service is an Application Layer service which is used to look up the IP address of a given domain name. Once a reply is received from the DNS server, it is then possible to form a Layer 3 connection to the third-party host. • 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 favorable channel conditions, which requires the MAC layer to obtain channel • The Internet application layer includes the OSI application layer, presentation layer, and most of the session layer. • Its end-to-end transport layer includes the graceful close function of the OSI session layer as well as the OSI transport layer. • The internetworking layer (Internet layer) is a subset of the OSI network layer. • The link layer includes the OSI data link layer and sometimes the physical layers, as well as some protocols of the OSI’s network layer. 5 These comparisons are based on the original seven-layer [9] Grigonis, Richard (2000). Computer telephony- encyclopaedia. CMP. p. 331. ISBN 9781578200450. protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of [10] G. Miao; G. Song (2014). Energy and spectrum efficient the network layer document. wireless network design. Cambridge University Press. ISBN 1107039886. The presumably strict layering of the OSI model as it is usually described does not present contradictions in [11] “ITU-T Recommendation Q.1400 (03/1993)], ArchitecTCP/IP, as it is permissible that protocol usage does not ture framework for the development of signaling and follow the hierarchy implied in a layered model. Such exOA&M protocols using OSI concepts". ITU. pp. 4, 7. amples exist in some routing protocols (e.g., OSPF), or in the description of tunneling protocols, which provide [12] ITU Rec. X.227 (ISO 8650), X.217 (ISO 8649). a link layer for an application, although the tunnel host [13] X.700 series of recommendations from the ITU-T (in parprotocol might well be a transport or even an applicationticular X.711) and ISO 9596. layer protocol in its own right. 7 See also • Hierarchical internetworking model [15] “3GPP specification: 36.300”. 3gpp.org. Retrieved 14 August 2015. • Management plane [16] RFC 3439 • Layer 8 [17] “RFC 3439 - Some Internet Architectural Guidelines and Philosophy”. ietf.org. Retrieved 14 August 2015. • Protocol stack • Service layer • WAP protocol suite • List of information technology acronyms • IBM Systems Network Architecture • Internet protocol suite 8 [14] “Internetworking Technology Handbook - Internetworking Basics [Internetworking]". Cisco. 15 January 2014. Retrieved 14 August 2015. References [1] ITU-T X-Series Recommendations [2] “Publicly Available Standards”. Standards.iso.org. 201007-30. Retrieved 2010-09-11. [3] “The OSI Model’s Seven Layers Defined and Functions Explained”. Microsoft Support. Retrieved 2014-12-28. [4] RFC 4253 [5] “ITU-T Recommendataion X.800 (03/91), Security architecture for Open Systems Interconnection for CCITT applications". ITU. Retrieved 14 August 2015. [6] “5.2 RM description for end stations”. IEEE Std 8022014, IEEE Standard for Local and Metropolitan Area Networks: Overview and Architecture. ieee. [7] International Organization for Standardization (1989-1115). “ISO/IEC 7498-4:1989 -- Information technology -Open Systems Interconnection -- Basic Reference Model: Naming and addressing”. ISO Standards Maintenance Portal. ISO Central Secretariat. Retrieved 2015-08-17. [8] “ITU-T Recommendation X.224 (11/1995) ISO/IEC 8073, Open Systems Interconnection - Protocol for providing the connection-mode transport service". ITU. [18] Walter Goralski. The Illustrated Network: How TCP/IP Works in a Modern Network (PDF). Morgan Kaufmann. p. 26. ISBN 978-0123745415. 9 External links • Microsoft Knowledge Base: The OSI Model’s Seven Layers Defined and Functions Explained • ISO/IEC standard 7498-1:1994 (PDF document inside ZIP archive) (requires HTTP cookies in order to accept licence agreement) • ITU-T X.200 (the same contents as from ISO) • The ISO OSI Reference Model , Beluga graph of data units and groups of layers • Zimmermann, Hubert (April 1980). “OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection”. IEEE Transactions on Communications. 28 (4): 425–432. doi:10.1109/TCOM.1980.1094702. CiteSeerX: 10.1.1.136.9497. • Cisco Systems Internetworking Technology Handbook • Osi Model : 7 Layer Of The Network Communication 6 10 10 10.1 TEXT AND IMAGE SOURCES, CONTRIBUTORS, AND LICENSES Text and image sources, contributors, and licenses Text • OSI model Source: https://en.wikipedia.org/wiki/OSI_model?oldid=740931280 Contributors: Damian Yerrick, AxelBoldt, Bryan Derksen, The Anome, Stephen Gilbert, Manning Bartlett, Amillar, Andre Engels, Gsl, Aldie, Nate Silva, M~enwiki, William Avery, DavidLevinson, Heron, B4hand, Edward, PhilipMW, Michael Hardy, Chris-martin, MartinHarper, Todd, Looxix~enwiki, Mkweise, Ahoerstemeier, Haakon, Ronz, Nanshu, Glenn, Bogdangiusca, Netsnipe, IMSoP, Evercat, EdH, JidGom, Mulad, Markb, Emperorbma, Fuzheado, Atreyu42, Markhurd, Tpbradbury, Grendelkhan, Jnc, Ed g2s, Rnbc, Nickshanks, Joy, Fvw, Johnleemk, Finlay McWalter, Phil Boswell, Robbot, TomPhil, Friedo, Fredrik, Dumbledad, RedWolf, Postdlf, Wjhonson, Rfc1394, Puckly, Rursus, 75th Trombone, Hadal, David Edgar, Rsduhamel, Filemon, David Gerard, Enochlau, Giftlite, Graeme Bartlett, DavidCary, Cokoli, Jpta~enwiki, Inkling, Tom harrison, Lupin, Herbee, 0x6D667061, Everyking, CyborgTosser, Itpastorn, EJDyksen, Mboverload, AlistairMcMillan, Alvestrand, Tagishsimon, Dave2, Gadfium, Pamri, Mendel, Inversetime, Ravikiran r, Eldiablo~enwiki, Sridev, Parakalo~enwiki, Tooki, Kramerino, Lazarus666, Paparodo, Jcw69, Slrobertson, Kevin Rector, Trevor MacInnis, Moxfyre, Safety Cap, EagleOne, DmitryKo, Gazpacho, Mike Rosoft, Venu62, Hes Nikke, Hgerstung, RossPatterson, Discospinster, Rich Farmbrough, Rhobite, DavidBarak, Lulu of the Lotus-Eaters, Demitsu, Paul August, Techtoucian, BACbKA, Kaszeta, CanisRufus, Charm, MBisanz, Lankiveil, Bletch, GarethGilson, Sietse Snel, RoyBoy, Triona, Causa sui, Rlaager, Bobo192, MarkWahl, Smalljim, John Vandenberg, Dreish, Che090572, AtomicDragon, Elipongo, Mikel Ward, Giraffedata, SpeedyGonsales, Chirag, Nk, Bdamokos, Wrs1864, Helix84, Krellis, Nsaa, Mdd, Wayfarer, Ogress, Alansohn, Gary, Ghostalker, SnowFire, Guy Harris, Free Bear, Lectonar, OSUKid7, Caesura, Snowolf, Simonfl, Wtmitchell, Rebroad, Wtshymanski, Rick Sidwell, Cburnett, Stephan Leeds, Suruena, RainbowOfLight, Drat, Mikeo, Pethr, H2g2bob, MIT Trekkie, Pytom, Cxxl, Richwales, Beelaj, Mahanga, Ott, Feezo, Stemonitis, OwenX, Woohookitty, Camw, LOL, Blueskies238, Ilario, Robert K S, Hdante, Kgrr, Cbustapeck, Eyreland, Prashanthns, Andybryant, Palica, Marudubshinki, Kesla, Runis57, Jannetta, Chupon, Jetekus, Josh Parris, Casey Abell, Ketiltrout, Dpark, Johnblade, Jake Wartenberg, Kinu, Inkhorn, CraSH, TAS, Tangotango, SMC, Vegaswikian, Kazrak, BDerrly, ElKevbo, N-Man, Fred Bradstadt, Syced, Yamamoto Ichiro, Audunv, StuartBrady, FlaBot, Nivix, Ewlyahoocom, Eric Soyke, RobyWayne, Intgr, Fresheneesz, Alphachimp, Chfalcao, Imnotminkus, Jmorgan, Chobot, Hatch68, Visor, DVdm, JesseGarrett, Bgwhite, Adoniscik, Gwernol, FrankTobia, Roboto de Ajvol, McGinnis, YurikBot, Albanaco, Borgx, Sceptre, Josef Sábl cz, Joodas, Jonwatson, Bhny, Tyler.szabo, SpuriousQ, Kirill Lokshin, Joebeone, Grubber, Shell Kinney, CambridgeBayWeather, Eleassar, Pseudomonas, Abarry, Friday, NawlinWiki, ENeville, Grafen, RazorICE, Nick, Marvin01, Brandon, Moe Epsilon, Kaz219~enwiki, Voidxor, Lomn, Davetrainer, Tony1, PyreneesJIM, Nethgirb, Rjstinyc, Jessemerriman, Caerwine, MarkBrooks, Wknight94, FF2010, Whitejay251, Lt-wiki-bot, YolanCh, Ageekgal, Closedmouth, Iambk, Fang Aili, Pb30, Abune, Bariswheel, JoanneB, ThunderBird, Allens, Honeyman, CIreland, Luk, Hiddekel, EJSawyer, Sardanaphalus, SmackBot, Mmernex, Bonobosarenicer, Irnavash, ThreeDee912, Reedy, KnowledgeOfSelf, Bjelleklang, Brick Thrower, Delldot, Nejko, Jonathanwagner, Gilliam, Psiphiorg, @modi, Djib, Delfeye, Alucard 16, Tree Biting Conspiracy, Stevage, Arroww, Octahedron80, Nbarth, Ctbolt, Adibob, Thief12, TripleF, Can't sleep, clown will eat me, YamiKaitou, ZachPruckowski, JonHarder, Addshore, Kcordina, Edivorce, Meepster, SundarBot, Adamantios, UU, Dohzer, Soosed, Apocryphite, Cybercobra, Fullstop, Valenciano, Shadow1, Tompsci, HarisM, Insineratehymn, Weregerbil, Mwtoews, WoiKiCK, Ged UK, Dino.korah, Umair ahmed123, H34d~enwiki, Kuru, Microchip08, Natarajuab, Rejax, Martinkop, Adj08, Sir Nicholas de Mimsy-Porpington, Tim Q. 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