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'{{OSIstack}} The '''Open System Interconnection Reference Model''' (OSI Reference Model or '''OSI Model''') is an abstract description for layered communications and computer [[network protocol]] design. It was developed as part of the [[Open Systems Interconnection]] (OSI) initiative.<ref> [http://www.itu.int/rec/T-REC-X.200-199407-I/en X.200 : Information technology - Open Systems Interconnection - Basic Reference Model: The basic model<!-- Bot generated title -->] </ref> In its most basic form, it divides network architecture into seven layers which, from top to bottom, are the Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers. It is therefore often referred to as the '''OSI Seven Layer Model'''. A layer is a collection of conceptually similar functions that provide services to the layer above it and receives service from the layer below it. On each layer an ''[[instance]]'' provides services to the instances at the layer above and requests service from the layer below. For example, a layer that provides error-free 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 make up the contents of the path. Conceptually two instances at one layer are connected by a horizontal protocol connection on that layer.<br> [[File:OSI-model-Communication.svg|thumb|400px|Communication in the OSI-Model (Example with layers 3 to 5)]] ==History== In 1970, work on a layered model of network architecture was started and the [[International Organization for Standardization]] (ISO) began to develop its OSI framework architecture. OSI has two major components: an abstract model of networking, called the Basic Reference Model or seven-layer model, and a set of specific protocols. Note: The standard documents that describe the OSI model can be freely downloaded from the ITU-T as the '''X.200'''-series of recommendations.<ref> [http://www.itu.int/rec/T-REC-X/en ITU-T X-Series Recommendations]. </ref> A number of the protocol specifications are also available as part of the ITU-T X series. The equivalent ISO and ISO/IEC standards for the OSI model are available from ISO, but only some of them at no charge.<ref> [http://standards.iso.org/ittf/PubliclyAvailableStandards/index.html Publicly Available Standards<!-- Bot generated title -->] </ref> All aspects of OSI design evolved from experiences with the [[CYCLADES]] network, which also influenced Internet design. The new design was documented in ISO 7498 and its various addenda. In this model, a networking system is divided into layers. Within each layer, one or more entities implement its functionality. Each entity interacts directly only with the layer immediately beneath it, and provides 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 describe the functionality provided to an (N)-layer by an (N-1) layer, where N is one of the seven layers of protocols operating in the local host. ==Description of OSI layers== {|class="wikitable" border="1" style="float:right;margin:0 0 1em 1em" |- ! colspan="5" | OSI Model |- ! !Data unit !Layer !style="width:9em;"|Function |- !rowspan="4"|Host<br />layers | bgcolor="#D8EC9B" rowspan="3"|Data | bgcolor="#D8EC9B" | 7. [[Application Layer|Application]] | bgcolor="#D8EC9B" |<small>Network process to application</small> |- | bgcolor="#D8EC9B" |6. [[Presentation Layer|Presentation]] | bgcolor="#D8EC9B" |<small>Data representation and encryption</small> |- | bgcolor="#D8EC9B" |5. [[Session Layer|Session]] | bgcolor="#D8EC9B" |<small>Interhost communication</small> |- | bgcolor="#E7ED9C" |Segment | bgcolor="#E7ED9C" |4. [[Transport Layer|Transport]] | bgcolor="#E7ED9C" |<small>End-to-end connections and reliability</small> |- !rowspan="3"|Media<br />layers | bgcolor="#EDDC9C" |Packet | bgcolor="#EDDC9C" |3. [[Network Layer|Network]] | bgcolor="#EDDC9C" |<small>Path determination and [[logical address]]ing</small> |- | bgcolor="#E9C189" |Frame | bgcolor="#E9C189" |2. [[Data Link Layer|Data Link]] | bgcolor="#E9C189" |<small>Physical addressing</small> |- | bgcolor="#E9988A" |Bit | bgcolor="#E9988A" |1. [[Physical Layer|Physical]] | bgcolor="#E9988A" |<small>Media, signal and binary transmission</small> |} ===Layer 1: Physical Layer=== {{Main| Physical Layer}} The [[Physical Layer]] defines the electrical and physical specifications for devices. In particular, it defines the relationship between a device and a physical medium. This includes the layout of [[Lead (electronics)|pins]], [[voltage]]s, [[cable]] [[specification]]s, [[network hub|hub]]s, [[repeater]]s, [[network card|network adapter]]s, [[host adapter|host bus adapter]]s (HBAs used in [[storage area network]]s) and more. To understand the function of the Physical Layer in contrast to the functions of the Data Link Layer, think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, where the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. The Physical Layer will tell one device how to transmit to the medium, and another device how to receive from it (in most cases it does not tell the device how to connect to the medium). Standards such as [[RS-232]] do use physical wires to control access to the medium. The major functions and services performed by the Physical Layer are: *Establishment and termination of a [[electrical connector|connection]] to a [[communication]]s [[transmission medium|medium]]. *Participation in the process whereby the communication resources are effectively shared among multiple users. For example, [[Computer networking|contention]] resolution and [[flow control]]. *[[Modulation]], or conversion between the representation of [[digital data]] in user equipment and the corresponding signals transmitted over a communications [[channel (communications)|channel]]. These are signals operating over the physical cabling (such as copper and [[optical fiber]]) or over a [[electromagnetic wave#Radio waves|radio link]]. [[Parallel SCSI]] buses operate in this layer, although it must be remembered that the logical [[SCSI]] protocol is a Transport Layer protocol that runs over this bus. Various Physical Layer Ethernet standards are also in this layer; Ethernet incorporates both this layer and the Data Link Layer. The same applies to other local-area networks, such as [[IBM token ring|token ring]], [[fiber distributed data interface|FDDI]], [[ITU-T]] [[G.hn]] and [[IEEE 802.11]], as well as personal area networks such as [[Bluetooth]] and [[IEEE 802.15#Task group 4 (Low Rate WPAN)|IEEE 802.15.4]]. ===Layer 2: Data Link Layer=== {{Main| Data Link Layer}} The [[Data Link Layer]] provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in [[IEEE 802|IEEE Project 802]]. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as [[Point-to-Point Protocol]] (PPP), and, on local area networks, the IEEE 802.2 [[Logical Link Control|LLC]] layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as [[Transmission Control Protocol|TCP]], but is still used in niches where [[X.25]] offers performance advantages. 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]] which provides both error correction and flow control by means of a [[selective repeat]] [[Sliding Window Protocol]]. Both WAN and LAN services arrange bits, from the Physical Layer, into logical sequences called frames. Not all Physical Layer bits necessarily go into frames, as some of these bits are purely intended for Physical Layer functions. For example, every fifth bit of the [[FDDI]] bit stream is not used by the Layer. ====WAN Protocol architecture==== [[Connection-oriented]] WAN data link protocols, in addition to framing, detect and may correct errors. They are also capable of controlling the rate of transmission. A WAN Data Link Layer might implement a [[sliding window]] flow control and acknowledgment mechanism to provide reliable delivery of frames; that is the case for [[Synchronous Data Link Control|SDLC]] and [[HDLC]], and derivatives of HDLC such as [[LAPB]] and [[Link Access Procedures, D channel|LAPD]]. ====IEEE 802 LAN architecture==== Practical, [[connectionless]] LANs began with the pre-IEEE [[Ethernet]] specification, which is the ancestor of [[IEEE 802.3]]. This layer manages the interaction of devices with a shared medium, which is the function of a [[Media Access Control]] sublayer. Above this MAC sublayer is the media-independent [[IEEE 802.2]] [[Logical Link Control]] (LLC) sublayer, which deals with addressing and multiplexing on multiaccess media. While IEEE 802.3 is the dominant wired LAN protocol and [[IEEE 802.11]] the wireless LAN protocol, obsolescent MAC layers include [[Token Ring]] and [[FDDI]]. The MAC sublayer detects but does not correct errors. ===Layer 3: Network Layer=== {{Main| Network Layer}} The [[Network Layer]] provides the functional and procedural means of transferring variable length [[data]] sequences from a source to a destination via one or more networks, while maintaining the [[quality of service]] requested by the Transport Layer. The Network Layer performs network [[routing]] functions, and might also perform fragmentation and reassembly, and report delivery errors. [[Router]]s operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is hierarchical. <!-- STRANGE THAT A PROTOCOL THAT DOESN'T ACUTALLY BELONG TO THE OSI MODEL IS ITS BEST KNOWN EXAMPLE, WHY NOT DISCUSS OSI ON ITS OWN MERITS? -->The best-known example of a Layer 3 protocol is the [[Internet Protocol]] (IP). It manages the [[Connectionless protocol|connectionless]] transfer of data one hop at a time, from end system to [[ingress router]], router to router, and from [[egress router]] to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of errored packets so they may be discarded. When the medium of the next hop cannot accept a packet in its current length, IP is responsible for '''fragmenting''' the packet into sufficiently small packets that the medium can accept. 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. ===Layer 4: Transport Layer=== {{Main| Transport Layer}} The [[Transport Layer]] provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state and connection oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the [[Transmission Control Protocol]] (TCP) and [[User Datagram Protocol]] (UDP). Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least error recovery) 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, both of which TCP is incapable. 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 | url = http://www.itu.int/rec/T-REC-X.224-199511-I/en/ }}</ref> {| class="wikitable" border="1" |- ! 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}} | {{No}} | {{Yes}} | {{Yes}} |- | Reinitiate connection (if an excessive number of [[Protocol data unit|PDUs]] are unacknowledged) | {{No}} | {{Yes}} | {{No}} | {{Yes}} | {{No}} |- | multiplexing and demultiplexing over a single [[virtual circuit]] | {{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}} |} Perhaps 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. Do remember, 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 that can be read by the addressee only. Roughly speaking, [[tunneling protocol]]s operate at the Transport Layer, such as carrying non-IP protocols such as [[IBM]]'s [[IBM Systems Network Architecture|SNA]] or [[Novell]]'s [[Internetwork Packet Exchange|IPX]] over an IP network, or end-to-end encryption with [[IPsec]]. While [[Generic Routing Encapsulation]] (GRE) might seem to be a Network Layer protocol, if the encapsulation of the payload takes place only at endpoint, GRE becomes closer to a transport protocol that uses IP headers but contains complete frames or packets to deliver to an endpoint. [[Layer 2 Tunneling Protocol|L2TP]] carries [[Point-to-Point Protocol|PPP]] frames inside transport packet. ===Layer 5: Session Layer=== {{Main| Session Layer}} 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 [[Duplex (telecommunications)|full-duplex]], [[half-duplex]], or [[Simplex communication|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 call]]s. '' ===Layer 6: Presentation Layer=== {{Main| Presentation Layer}} The [[Presentation Layer]] establishes a context between Application Layer entities, in which the higher-layer entities can use different syntax and semantics, as long as the presentation service understands both and the mapping between them. The presentation service data units are then encapsulated into Session Protocol data units, and moved down the stack. This layer provides independence from differences in data representation (e.g., encryption) by translating from application to network format, and vice versa. The presentation layer works to transform data into the form that the application layer can accept. This layer formats and encrypts data to be sent across a network, providing freedom from compatibility problems. It is sometimes called the syntax layer. The original presentation structure used the basic encoding rules of [[Abstract Syntax Notation One]] (ASN.1), with capabilities such as converting an [[EBCDIC]]-coded text [[computer file|file]] to an [[ASCII]]-coded file, or [[serialization]] of [[object (computer science)|object]]s and other [[data structure]]s from and to [[XML]]. ===Layer 7: Application Layer=== {{Main| Application Layer}} The application layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication 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 or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer. Some examples of application layer implementations include Hypertext Transfer Protocol (HTTP), File Transfer Protocol (FTP), Simple Mail Transfer Protocol (SMTP) and X.400 Mail... ==Interfaces== Neither the OSI Reference Model nor OSI protocols specify any programming interfaces, other than as deliberately abstract service specifications. Protocol specifications precisely define the interfaces between different computers, but the software interfaces inside computers are implementation-specific. For example [[Microsoft Windows]]' [[Winsock]], and [[Unix]]'s [[Berkeley sockets]] and [[UNIX System V|System V]] [[Transport Layer Interface]], are interfaces between applications (Layer 5 and above) and the transport (Layer 4). [[Network Driver Interface Specification|NDIS]] and [[Open Data-Link Interface|ODI]] are interfaces between the media (Layer 2) and the network protocol (Layer 3). Interface standards, except for the Physical Layer to media, are approximate implementations of OSI Service Specifications. ==Examples== {| class="wikitable" border="1" |- !colspan="2"| Layer !rowspan="2"| [[Open Systems Interconnection|OSI]] protocols !rowspan="2"| [[Internet Protocol Suite|TCP/IP protocols]] !rowspan="2"| [[Signaling System 7]]<ref>[http://www.itu.int/rec/T-REC-Q.1400/en/ ITU-T Recommendation Q.1400 (03/1993)], ''Architecture framework for the development of signalling and OA&M protocols using OSI concepts'', pp 4, 7.</ref> !rowspan="2"| [[AppleTalk]] !rowspan="2"| [[IPX]] !rowspan="2"| [[Systems Network Architecture|SNA]] !rowspan="2"| [[UMTS]] !rowspan="2"| Misc. examples |- ! # ! Name |- | 7 | Application | [[FTAM]], [[X.400]], [[X.500]], [[Directory Access Protocol|DAP]], [[ROSE]], [[RTSE]], [[ACSE]] | [[Network News Transfer Protocol|NNTP]], [[Session Initiation Protocol|SIP]], [[Simple Sensor Interface protocol|SSI]], [[Domain Name System|DNS]], [[File transfer protocol|FTP]], [[Gopher (protocol)|Gopher]], [[HTTP]], [[Network File System (protocol)|NFS]], [[Network Time Protocol|NTP]], [[Dynamic Host Configuration Protocol|DHCP]], [[Short Message Peer-to-Peer Protocol|SMPP]], [[Simple Mail Transfer Protocol|SMTP]], [[Simple network management protocol|SNMP]], [[Telnet]], [[Routing Information Protocol|RIP]], [[Border Gateway Protocol|BGP]] | [[INAP]], [[Mobile Application Part|MAP]], [[Transaction Capabilities Application Part|TCAP]], [[ISDN User Part|ISUP]], [[Telephone User Part|TUP]] | [[Apple Filing Protocol|AFP]], [[Zone Information Protocol|ZIP]], [[Routing Table Maintenance Protocol|RTMP]], [[AppleTalk|NBP]] | RIP, [[Service Advertising Protocol|SAP]] | [[IBM Advanced Program-to-Program Communication|APPC]] | | [[HL7]], [[Modbus]] |- | 6 | Presentation | ISO/IEC&nbsp;8823, X.226, ISO/IEC&nbsp;9576-1, X.236 | [[MIME]], [[Secure Sockets Layer|SSL]], [[Transport Layer Security|TLS]], [[External Data Representation|XDR]] | [[Apple Filing Protocol|AFP]] | | | | [[Tabbed Document Interface|TDI]], [[ASCII]], [[EBCDIC]], [[Musical Instrument Digital Interface|MIDI]], [[MPEG]] |- | 5 | Session | ISO/IEC&nbsp;8327, X.225, ISO/IEC&nbsp;9548-1, X.235 | Sockets. Session establishment in [[Transmission Control Protocol|TCP]], [[Session Initiation Protocol|SIP]], [[Real-time Transport Protocol|RTP]] | | [[AppleTalk|ASP]], [[AppleTalk|ADSP]], [[Printer Access Protocol|PAP]] | [[NWLink]] | [[Data Link Control|DLC]]? | | [[Named pipes]], [[NetBIOS]], [[Session Announcement Protocol|SAP]], [[half duplex]], [[full duplex]], [[simplex]], [[Session Description Protocol|SDP]], [[Remote Procedure Call|RPC]] |- | 4 | Transport | ISO/IEC&nbsp;8073, TP0, TP1, TP2, TP3, TP4 (X.224), ISO/IEC&nbsp;8602, X.234 | [[Transmission Control Protocol|TCP]], [[User Datagram Protocol|UDP]], [[Stream Control Transmission Protocol|SCTP]] | | | [[Datagram Delivery Protocol|DDP]], [[Sequenced packet exchange|SPX]] | | | [[NetBIOS Frames protocol|NBF]] |- | 3 | Network | ISO/IEC&nbsp;8208, [[X.25]] ([[Packet-Layer Protocol|PLP]]), ISO/IEC&nbsp;8878, [[CONS|X.223]], ISO/IEC&nbsp;8473-1, [[CLNS|CLNP]] X.233. | [[Internet Protocol|IP]], [[IPsec]], [[Internet Control Message Protocol|ICMP]], [[Internet Group Management Protocol|IGMP]], [[Open Shortest Path First|OSPF]] | [[Signalling Connection Control Part|SCCP]], [[Message Transfer Part|MTP]] | [[AppleTalk|ATP]] ([[TokenTalk]] or [[EtherTalk]]) | [[IPX]] | | [[Radio Resource Control|RRC]] ([[Radio Resource Control]]) [[PDCP|Packet Data Convergence Protocol]] ([[PDCP]]) and [[Broadcast/Multicast Control|BMC]] ([[Broadcast/Multicast Control]]) | [[NetBIOS Frames protocol|NBF]], [[Q.931]], [[IS-IS]] |- | 2 | Data Link | ISO/IEC&nbsp;7666, [[X.25]] ([[LAPB]]), [[Token Bus]], X.222, ISO/IEC&nbsp;8802-2 [[IEEE 802.2|LLC]] Type 1 and 2 | [[Point-to-Point Protocol|PPP]], [[SLIP]], [[PPTP]], [[L2TP]] | [[Message Transfer Part|MTP]], [[Message Transfer Part|Q.710]] | [[LocalTalk]], [[AppleTalk Remote Access]], [[Point-to-Point Protocol|PPP]] | [[IEEE 802.3]] framing, [[Ethernet II framing]] | [[Synchronous Data Link Control|SDLC]] | [[Logical Link Control|LLC]] ([[Logical Link Control]]), [[Media Access Control|MAC]] ([[Media Access Control]]) | [[Ethernet|802.3 (Ethernet)]], [[IEEE 802.11|802.11a/b/g/n MAC/LLC]], [[IEEE 802.1Q|802.1Q (VLAN)]], [[Asynchronous Transfer Mode|ATM]], [[Hopling Discovery Protocol|HDP]], [[FDDI]], [[Fibre Channel]], [[Frame Relay]], [[HDLC]], [[Cisco Inter-Switch Link|ISL]], [[Point-to-Point Protocol|PPP]], [[Q.921]], [[Token Ring]], [[Cisco Discovery Protocol|CDP]], [[Address resolution protocol|ARP]] (maps layer 3 to layer 2 address), [[G.hn|ITU-T G.hn DLL]] |- | 1 | Physical | [[X.25]] ([[X.21bis]], [[EIA/TIA-232]], [[EIA/TIA-449]], [[EIA-530]], [[G.703]]) | | [[Message Transfer Part|MTP]], [[Message Transfer Part|Q.710]] | [[RS-232]], [[RS-422]], [[Twisted pair#Shielded Twisted Pair (STP)|STP]], [[PhoneNet]] | | [[Twinaxial cabling|Twinax]] | [[UMTS Physical Layer|UMTS L1]] ([[UMTS Physical Layer]]) | [[RS-232]], [[Full duplex]], [[RJ45]], [[V.35]], [[V.34]], [[I.430]], [[I.431]], [[T-carrier|T1]], [[E-carrier|E1]], [[10BASE-T]], [[100BASE-TX]], [[Plain old telephone service|POTS]], [[Synchronous optical networking|SONET]], [[Synchronous digital hierarchy|SDH]], [[Digital Subscriber Line|DSL]], [[IEEE 802.11|802.11a/b/g/n PHY]], [[G.hn|ITU-T G.hn PHY]] |} ==Comparison with TCP/IP== In the [[TCP/IP model]] of the Internet, protocols are deliberately not as rigidly designed into strict layers as the OSI model.<ref>RFC 3439</ref> RFC 3439 contains a section entitled "Layering [[considered harmful]]." However, TCP/IP does recognize four broad layers of functionality which are derived from the operating scope of their contained protocols, namely the scope of the software application, the end-to-end transport connection, the internetworking range, and lastly the scope of the direct links to other nodes on the local network. Even though the concept is different than in OSI, these layers are nevertheless often compared with the OSI layering scheme in the following way: 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, while the [[Link Layer]] includes the OSI Data Link and Physical Layers, as well as parts of OSI's Network Layer. These comparisons are based on the original seven-layer protocol model as defined in ISO 7498, rather than refinements in such things as the internal organization of the Network Layer document. The presumably strict consumer/producer layering of OSI as it is usually described does not present contradictions in TCP/IP, as it is permissible that protocol usage does not follow the hierarchy implied in a layered model. Such examples exist in some routing protocols (e.g., OSPF), or in the description of [[tunneling protocol]]s, which provide a Link Layer for an application, although the tunnel host protocol may well be a Transport or even an Application Layer protocol in its own right. The TCP/IP design generally favors decisions based on simplicity, efficiency and ease of implementation.{{Citation needed|date=October 2009}} --~~~~--~~~~--~~~~<nowiki>[[Media:Insert non-formatted text here]][[File: == Example.jpg == ]]</nowiki>==See also== * [[Cognitiv,m,,,,,,,,,,,, hdfdyyhn ne networks]] * [[Hierarchical internetworking model]] * [[Internet protocol suite]] * [[Layer 8]] * [[OSI protocol suite]] * [[Protocol stack]] * [[Service layer]] * [[TCP/IP model]] * [[X.25 protocol suite]] * [[WAP protocol suite]] ==References== <references/> ==External links== * [http://standards.iso.org/ittf/PubliclyAvailableStandards/s020269_ISO_IEC_7498-1_1994(E).zip ISO/IEC standard 7498-1:1994] ([[Portable Document Format|PDF document]] inside [[ZIP (file format)|ZIP archive]]) (requires [[HTTP cookies]] in order to accept licence agreement) * [http://www.itu.int/rec/dologin_pub.asp?lang=e&id=T-REC-X.200-199407-I!!PDF-E&type=items ITU-T X.200 (the same contents as from ISO)] * [http://infchg.appspot.com/usr?at=1263939371 The ISO OSI Reference Model , Beluga graph of data units and groups of layers] * {{PDFlink | 1 = [http://www.comsoc.org/livepubs/50_journals/pdf/RightsManagement_eid=136833.pdf OSI Reference Model — The ISO Model of Architecture for Open Systems Interconnection] | 2 = 776&nbsp;KB }}, Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp.&nbsp;425 - 432. * [http://www.cisco.com/en/US/docs/internetworking/technology/handbook/Intro-to-Internet.html Internetworking Basics] {{ISO standards}} {{DEFAULTSORT:Osi Model}} [[Category:Network architecture]] [[Category:ISO standards]] [[Category:ITU-T recommendations]] [[Category:OSI protocols]] [[Category:Reference models]] [[af:OSI-model]] [[ar:مرجع أو إس آي]] [[bs:OSI model]] [[br:OSI]] [[bg:OSI модел]] [[ca:OSI]] [[cs:Referenční model ISO/OSI]] [[da:OSI-model]] [[de:OSI-Modell]] [[et:OSI raammudel]] [[el:Μοντέλο αναφοράς OSI]] [[es:Modelo OSI]] [[eu:OSI eredua]] [[fa:مدل مرجع اتصال داخلی سیستم‌های باز]] [[fr:Modèle OSI]] [[gl:Modelo OSI]] [[ko:OSI 모델]] [[hi:ओएसआई प्रतिमान]] [[hr:OSI model]] [[id:Model OSI]] [[it:Open Systems Interconnection]] [[he:מודל ה-OSI]] [[ka:OSI მოდელი]] [[lv:Atvērto sistēmu sadarbības bāzes etalonmodelis]] [[lt:OSI modelis]] [[hu:OSI modell]] [[mk:OSI модел]] [[ml:ഒ.എസ്.ഐ. മാതൃക]] [[ms:Model OSI]] [[nl:OSI-model]] [[ja:OSI参照モデル]] [[no:OSI-modellen]] [[nn:OSI-modellen]] [[pl:Model OSI]] [[pt:Modelo OSI]] [[ro:Modelul OSI]] [[ru:Сетевая модель OSI]] [[sk:OSI model]] [[sl:ISO/OSI referenčni model]] [[sr:ОСИ модел]] [[sh:OSI model]] [[fi:OSI-malli]] [[sv:OSI-modellen]] [[ta:திறந்த முறைமை வலைப்பின்னல் மாதிரி]] [[th:OSI Model]] [[tr:OSI modeli]] [[uk:Модель OSI]] [[zh:OSI模型]]'