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OSI (Open Systems Interconnection)Source: http://searchnetworking.techtarget.com

OSI (Open Systems Interconnection) is a standard description or "reference model" for how messages should betransmitted between any two points in a telecommunication network. Its purpose is to guide product implementers

so that their products will consistently work with other products. he reference model defines se!en layers offunctions that take place at each end of a communication. lthough OSI is not always strictly adhered to in terms ofkeeping related functions together in a well#defined layer$ many if not most products in!ol!ed in telecommunicationmake an attempt to describe themsel!es in relation to the OSI model. It is also !aluable as a single reference !iew ofcommunication that furnishes e!eryone a common ground for education and discussion.%e!eloped by representati!es of ma&or computer and telecommunication companies beginning in ' *$ OSI wasoriginally intended to be a detailed specification of interfaces. Instead$ the committee decided to establish a commonreference model for which others could de!elop detailed interfaces$ that in turn could become standards. OSI wasofficially adopted as an international standard by the International Organi+ation of Standards ( ISO ). ,urrently$ it is-ecommendation . 00 of the I 1# S.

he main idea in OSI is that the process of communication between two end points in a telecommunication networkcan be di!ided into layers$ with each layer adding its own set of special$ related functions. 2ach communicating user or program is at a computer e3uipped with these se!en layers of function. So$ in a gi!en message between users$

there will be a flow of data through each layer at one end down through the layers in that computer and$ at the otherend$ when the message arri!es$ another flow of data up through the layers in the recei!ing computer and ultimatelyto the end user or program. he actual programming and hardware that furnishes these se!en layers of function isusually a combination of the computer operating system$ applications (such as your 4eb browser)$ ,5/I5 oralternati!e transport and network protocols$ and the software and hardware that enable you to put a signal on one ofthe lines attached to your computer.OSI di!ides telecommunication into se!en layers. he layers are in two groups. he upper four layers are usedwhene!er a message passes from or to a user. he lower three layers (up to the network layer) are used when anymessage passes through the host computer. 6essages intended for this computer pass to the upper layers. 6essagesdestined for some other host are not passed up to the upper layers but are forwarded to another host. he se!enlayers are:Layer 7: The application layer ... his is the layer at which communication partners are identified$ 3uality of ser!iceis identified$ user authentication and pri!acy are considered$ and any constraints on data synta7 are identified. ( his

layer is not the application itself$ although some applications may perform application layer functions.)Layer 6: The presentation layer ... his is a layer$ usually part of an operating system$ that con!erts incoming andoutgoing data from one presentation format to another (for e7ample$ from a te7t stream into a popup window withthe newly arri!ed te7t). Sometimes called the synta7 layer.Layer 5: The session layer ... his layer sets up$ coordinates$ and terminates con!ersations$ e7changes$ and dialogs

between the applications at each end. It deals with session and connection coordination.Layer 4: The transport layer ... his layer manages the end#to#end control (for e7ample$ determining whether all

packets ha!e arri!ed) and error#checking. It ensures complete data transfer.Layer 3: The network layer ... his layer handles the routing of the data (sending it in the right direction to the rightdestination on outgoing transmissions and recei!ing incoming transmissions at the packet le!el). he network layerdoes routing and forwarding.Layer 2: The data-link layer ... his layer pro!ides synchroni+ation for the physical le!el and does bit#stuffing forstrings of '8s in e7cess of 9. It furnishes transmission protocol knowledge and management.

Layer 1: The physical layer ... his layer con!eys the bit stream through the network at the electrical andmechanical le!el. It pro!ides the hardware means of sending and recei!ing data on a carrier.

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OSI -eference 6odel illustrated

Open Systems Interconnection ( OSI ) is a standard reference model for communication between two end users in anetwork. It is used in de!eloping products and understanding networks. his figure shows where commonly#usedInternet products and ser!ices fit within the model. lso see the notes below the figure.

Notes: he OSI -eference 6odel describes se!en layers of related functions that are needed at each end when a

message is sent from one party to another party in a network. n e7isting network product or program can bedescribed in part by where it fits into this layered structure. or e7ample$ ,5/I5 is usually packaged with otherInternet programs as a suite of products that support communication o!er the Internet. his suite includes the ile

ransfer 5rotocol ( 5 )$ elnet $ the ;yperte7t ransfer 5rotocol ( ; 5 )$ e#mail protocols$ and sometimes others.lthough ,5 fits well into the ransport layer of OSI and I5 into the