This 2000 word essay will be on the OSI and TCP/IP Models, what they areand how some parts of them can be compared.During  and before the 1980s, there were only ahandful of protocols and standards that were around and they belonged todifferent manufacturers who didn’t have much dialogue with each other. Though eventuallycomputer science and technology continued to be further innovated and becomemore readily available to companies and the public, it became necessary that awidespread standard would be needed in place to ensure compatibility betweenall machines. This was especially true about networks, and networkingtechnology. A network is designed to share data and information betweenmachines, a standard that dictates how this information is formatted,transmitted, and received would make it easy for data to be shared openly, evenwhen sending or receiving from networks that are not similar. Requirementsfor a new standard of implementing open communications led the InternationalOrganization for Standardization (ISO) and American Nation Standards Institute(ANSI) to develop a 7 layer network communications model known as Open SystemsInterconnect, or the OSI. The OSI became a link that allows data to be reliablyexchanged and transmitted since guidelines were created to set a standard inhow network equipment is manufactured and network OS’s communicate to each otheron a network.

The OSI model doesn’t actually perform any tasks or functions butit dictates HOW the work should be performed by other hardware or softwarebetween networks so that communication can occur.TheOSI model is made up of these seven layers; physical layer, data link layer,network layer, transport layer, session layer, presentation layer andapplication layer. These 7 parts of the model are called a ‘stack’.Here are the 7 layers in further detail: Physical LayerThe physical layer defines all the mechanical, procedural,functional and electrical specifications for activating, deactivating andmaintaining the link between each system . Such characteristics as voltagelevels, timing of voltage changes, physical data rates, maximum transmissiondistances, and physical connectors, are defined by physical layerspecifications.Data Link LayerThe data-link layer provides error-free transfer of dataframes from one computer to another over the physical layer.

The layers abovethis layer can assume virtually error-free transmission over the network. Thedata-link layer provides the following functions. – Establishing and terminating a logical link between two computersidentified by their unique network interface card. – Controlling frame flow by instructing the transmittingcomputer not to transmit frame buffers – Sequentially transmitting and receiving frames – Providing and expecting frame-acknowledgment, and detectingand recovering from errors that occur in the physical layer by retransmittingnon-acknowledged frames and handling duplicate frame receipts – Managing media access to determine when the computer ispermitted to use the physical medium – Eliminating frames to create and recognize frame boundaries – Error-checking frames to confirm the integrity of thereceived frame – Inspecting the destination address of each received frameand determining if the frame should be directed to the layer above Network LayerThenetwork layer knows the address of the adjacent nodes, controls the subnetoperations, packages output with the correct network address information,selects routes and quality of service, and recognizes and forwards to theTransport layer incoming messages for local host domains.- Controlling subnet traffic to allow an intermediate systemto instruct a sending station not to transmit its frame when the router’sbuffer fills up. If the router is busy, the network layer can instruct thesending station to use an alternate destination station.

– Resolving the logical computer address with the physicalnetwork interface card address. – Keeping an accounting record of frames forwarded to producebilling information Transport LayertheTransport layer makes sure the arrival of messages goes well and provides errorchecking and data flow controls such as stopping duplication or loss. It removes theconcern from the higher layer protocols about data transfer between the higherlayer and its peers. The size and complexity of a transport protocol depends onthe type of service it can get from the network layer or data link layer. For areliable network layer a minimal transport layer is required. Functions of thetransport layer include the following.- Accepting messages from the layer above and, if necessary,splitting them into frames – Providing reliable, end-to-end message delivery withacknowledgments – Instructing the transmitting computer not to transmit whenno receive buffers are available – Multiplexing several process-to-process message streams orsessions onto one logical link and keeping track of which messages belong towhich sessions Session LayerThe session layer establishes a communications sessionbetween processes running on different computers, and can support message-modedata transfer. Functions of the session layer include:- Allowing application processes to register unique processaddresses.

It provides the means by which these process addresses can beresolved to the network-layer or data-link-layer NIC addresses, if necessary.- Terminating, establishing and monitoring a virtual-circuit betweentwo processes identified by their specific addresses. A virtual-circuit sessionis a direct link that exists between the sender and receiver to add headerinformation that indicates where a message starts and ends. The receivingsession layer can then refrain from indicating any message data to theoverlying application until the entire message has been received. – Informing the receiving application when buffer space isinsufficient for the entire message and that the message is incomplete.

Thereceiving session layer may also use a control frame to inform the sendingsession layer how many bytes of the message have been successfully received.The sending session layer can then resume sending data at the byte followingthe last byte acknowledged as received. When the application provides anotherbuffer, the session layer can place the remainder of the message in that bufferand indicate to the application that the entire message has been received. Presentation LayerThe presentation layer ensures that information sent by theapplication layer of one system will be readable by the application layer ofanother system. If necessary, the presentation layer translates betweenmultiple data representation formats by using a common data representationformat.

The presentation layer concerns itself not only with the format andrepresentation of actual user data, but also with data structures used byprograms. In addition to actual data format transformation, the presentationlayer negotiates data transfer syntax for the application layer.Application LayerThe application layer is the OSI layer closest to the user.

It differs from the other layers because it does not provide services to anyother OSI layer, but rather to application processes lying outside the scope ofthe OSI model. Examples include spreadsheet programs, word-processing programs,banking terminal programs, etc. The application layer identifies andestablishes the availability of intended communication partners, synchronizescooperating applications, and establishes agreement on procedures for errorrecovery and control of data integrity. Also, the application layer determineswhether sufficient resources for the intended communication exist.The TCP/IP modelTransmission Control Protocol/Internet Protocol was developed in the 60’s as amethod that connect large mainframes computers together for the simple purposeof sharing data or information. In the present, most computer operating systemsmanufactures incorporate the TCP/IP suit into their software programs allowingfor each individual workstation to encompass the ability to transmit, receive,and share information through the largest mainframe available, the Internet. The TCP/IP model is made up of 4 layers, a fewlayers less than the OSI model.

The TCP/IP model consist of from highest tolowest: The Application layer, The Transport layer, The Internet layer and theLink layer or Subnet layer.The Application layerJust like the OSI model, the Application layer in the TCP/IPmodel performs the same sort of function. Only that the Application layer forthe TCP/IP corresponds to the Application layer, Presentation layer and Sessionlayer of the 7 layer OSI model.

The Transport layerTransport layers exist in both TCP/IP and OSI model. Eventhough both models have Transport layers they differ. The TCP/IP model consistof two standard transport protocols: Transmission Control Protocol (TCP) andUser Datagram Protocol (UDP). TCP uses a reliable data-stream protocol which isconnected oriented and UDP uses an unreliable data-stream protocol which isconnectionless oriented.The Internet layerThe Internet layer is a group of protocols and specificationsthat are used to transport packets from the host across a network, the hostspecified by a network address (IP address).

The Link layerThe lower level layer of the TCP/IP model, this layer is usedby a suite of protocols for the “Internet”. This is used to connect hosts ornodes to a network. This layer is compared to the “Data Link” layer and the  “Physical ” layer of the OSI model.TCP/IP Application Layer VS OSI Application, Presentation andSession layer.

The similarities in both models are comparable but differentat the same time. All though they exist in both, the approach each uses toconstruct applications is different. In the OSI model the Application layer,Presentation layer and Session layer correspond to the Application layer of theTCP/IP model.

They somewhat do the same job but use different protocols, TCP/IPuses: FTP, SMTP, TELNET, DNS and SNMP where the OSI model uses: FTAM, VT, MHS,DS, CMIP.TCP/IP Transport layer VS OSI Transport layer.UDP and TCP defined by TCP/IP Transport Layer correspond tomany of the requirements of the OSI Transport Layer. Some issues occur over therequirements in the session layer of OSI since sequence numbers and port valuescan help the Operating System to keep track of active sessions. Most of the TCPand UDP functions and specifications map to the OSI Transport Layer. The TCP/IPand OSI architecture models both employ all connection and connectionlessmodels at transport layer. The architecture calls the 2 models in TCP/IP simplyConnections and Datagrams. The OSI model uses the terms “Connection-mode” and”Connection-oriented” for the connection model and the term”Connectionless-mode” for the connectionless model.

TCP/IP Internet layer VS OSI Network layerThe Network layer of the OSI model is compared to the Internetlayer of the TCP/IP model. Both models support “Connectionless” networkservices, but only the Network layer in the OSI supports connected services.The OSI layer is a “catch-all” for all protocols that assist in networkfunctionality, where the “Internet” layer of the TCP/IP model assist ininternetworking using Internet Protocol.

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