Introducing NetworkStandardsIntroducing NetworkStandards
This lesson describes the Open SystemsInterconnection (OSI) reference model.This lesson describes the Open SystemsInterconnection (OSI) reference model.
The OSI reference model represents the sevenlayers of the process by which data is packagedand transmitted from a sending applicationthrough the physical wires to the receivingapplication. The OSI reference model represents the sevenlayers of the process by which data is packagedand transmitted from a sending applicationthrough the physical wires to the receivingapplication.
Network CommunicationsNetwork Communications
•Network activity involves sending data from onecomputer to another.•Network activity involves sending data from onecomputer to another.
•This complex process can be broken into discrete,sequential tasks. The sending computer must:•This complex process can be broken into discrete,sequential tasks. The sending computer must:
•1. Recognize the data.•1. Recognize the data.
•2. Divide the data into manageable chunks.•2. Divide the data into manageable chunks.
•3. Add information to each chunk to determinethe receiver.•3. Add information to each chunk to determinethe receiver.
•4. Add timing and error-checking information.•4. Add timing and error-checking information.
•5. Put the data on the network and send it.•5. Put the data on the network and send it.
Protocol StackProtocol Stack
•Network client software operates at many differentlevels within the sending and receiving computers.•Network client software operates at many differentlevels within the sending and receiving computers.
•Each of these levels, or tasks, is governed by oneor more protocols.•Each of these levels, or tasks, is governed by oneor more protocols.
•These protocols, or rules of behavior, are standardspecifications for formatting and moving the data.•These protocols, or rules of behavior, are standardspecifications for formatting and moving the data.
•When the sending and receiving computers followthe same protocols, communication is assured.•When the sending and receiving computers followthe same protocols, communication is assured.
•Because of this layered structure, this is oftenreferred to as the protocol stack.•Because of this layered structure, this is oftenreferred to as the protocol stack.
OSI Reference Model & Project 802OSI Reference Model & Project 802
•With the rapid growth of networking hardware andsoftware, a need arose for standard protocols thatcould allow hardware and software from differentvendors to communicate.•With the rapid growth of networking hardware andsoftware, a need arose for standard protocols thatcould allow hardware and software from differentvendors to communicate.
•In response, two primary sets of standards weredeveloped: the OSI reference model and amodification of that standard called Project 802.•In response, two primary sets of standards weredeveloped: the OSI reference model and amodification of that standard called Project 802.
•Acquiring a clear understanding of these models isan important first step in understanding thetechnical aspects of how a network functions.•Acquiring a clear understanding of these models isan important first step in understanding thetechnical aspects of how a network functions.
The OSI Reference ModelThe OSI Reference Model
•In 1978, the International Organization forStandardization (ISO) released a set ofspecifications that described network architecturefor connecting dissimilar devices.•In 1978, the International Organization forStandardization (ISO) released a set ofspecifications that described network architecturefor connecting dissimilar devices.
•In 1984, the ISO released a revision of this modeland called it the Open Systems Interconnection(OSI) reference model.•In 1984, the ISO released a revision of this modeland called it the Open Systems Interconnection(OSI) reference model.
•The 1984 revision has become an internationalstandard and serves as a guide for networking.•The 1984 revision has become an internationalstandard and serves as a guide for networking.
•It provides a description of how network hardwareand software work together in a layered fashion tomake communications possible.•It provides a description of how network hardwareand software work together in a layered fashion tomake communications possible.
A Layered ArchitectureA Layered Architecture
•The OSI reference modelarchitecture dividesnetwork communicationinto seven layers. Themodel defines how eachlayer communicates andworks with the layersimmediately above andbelow it.•The OSI reference modelarchitecture dividesnetwork communicationinto seven layers. Themodel defines how eachlayer communicates andworks with the layersimmediately above andbelow it.
•Each layer covers different network activities,equipment, or protocols. The layers are separatedfrom each other by boundaries called interfaces.•Each layer covers different network activities,equipment, or protocols. The layers are separatedfrom each other by boundaries called interfaces.
Relationships among OSI layersRelationships among OSI layers
•Each layer provides services to the next-higher layer andshields the upper layer from the details of how theservices below it are actually implemented.•Each layer provides services to the next-higher layer andshields the upper layer from the details of how theservices below it are actually implemented.
•At the same time, each layer appears to be in directcommunication with its associated layer on the othercomputer.•At the same time, each layer appears to be in directcommunication with its associated layer on the othercomputer.
•Thisprovides alogical, orvirtual,communication betweenpeer layers.•Thisprovides alogical, orvirtual,communication betweenpeer layers.
Application LayerApplication Layer
•Layer 7, the topmost layer of the OSI referencemodel, is the application layer.•Layer 7, the topmost layer of the OSI referencemodel, is the application layer.
•This layer relates to the services that directlysupport user applications, such as software for filetransfers, database access, and e-mail.•This layer relates to the services that directlysupport user applications, such as software for filetransfers, database access, and e-mail.
•In other words, it serves as a window, or userinterface, through which application processes canaccess network services.•In other words, it serves as a window, or userinterface, through which application processes canaccess network services.
•Application-layer protocols can be programs inthemselves, such as FTP; or they can be used byother programs, such as SMTP.•Application-layer protocols can be programs inthemselves, such as FTP; or they can be used byother programs, such as SMTP.
Presentation LayerPresentation Layer
•Layer 6, the presentation layer, defines the formatused to exchange data among networked computers.•Layer 6, the presentation layer, defines the formatused to exchange data among networked computers.
•Within the sending computer, the presentation layertranslates data from the format sent down from theapplication layer into a commonly recognized,intermediary format.•Within the sending computer, the presentation layertranslates data from the format sent down from theapplication layer into a commonly recognized,intermediary format.
•At the receiving computer, this layer translates theintermediary format into a format that can beuseful to that computer's application layer.•At the receiving computer, this layer translates theintermediary format into a format that can beuseful to that computer's application layer.
•Other tasks include protocols converting, dataencrypting, and data compressing.•Other tasks include protocols converting, dataencrypting, and data compressing.
Session LayerSession Layer
•Layer 5, the session layer, allows two applications ondifferent computers to open, use, and close aconnection called a session.•Layer 5, the session layer, allows two applications ondifferent computers to open, use, and close aconnection called a session.
•This layer is responsible for managing this dialog.•This layer is responsible for managing this dialog.
•It performs name-recognition and other functions,such as security, that are needed to allow twoapplications to communicate over the network.•It performs name-recognition and other functions,such as security, that are needed to allow twoapplications to communicate over the network.
•The session layer synchronizes user tasks byplacing checkpoints in the data stream. Thecheckpoints break the data into smaller groups forerror detection.•The session layer synchronizes user tasks byplacing checkpoints in the data stream. Thecheckpoints break the data into smaller groups forerror detection.
Transport LayerTransport Layer
•Layer 4, the transport layer, ensures that packetsare delivered error free, in sequence, and withoutlosses or duplications.•Layer 4, the transport layer, ensures that packetsare delivered error free, in sequence, and withoutlosses or duplications.
•This layer provides message repackaging,acknowledgment, retransmission, flow control anderror handling.•This layer provides message repackaging,acknowledgment, retransmission, flow control anderror handling.
•Transmission Control Protocol (TCP) andSequenced Packet Exchange (SPX) are examplesof transport-layer protocols.•Transmission Control Protocol (TCP) andSequenced Packet Exchange (SPX) are examplesof transport-layer protocols.
Network LayerNetwork Layer
•Layer 3, the network layer, is responsible foraddressing messages and translating logicaladdresses and names into physical addresses.•Layer 3, the network layer, is responsible foraddressing messages and translating logicaladdresses and names into physical addresses.
•This layer also determines the route from thesource to the destination computer.•This layer also determines the route from thesource to the destination computer.
• It also manages traffic problems on the network,such as switching and routing of packets andcontrolling the congestion of data.• It also manages traffic problems on the network,such as switching and routing of packets andcontrolling the congestion of data.
• This layer layer breaks data into packets forrouting and reassembles at destination. IP and IPXare examples of network-layer protocols.• This layer layer breaks data into packets forrouting and reassembles at destination. IP and IPXare examples of network-layer protocols.
Data-Link LayerData-Link Layer
•Layer 2, the data-link layer, sends data framesfrom the network layer to the physical layer.•Layer 2, the data-link layer, sends data framesfrom the network layer to the physical layer.
•This layerpackages rawbits from thephysical layerinto data framesin the receivingend, and do thebits errorchecking.•This layerpackages rawbits from thephysical layerinto data framesin the receivingend, and do thebits errorchecking.
Physical LayerPhysical Layer
•Layer 1, the bottom layer of the OSI referencemodel, is the physical layer.•Layer 1, the bottom layer of the OSI referencemodel, is the physical layer.
•This layer transmits the unstructured, raw bitstream over a physical medium.•This layer transmits the unstructured, raw bitstream over a physical medium.
• This layer is totally hardware-oriented and dealswith all aspects of establishing and maintaining aphysical link between communicating computers.• This layer is totally hardware-oriented and dealswith all aspects of establishing and maintaining aphysical link between communicating computers.
• This layer defines how the cable is attached to theNIC; and provides data encoding and bitsynchronization.• This layer defines how the cable is attached to theNIC; and provides data encoding and bitsynchronization.
Data PacketsData Packets
•Datapackets areassembledanddisassembled accordingto the OSIreferencemodel.•Datapackets areassembledanddisassembled accordingto the OSIreferencemodel.
OSI Layers and WindowsOSI Layers and Windows
•Windows compresses the seven layers into threelayers: file system drivers, transport protocols,and NIC drivers•Windows compresses the seven layers into threelayers: file system drivers, transport protocols,and NIC drivers
Software and ProtocolsSoftware and Protocols
•This figure shows how software and protocolsrelate to the OSI reference model and theWindows model.•This figure shows how software and protocolsrelate to the OSI reference model and theWindows model.
