HISTORYThe OSI Reference Model was a major advance in the teaching of network concepts. It promoted the idea of a consistent model of protocol layers, defining interoperability between network devices and software. The OSI model was established by ISO in 1984.
Why do we need OSI ModelOver the past couple of decades many of the networks thatwere built used different hardware and softwareimplementations, as a result they were incompatible and itbecame difficult for networks using different specifications tocommunicate with each other.To address the problem of networks being incompatible andunable to communicate with each other, the InternationalOrganisation for Standardisation (ISO) researched variousnetwork schemes.The ISO recognised there was a need to create a NETWORKMODEL that would help vendors create interoperable networkimplementations.
Why do we need OSI ModelThe OSI Reference Model is composed of sevenlayers, each specifying particular networkfunctions.The process of breaking up the functions ortasks of networking into layers reducescomplexity.Each layer provides a service to the layer aboveit in the protocol specification. Each layer communicates with the same layer’ssoftware or hardware on other computers.
WORKS OF DIFFERENT LAYERSThe lower 4 layers (transport, network, data linkand physical —Layers 4, 3, 2, and 1) areconcerned with the flow of data from end to endthrough the network.The upper four layers of the OSI model(application, presentation and session—Layers7, 6 and 5) are orientated more toward servicesto the applications.Data is Encapsulated with the necessaryprotocol information as it moves down the layersbefore network transit.
Physical LayerThe physical layer deals with the physicalcharacteristics of the transmission medium.It defines the electrical, mechanical, procedural,and functional specifications for activating,maintaining, and deactivating the physical linkbetween end systems.
Physical LayerSuch characteristics as voltage levels, timing of voltagechanges, physical data rates, maximum transmissiondistances, physical connectors, and other similarattributes are defined by physical layer specifications.Examples :- EIA/TIA-232, RJ45, NRZ.
Data Link LayerThe data link layer provides access to the networkingmedia and physical transmission across the media andthis enables the data to locate its intended destination ona network.The data link layer provides reliable transit of dataacross a physical link by using the Media Access Control(MAC) addresses.
Data Link Layer Sub layersLLC Upper sub layer Provides common interface Supplies reliability and flow control servicesMAC Lower sub layer Appends physical address of destination computer onto the frame
Data Link LayerThe data link layer uses the MAC address to define ahardware or data link address in order for multiplestations to share the same medium and still uniquelyidentify each other.Concerned with network topology, network access, errornotification, ordered delivery of frames, and flow control.Examples :- Ethernet, Frame Relay, FDDI.
FramesStructured package for moving dataIncludes raw data (or payload) along with sender’s andreceiver’s: Network addresses Error-checking and control information Figure 2-2: A simplified data frame
Network LayerThird layer of OSI ModelTranslates network addresses into theirphysical counterpartsDecides how to route data from sender toreceiver
Network LayerBecause Network layer handles routing,routers belong in Network layer To route means to direct data based on addressing, usage patterns, and availabilityNetwork layer protocols also accomplish: Segmentation ReassemblyExamples :- IP, IPX, AppleTalk.
Transport LayerFourth layer of OSI ModelEnsures that data are transferred betweenpoints reliably and without errorsHandles flow control Method of gauging appropriate rate of data transmission
Transport LayerSequencing Process of assigning a placeholder to each piece of a data block to allowing receiving node’s Transport layer to reassemble data in correct orderAcknowledgement (ACK) Response generated in Transport layer Confirms to sender that its frame was receivedLayer 4 protocols include TCP (TransmissionControl Protocol) and UDP (User DatagramProtocol).
Session LayerThe session layer defines how to start,control and end conversations (calledsessions) between applications.This includes the control and managementof multiple bi-directional messages usingdialogue control.It also synchronizes dialogue between twohosts presentation layers and managestheir data exchange.
Session LayerThe session layer offers provisions forefficient data transfer.Examples :- SQL, ASP (AppleTalkSession Protocol).
Presentation LayerThe presentation layer ensures that the informationthat the application layer of one system sends out isreadable by the application layer of another system. If necessary, the presentation layer translatesbetween multiple data formats by using a commonformat.Provides encryption and compression of data.Examples :- JPEG, MPEG, ASCII, EBCDIC, HTML.
Application LayerThe application layer is the OSI layer thatis closest to the user.It provides network services to the user’sapplications.It differs from the other layers in that itdoes not provide services to any otherOSI layer, but rather, only to applicationsoutside the OSI model.
Application LayerExamples of such applications arespreadsheet programs, word processingprograms, and bank terminal programs.The application layer establishes theavailability of intended communicationpartners, synchronizes and establishesagreement on procedures for errorrecovery and control of data integrity.
Communication Between Sender and Receiver How Data transferbetween two systems
Communication Between Sender and Receiver Data transformed through the OSI Model
SummaryStandards are documented agreementscontaining technical specifications or otherprecise criteria used as guidelines to ensurematerials, products, processes, and services suittheir purposeProminent standards organization include: ANSI EIA IEEE ISO ITU
SummaryOSI Model divides networking architectureinto seven layers: Physical layer Data Link layer Network layer Transport layer Session layer Presentation layer Application layer
SummaryA data request from a software program is receivedby Application layer services and is transferreddown through layers of OSI Model until it reachesthe Physical layerData frames are small blocks of data with control,addressing, and handling information attached tothemEach node on a network can be identified by twotypes of addresses: Network layer address Data Link layer address
SummaryIn addition to frame types, IEEE networkingspecifications apply to connectivity,networking media, error checking algorithms,encryption, emerging technologies, and moreISO expanded OSI Model by separating theData Link layer into sublayers: LLC layer MAC layer