2ISO – ORG FOR STANDARDISATION The International Organisation for Standardisation (ISO) is an International standards organisation responsible for a wide range of standards, including many that are relevant to networkingIn 1984 in order to aid network interconnection without necessarily requiring complete redesign, the Open Systems Interconnection (OSI) reference model was approved as an international standard for communications architecture
3OSI ModelAn ISO standard that covers all aspects of Network Communication is the Open System InterconnectionAn open system allows two different systems to communicate with each other regardless of their underlying architectureISO is an OrganizationOSI is a model
4THE NEED FOR STANDARDSPreviously Networks used different Hardware & Software implementationsResulting in incompatibilitiesWith different specifications nodes were unable to communicate with each otherTo address problem ISO researched various network schemesNeed to create a NETWORK MODELHelp vendors create interoperable network implementations
5THE OSI REFERENCE MODEL OSI is short for Open System InterconnectionTask : Moving info between computers over a networkDivide the task into SEVEN smaller / more manageable tasksSeparation into smaller more manageable functions is known as layeringWHAT needs to be done to send data from one computer to another and not HOW it should be done
7A LAYERED NETWORK MODEL Each of 7 layers has particular Network functionsEach layer provides a service to the layer above itEach layer correspond with the layers above and below it and also the Peer layer.Through agreed upon rules called ProtocolsPeer to Peer processEach layer add info to message it receives from layer above it called Header & Trailer (Data link layer only)By breaking up tasks into layers reduces complexityEnhance professionalism
8Organization of Layers Lower 3 layers are Network Support LayersConcerned with flow of data from end to end through NetworkCombination of HW & SWPhysical layer always implemented in HardwareUpper 3 layers are User Support LayersAlways implemented in Software.Layer 4 ensures end to end reliable data transmissionLayer 2 ensures reliable transmission on a single linkLayers glued together by interfacesEach interface defines what info & services it must provide for the above layerHeaders & Trailers are added as data moves from Application to Physical and removed vice versaData is Encapsulated with the necessary protocol information as it moves down the layers before network transit.
9Only last 3 layers involved as data moved through Intermediate nodes OSI LayersOnly last 3 layers involved as data moved through Intermediate nodes
10Exchange Using the OSI Model Header & Trailer being added & removed
11LAYER 7: APPLICATIONThe application layer is responsible for providing services to the userClosest to the user and provides user interfaceEstablishes the availability of intended communication partnersDoes not provide services to any other OSI layerExamples: Spreadsheet programs, word processing programs, electronic mail, remote file access & transfer and share database management and other distributed information servicesExamples of Application layer protocols are: Telnet, SMTP, FTP, SNMP
12LAYER 6: PRESENTATIONPresentation layer is concerned with the syntax and semantics of the information exchanged between two systemsThis layer is primarily responsible for the translation, encryption and compression of dataIt ensures that the information that the application layer of one system sends out is readable by the application layer of another system. Defines coding and conversion functionsThis layer also manages security issues by providing services such as data encryption and data compressionExamples of these formats and schemes are: MPEG, QuickTime, ASCII, EBCDIC, GIF, TIFF, JPEG
13LAYER 5: SESSIONThe session layer defines how to start, control and end conversations (called sessions) between applicationsEstablishes dialog control between the two computers in a session, regulating which side transmits, plus when and how long it transmits (Full duplex)Synchronization: Allows processes to add check points. E.g. Insert check point at every 100 page of 2000 page file to ensure that each 100-page unit is received & acknowledgedTransmits Data
14LAYER 4: TRANSPORTIt regulates information flow to ensure process-to- process connectivity between host applications reliably and accuratelyAdds service point address or Port addressSegmentation & Re-assembly: SEGMENTS data from sending node and reassembles data on receiving nodeFlow control / Error control at Source to destination levelConnection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplicationsEstablishes, maintains and terminates virtual circuitsConnection oriented / Connectionless: TCP (Reliable, provides guaranteed delivery), UDP (Unreliable, less overhead, reliability can be provided by the Application layer)Provides multiplexing; the support of different flows of data to different applications on the same host
15LAYER 3: NETWORKDefines source to destination delivery of packets across NWsDefines logical addressingNo need if 2 systems on same LinkTreat each packet independentlyDefines how routing works and how routes are learnedConverts frames to packetsRouted protocols ( encapsulate data into packets) and Routing protocols (create routing tables) work on this layerExamples of Routed protocols are: IP, IPX, AppleTalk and Routing protocols are OSPF, IGRP/EIGRP, RIP, BGPRelieves higher layers from knowing about underlying data transmission and switching technologies used to interconnect systemsMay use packet switched / circuit switched networksRouters operate at Layer 3.
16LAYER 2: DATA LINKPackages raw bits from the physical layer into FRAMESThe data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addressesSource & Destination ( address of device that connects one Network to next) addressFlow Control: Prevent overwhelming of Receiving NodeError Control: Through TrailerAccess Control: Which device to have controlData Link LAN specifications: Fast Ethernet, Token Ring, FDDIData Link WAN specifications are: Frame Relay, PPP, X.25Bridges and Switches operate at this layer
17LAYER 1: PHYSICALDeals with the physical characteristics of the Transmission mediumTransmits bits from one computer to anotherElectrical: Voltage levels / timing of voltage changesMechanical: Physical connectors (DB-25)Procedural: Sequence of events by which bit stream is exchanged across Physical mediumData rates / Maximum transmission distancesPhysical TopologySynchronization of bits: Tx & Rx syncLine configuration: Pt –to –Pt / Multi PointTransmission modes & Repeaters operate at this layer
18Data EncapsulationData Encapsulation is the process of adding a header to wrap the data that flows down the OSI model.The 5 Steps of Data Encapsulation are:1. The Application, Presentation and Session layers create DATA from users' input.2. The Transport layer converts the DATA to SEGMENTS3. The NW layer converts the Segments to Packets (datagram)4. The Data Link layer converts the PACKETS to FRAMES5. The Physical layer converts the FRAMES to BITS.
19Important Data-Link: Communication between nodes on same network Network: Comm between nodes on possibly different NWsTransport: Communication between processes (running on machines on possibly different networks)Connecting Networks DevicesRepeater: Physical layerBridge: Data link layerRouter: Network layerGateway: Transport layer and above
20SUMMARY No standard for networks in the early days Difficult for Networks to communicate with each otherISO recognised this and researched various network schemes, and in 1984 introduced OSI reference modelThe OSI ensure greater compatibility and interoperability between various types of network technologiesOrganizes network functions into seven numbered layersEach layer provides a service to the layer above it and communicates with the same layer on other computersLayers 1-4 are concerned with the flow of data from end to end through the network and Layers 5-7 are concerned with services to the applications