CSCI1412 - Network Layer Models

CSCI1412Lecture 16Networks 2Network Layer ModelsDr John Cowell

phones off (please)1OverviewProtocol layer modelswhat is a layer model and why use it?standards and standards agenciesThe OSI seven layer modeloverview of the layer modelthe seven layers explainedThe real worldhow the theoretical model applies to the Internet2Layer Models3Protocol Layer Models4

Head of StateFormulate messagein countrys interest

Ambassador/DiplomatsRephrase in diplomatic language

TranslatorTranslate to intermediate language

Communicate, but no concept of meaningTranslate into national language

Rephrase usingappropriate language

Receive message and decide uponresponseCommunicate ideasno generation or change of meaningHigh-level communicationno common languageProtocol StandardsA protocol is a set of rules governing communication issues - a standarddiplomats have protocolsso do networksso do computer networks!Two types of standardsde-facto - devolved standardse.g. many IBM products, Microsoft Office, TCP/IPdesigned - written standardssubmitted to a standards agency for approvalagreed by a group of companies or organisations5AgenciesProtocols often written/regulated by agencies, e.g.ANSIAmerican National Standards InstituteFDDI, ASCIICCITTComit Consultatif Internationale de Tlgraphic et TlphoniqueV series for telephone communications (modems)X series for network interfaces & public NetworksIEEEInstitute of Electrical & Electronic EngineersProject 802 - LAN standardsIABInternet Activities Boardset and document internet policies and standards, e.g. TCP/IP6Who is ISO?International Organisation for Standardisationa worldwide federation of national standards bodies from 130 countries, one from each countrya non-governmental organization established in 1947to promote the development of standardization ISOs work results in international agreements which are published as International StandardsISOs name shouldnt the acronym be IOS?yes, if it were an acronym; but it is notISO is a word, from the Greek isos, meaning equal7ISOs OSI ModelOpen Systems Interconnectiona standard description or reference model for how messages should be transmitted between any two points in a telecommunication networkany two computers can communicate when connectedregardless of underlying architectureThe seven layer modelthe process of communication between two users in a comms. network can be divided into layers, with each layer adding its own set of special, related functionswritten standard not implemented8The OSI 7 Layer Model9Origins of the ModelLate 1970s 2 separate projects byISOCCITT To standardise a framework for emerging network technologies.1984 published as ISO 7498 and CCITT as X.200

The Internet and growth of TCP/IP (Transmission Control Protocol/ Internet Protocol) became the de-facto standard.10The Seven Layers11Application layerPresentation layerSession layerTransport layerNetwork layerData Link layerPhysical layerUser services, application, activitiesActual data transmissioncommunications subnetWhat They DoThe layers are in two groupsthe upper four layers are used whenever a message passes from or to a userwithin the same computer or to another computerlower three layers used when any message passes through the host computer to anothermessages destined for some other host are not passed up to the upper layers but are forwarded to another hostPurpose is to separate specific functionsmakes implementation transparent to other componentsallows independent testing & maintenance of each layer12Information Processing13Application layerPresentation layerSession layerTransport layerNetwork layerData Link layerPhysical layerApplication layerPresentation layerSession layerTransport layerNetwork layerData Link layerPhysical layerProcess sending informationProcess receiving informationLogicalcommunicationbetweenlayerssame computerother computersData Unit at Different LayersEach layer deals with different grouping of data

Data - Application, Presentation, SessionSegment TransportPacket Network Frame Data LinkBit - Physical14Application (Layer 7)The application layerinteracts with applications that offer some form of communication componentnot an application programProvides services such asidentifying communicating partnersdetermining the identity and availability of partnersdetermining resource availabilitydecide whether sufficient networking resources are availablesynchronising communicationall communication between applications requires cooperation15Presentation (Layer 6)The presentation layerpresents data in user understandable formatallows user to see information, not datatranslates between formatsProvides a variety of coding and conversion functions that are applied to application layer datadata representatione.g. JPEG, GIF, TIFF formats for graphicscharacter representatione.g. ASCII, EBCDIC encodings for charactersdata compression schemesdata encryption schemes16Session (Layer 5)The session layerresponsible for managing connections between two machines during the course of communication determines whether all pertinent information for the session has been transmitted and/or receivedThe layerestablishes logical connectioncontrols synchronisation between two endsmanages acknowledgements and retransmissionsreconnects on premature termination (at lower level)releases logical connection17Transport (Layer 4)The transport layerend-to-end communicationsdecides upon network to useaccording to data type, e.g. packets or continuous bit streamTransport layer functions typically includeflow controlensuring there is enough space to send/receive messagesmultiplexingcombining more than one message on the same connectionpacketingbreaking larger units down into small pieces to be transferrederror checking and recovery18Network (Layer 3)The network layerprovides routing and addressing functions that enable multiple data links to be combined into a networksending it in the right direction to the right destination on outgoing transmissionsreceiving incoming transmissions at the packet levelIt provides functions such asrouting strategiesdeciding which route to use and whyforwardingsending on packets destined for other hostscontrolling communications subnet19Data Link (Layer 2)The data link layerprovides the first level of organization of the data bits into a rudimentary structure called a datalink framethis is organized into fields of information that convey the beginning and ending of the framethe address of the senderthe address of the receivera method to ensure that the frame does not contain errorsan area to provide some basic administrative functionIt is sometimes broken down in two sub layerslogical link control layer (LLC)media access control layer (MAC)20Physical (Layer 1)The physical layerdefines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating the physical link between communicating network systemstransmits/receives bits as electrical impulsesThis layer describesthe network topology (shape, structure)the transmission media (cables, broadcasts)the transmission devices (network cards, modems)the electrical or optical signalling characteristics21The Real World22The Real World!Loosely, a five layer model23application layerpresentation layersession layertransport layernetwork layerdata link layerphysical layerOSI7654321world wide web (HTML/HTTP)email (POP / SMTP)file transfers (FTP) remote sessions (TELNET)transmission control protocol (TCP)internet protocol (IP)(physical level)ethernetThe Real World54321PPPmodemApplication LayerTop layer (5) comprises other protocols such as HTML / HTTP (the World Wide Web)POP / SMTP (email)FTP (file transfer)TELNET (remote sessions)DNS (Domain Name Server)maps domain names to IP addresses146.227.102.2 http://www.cms.dmu.ac.uk/This application layer roughly corresponds to layers 5, 6, and 7 of the OSI model24TCP/IPTCP/IP is the basic communication protocol (language) of the InternetTransmission Control Protocolmanages assembling a message or file into smaller parts (packets) that will be transmitted over a networkreassembles packets when receivedTCP OSI layer 4 (transport)Internet Protocolhandles the address part of each packet to ensure that it gets to the correct destinationIP OSI layer 3 (network)25Lower LevelsThree of the most common methods of connecting to the Internet areEthernetLAN protocol developed by Xerox, DEC & Intel in 1976specifies the way data is transmitted as well as the physical connection optionsincorporates layers 1 and 2 of the OSI modelPPP over a modem linkpoint-to-point protocol controls how the data is sent (level 2)now included in Windows and other operating systemsmodem handles transmission of data over telephone line (level 1)DSL (Broadband)utilises unused bandwidth on standard telephone wiresneed frequency divider (DSL modem) to separate voice call from data signal26SummaryProtocol layer modelswhat is a layer model and why use it?standards and standards agenciesThe OSI seven layer modeloverview of the layer modelthe seven layers explainedThe real worldhow the theoretical model applies to the Internet27