Open System Interconnection

ISO – 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 networking In 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

OSI Model An ISO standard that covers all aspects of Network Communication is the Open System Interconnection An open system allows two different systems to communicate with each other regardless of their underlying architecture ISO is an Organization OSI is a model

THE NEED FOR STANDARDS Previously Networks used different Hardware & Software implementations Resulting in incompatibilities With different specifications nodes were unable to communicate with each other To address problem ISO researched various network schemes Need to create a NETWORK MODEL Help vendors create interoperable network implementations

THE OSI REFERENCE MODEL OSI is short for Open System Interconnection Task : Moving info between computers over a network Divide the task into SEVEN smaller / more manageable tasks Separation into smaller more manageable functions is known as layering WHAT needs to be done to send data from one computer to another and not HOW it should be done

OSI Model

A LAYERED NETWORK MODEL Each of 7 layers has particular Network functions Each layer provides a service to the layer above it Each layer correspond with the layers above and below it and also the Peer layer. Through agreed upon rules called Protocols Peer to Peer process Each 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 complexity Enhance professionalism

Organization of Layers Lower 3 layers are Network Support Layers Concerned with flow of data from end to end through Network Combination of HW & SW Physical layer always implemented in Hardware Upper 3 layers are User Support Layers Always implemented in Software. Layer 4 ensures end to end reliable data transmission Layer 2 ensures reliable transmission on a single link Layers glued together by interfaces Each interface defines what info & services it must provide for the above layer Headers & Trailers are added as data moves from Application to Physical and removed vice versa Data is Encapsulated with the necessary protocol information as it moves down the layers before network transit.

Only last 3 layers involved as data moved through Intermediate nodes OSI Layers Only last 3 layers involved as data moved through Intermediate nodes

Exchange Using the OSI Model Header & Trailer being added & removed

LAYER 7: APPLICATION The application layer is responsible for providing services to the user Closest to the user and provides user interface Establishes the availability of intended communication partners Does not provide services to any other OSI layer Examples: Spreadsheet programs, word processing programs, electronic mail, remote file access & transfer and share database management and other distributed information services Examples of Application layer protocols are: Telnet, SMTP, FTP, SNMP

LAYER 6: PRESENTATION Presentation layer is concerned with the syntax and semantics of the information exchanged between two systems This layer is primarily responsible for the translation, encryption and compression of data It 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 functions This layer also manages security issues by providing services such as data encryption and data compression Examples of these formats and schemes are: MPEG, QuickTime, ASCII, EBCDIC, GIF, TIFF, JPEG

LAYER 5: SESSION The session layer defines how to start, control and end conversations (called sessions) between applications Establishes 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 & acknowledged Transmits Data

LAYER 4: TRANSPORT It regulates information flow to ensure process-to- process connectivity between host applications reliably and accurately Adds service point address or Port address Segmentation & Re-assembly: SEGMENTS data from sending node and reassembles data on receiving node Flow control / Error control at Source to destination level Connection oriented transport service ensures that data is delivered error free, in sequence with no losses or duplications Establishes, maintains and terminates virtual circuits Connection 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

LAYER 3: NETWORK Defines source to destination delivery of packets across NWs Defines logical addressing No need if 2 systems on same Link Treat each packet independently Defines how routing works and how routes are learned Converts frames to packets Routed protocols ( encapsulate data into packets) and Routing protocols (create routing tables) work on this layer Examples of Routed protocols are: IP, IPX, AppleTalk and Routing protocols are OSPF, IGRP/EIGRP, RIP, BGP Relieves higher layers from knowing about underlying data transmission and switching technologies used to interconnect systems May use packet switched / circuit switched networks Routers operate at Layer 3.

LAYER 2: DATA LINK Packages raw bits from the physical layer into FRAMES The data link layer provides reliable transit of data across a physical link by using the Media Access Control (MAC) addresses Source & Destination ( address of device that connects one Network to next) address Flow Control: Prevent overwhelming of Receiving Node Error Control: Through Trailer Access Control: Which device to have control Data Link LAN specifications: Fast Ethernet, Token Ring, FDDI Data Link WAN specifications are: Frame Relay, PPP, X.25 Bridges and Switches operate at this layer

LAYER 1: PHYSICAL Deals with the physical characteristics of the Transmission medium Transmits bits from one computer to another Electrical: Voltage levels / timing of voltage changes Mechanical: Physical connectors (DB-25) Procedural: Sequence of events by which bit stream is exchanged across Physical medium Data rates / Maximum transmission distances Physical Topology Synchronization of bits: Tx & Rx sync Line configuration: Pt –to –Pt / Multi Point Transmission modes & Repeaters operate at this layer

Data Encapsulation Data 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 SEGMENTS 3. The NW layer converts the Segments to Packets (datagram) 4. The Data Link layer converts the PACKETS to FRAMES 5. The Physical layer converts the FRAMES to BITS.

Important Data-Link: Communication between nodes on same network Network: Comm between nodes on possibly different NWs Transport: Communication between processes (running on machines on possibly different networks) Connecting Networks Devices Repeater: Physical layer Bridge: Data link layer Router: Network layer Gateway: Transport layer and above

SUMMARY No standard for networks in the early days Difficult for Networks to communicate with each other ISO recognised this and researched various network schemes, and in 1984 introduced OSI reference model The OSI ensure greater compatibility and interoperability between various types of network technologies Organizes network functions into seven numbered layers Each layer provides a service to the layer above it and communicates with the same layer on other computers Layers 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