(Source: Free Dictionary)
(Source: Free Dictionary)

OSI Model
  • OSI : Open System Interconnection
    • What is it?
      • network protocol stack
        • set of rules for communication of data
        • protocols implemented
      • also defines network devices
      • conceptual framework to understand complex interactions
  • control passed from one layer to the next
  • networking framework
  • not a physical existing system
    • only a concept used to guide system programmers, so that the different operating systems can communicate per network
  • seven layers with specific operational functions
    • sending data: moving from the application layer to the physical level
    • receiving data: moving from the physical layer to the application level
  • The 7 layers:
  • 7) Application Layer
    • consists of network applications
      1. e.g. email, browser, web pages, file sharing services
    • applications need connectivity of network
    • identifies: communication partner, quality of service, constraints of syntax
    • considers: user authentication, privacy
  • 6) Presentation Layer
    • configures data
      1. encryption, compression, translation
      2. translates from application to network format (encrypted)
        • translates so application layer accepts format
        • no compatibility problems
  • 5) Session Layer
    • controls communications
      1. login rights, permissions, rights
      2. establishes, manages, ends connections between applications
        • e.g. Video streamed: combines audio & video
  • 4) Transport Layer
    • guarantees delivery of data
      1. end-to-end error recovery → complete transfer
    • transfer of data
      1. received from session layer → divided into “messages” (smaller data units)
        • passed to network layer
  • 3) Network Layer
    • finds shortest path to destination (shortest time)
      1. different routing protocols
    • switching/routing technology
    • logical paths → virtual circuits for transmitting from node to node
    • data received turned into packets and routes them
  • 2) Data Link Layer
    • bus arbitration → decides whose turn to communicate
    • locates physical devices on network
    • packets encoded → turned into bits
    • transmission protocol
    • handles errors of physical layer
    • 2 sub layers
      1. MAC → Media Access Control
        • controls how network gains access to data and transfer permission
      2. LLC → Logical Link Control
        • controls error checking
    • error free data transmission
      1. packets turned into frames and back → physical layer transmits
  • 1) Physical Layer
    • physical parts → cables, voltages, connectors, bits, transfer rate, etc.
      1. electrical/mechanical level
      2. hardware for sending/recieving data
    • conveys bit stream → transmission of raw bits
      1. encoding methods for bits so it can be transferred over hardware
      2. in frame form




Flowchart Example:


OSI Model Flowchart.jpg





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Summary:

  • OSI model was established as a conceptual framework for constructing networks. It describes the connection between networks/devices.
  • What is it?
    • Concept — does not exist physically
    • A set of rules for communicating data.
    • Protocols are implemented.
    • Also describes devices.
    • 7 layers (read from bottom to top)
      • 7) Application Layer
      • 6) Presentation
      • 5) Session
      • 4) Transport
      • 3) Network
      • 2) Data Link
      • 1) Physical
    • each layer controls a specific aspect of the process — control is given from one layer to the next
  • The Layers in Detail:
    • 7) Application
      • network applications (receive input)
    • 6) Presentation
      • configures data — encryption/decryption
    • 5) Session
      • controls communication — login rights, session timeout, permissions
    • 4) Transport
      • transfer of data — divides data into smaller packets “messages"
      • guarantees delivery
    • 3) Network
      • routing protocols — find shortest path in terms of time
      • switching/routing
      • creates packets
    • 2) Data Link
      • locates physical device (MAC - Media Access Control & LLC - Logical Link Control)
      • bus arbitration — who’s turn to talk?
      • encoding — turned into bits
      • transmission protocol — error free transmission
    • 1) Physical
      • physical components (hardware) — cables, routers, etc.
      • conveys bit stream — encodes bits so can be send over hardware



Created By: Lucie Magister
Last update: 13/01/2015

Sources: