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Open System Interconnection (OSI) Model

Open System Interconnection (OSI) Model. Objectives. Explain the OSI reference model, which sets standards for LAN and WAN communications Discuss communication between OSI stacks when two computers are linked through a network Apply the OSI model to realistic networking situations.

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Open System Interconnection (OSI) Model

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  1. Open System Interconnection (OSI) Model

  2. Objectives • Explain the OSI reference model, which sets standards for LAN and WAN communications • Discuss communication between OSI stacks when two computers are linked through a network • Apply the OSI model to realistic networking situations

  3. OSI Reference Model • Foundation that brings continuity to LAN and WAN communications • Product of two standards organizations: • ISO • ANSI • Developed in 1974 • Set of communication guidelines for hardware and software design

  4. OSI Guidelines Specify… • How network devices contact each other; how devices using different protocols communicate • How a network device knows when to transmit and not transmit data • How physical network network devices are arranged and connected

  5. OSI Guidelines Specify… • Methods to ensure that network transmissions are received correctly • How network devices maintain a consistent rate of data flow • How electronic data is represented on network media

  6. OSI Layers

  7. Bottom layers Support for physical connectivity, frame formation, encoding, and signal transmission Middle layers Establish and maintain a communication session between two network nodes Monitor for error conditions Uppermost layers Application/software support for encrypting data and assuring interpretation/presentation of data OSI Layers

  8. Physical Layer Functions • Provides transfer medium (eg, cable) • Translates data into a transmission signal • Sends signal along the transfer medium • Includes physical layout of network • Monitors for transmission errors • Determines voltage levels for data signal transmissions and to synchronize transmissions • Determines signal type (eg, digital or analog)

  9. Analog Signals

  10. Digital Signals

  11. Data Link Layer Functions • Constructs data frames • Creates CRC information; checks for errors • Retransmits data if there is an error • Initiates communications link; makes sure it is not interrupted (ensures node-to-node physical reliability) • Examines device addresses • Acknowledges receipt of a frame

  12. Data Link Layer • Data link frame contains fields consisting of address and control information • Two important sublayers • Logical link control (LLC) • Media access control (MAC) • Connectionless service versus connection-oriented service

  13. Network Layer Functions • Determines network path for routing packets • Helps reduce network congestion • Establishes virtual circuits • Routes packets to other networks, resequencing packet transmissions when needed • Translates between protocols

  14. Transport Layer Functions • Ensures reliability of packet transmissions • Ensures data is sent and received in the same order • Sends acknowledgement when packet is received • Monitors for packet transmission errors and resends bad packets • Breaks large data units into smaller ones and reconstructs them at the receiving end for networks using different protocols

  15. Session Layer Functions • Establishes and maintains communications link • Determines which node transmits at any point in time • Disconnects when communication session is over • Translates node addresses

  16. Presentation Layer Functions • Translates data to a format the receiving node understands (eg, from EBCDIC to ASCII) • Performs data encryption • Performs data compression

  17. Application Layer Functions • Enables sharing remote drivers and printers • Handles e-mail messages • Provides file transfer services • Provides file management services • Provides terminal emulation services

  18. Communicating Between Stacks • OSI model provides standards for: • Communicating on a LAN • Communicating between LANs • Internetworking between LANs and WANs and between WANs and WANs

  19. Peer Protocols

  20. Applying the OSI Model

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