Medium Access Control for IEEE 802.11

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A detailed presentation about Medium Access Control for IEEE 802.11

A detailed presentation about Medium Access Control for IEEE 802.11

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  • 1. Medium Access Control for IEEE 802.11
  • 2. By. P. Victer Paul Dear, We planned to share our eBooks and project/seminar contents for free to all needed friends like u.. To get to know about more free computerscience ebooks and technology advancements in computer science. Please visit.... http://free-computerscience-ebooks.blogspot.com/ http://recent-computer-technology.blogspot.com/ http://computertechnologiesebooks.blogspot.com/ Please to keep provide many eBooks and technology news for FREE. Encourage us by Clicking on the advertisement in these Blog.
  • 3.  
  • 4. Functional Areas
    • MAC layer covers three functional areas:
      • Reliable data delivery
      • Access control
      • Security
  • 5. Reliable Data Delivery
    • A wireless LAN using the IEEE 802.11 physical & MAC layers is subject to unreliable.
    • Noise, interference & other propagation effects result in loss of significant no. of frames.
    • This situation can be dealt with by reliability mechanisms at a higher layer, such as TCP.
  • 6. Reliable Data Delivery
    • For this purpose, IEEE 802.11 includes a frame exchange protocol.
    • Frame exchange protocol
      • Source station transmits data
      • Destination responds with acknowledgment (ACK)
      • If source doesn’t receive ACK, it retransmits frame
  • 7. Reliable Data Delivery
    • To further enhance reliability, a four-frame exchange may be used.
    • Four frame exchange
      • Source issues request to send (RTS)
      • Destination responds with clear to send (CTS)
      • Source transmits data
      • Destination responds with ACK
  • 8. C F B E D RTS RTS = Request-to-Send Pretending a circular range
  • 9. C F A B E D RTS RTS = Request-to-Send NAV = 10 NAV = remaining duration to keep quiet
  • 10. C F A B E D CTS CTS = Clear-to-Send
  • 11. C F A B E D CTS CTS = Clear-to-Send NAV = 8
  • 12. C F A B E D DATA
    • DATA packet follows CTS. Successful data reception acknowledged using ACK .
  • 13. C F A B E D ACK
  • 14. Functional Areas
    • MAC layer covers three functional areas:
      • Reliable data delivery
      • Access control
      • Security
  • 15. MEDIUM ACCESS CONTROL
    • IEEE 802.11 considered 2 types of MAC algorithm:
      • Distributed Access protocols
      • Centralized Access protocols.
    • End result for 802.11 is a MAC algorithm called DFWMAC (Distributed Foundation Wireless MAC).
  • 16.  
  • 17. Distributed Coordination Function
    • DCF makes use of simple CSMA algorithm.
      • If a station has MAC frame to transmit, it listens to the medium.
      • If the medium is idle, station may transmit.
      • Otherwise it must wait until current transmission is complete.
    • DCF does not include a Collision detection function.
  • 18. Distributed Coordination Function
    • To ensure smooth & fair functioning of this algorithm, DCF includes a set of delays that amounts to a priority scheme.
    • Let us consider a single delay known as an Inter Frame Space (IFS).
  • 19.  
  • 20. 3 Different IFS values
    • SIFS (Short IFS): The Shortest IFS, Used for immediate response actions
    • PIFS (Point Coordination Function IFS): A mid-length IFS, used by centralized controller in the PCF scheme.
    • DIFS (Distributed Coordination Function IFS): The longest IFS, used as a minimum delay for asynchronous frames.
  • 21. IFS Usage
    • SIFS
      • Acknowledgment (ACK)
      • Clear to send (CTS)
      • Poll response
    • PIFS
      • Used by centralized controller in issuing polls
      • Takes precedence over normal contention traffic
    • DIFS
      • Used for all ordinary asynchronous traffic
  • 22. Point Coordination Function
  • 23. MAC Frame Fields
    • Frame Control – frame type, control information
    • Duration/connection ID – channel allocation time
    • Addresses – context dependant, types include source and destination
    • Sequence control – numbering and reassembly
    • Frame body – MSDU or fragment of MSDU
    • Frame check sequence – 32-bit CRC
  • 24. Frame Control Fields
    • Protocol version – 802.11 version
    • Type – control, management, or data
    • Subtype – identifies function of frame
    • To DS – 1 if destined for DS
    • From DS – 1 if leaving DS
    • More fragments – 1 if fragments follow
    • Retry – 1 if retransmission of previous frame
  • 25. Frame Control Fields
    • Power management – 1 if transmitting station is in sleep mode
    • More data – Indicates that station has more data to send
    • WEP – 1 if wired equivalent protocol is implemented
    • Order – 1 if any data frame is sent using the Strictly Ordered service
  • 26. Thank You