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Medium Access Control for IEEE 802.11
 

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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    Medium Access Control for IEEE 802.11 Medium Access Control for IEEE 802.11 Presentation Transcript

    • Medium Access Control for IEEE 802.11
    • 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.
    •  
    • Functional Areas
      • MAC layer covers three functional areas:
        • Reliable data delivery
        • Access control
        • Security
    • 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.
    • 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
    • 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
    • C F B E D RTS RTS = Request-to-Send Pretending a circular range
    • C F A B E D RTS RTS = Request-to-Send NAV = 10 NAV = remaining duration to keep quiet
    • C F A B E D CTS CTS = Clear-to-Send
    • C F A B E D CTS CTS = Clear-to-Send NAV = 8
    • C F A B E D DATA
      • DATA packet follows CTS. Successful data reception acknowledged using ACK .
    • C F A B E D ACK
    • Functional Areas
      • MAC layer covers three functional areas:
        • Reliable data delivery
        • Access control
        • Security
    • 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).
    •  
    • 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.
    • 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).
    •  
    • 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.
    • 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
    • Point Coordination Function
    • 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
    • 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
    • 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
    • Thank You