Medium Access Control for IEEE 802.11


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Medium Access Control for IEEE 802.11

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