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
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  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

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