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WiFi MAC Protocol

WiFi MAC Protocol. History. WiFi is used to “describe the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications WiFi developed by Kyle Brown

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WiFi MAC Protocol

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  1. WiFi MAC Protocol

  2. History • WiFi is used to “describe the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications • WiFi developed by Kyle Brown • IEEE 802.11 “denotes a set of Wireless LAN/WLAN standards developed by working group 11 of the IEEE LAN/MAN Standards Committee (IEEE 802)”

  3. First WiFi • Intended for cashiers • Called WaveLAN • Vic Hayes • “Father of WiFi” • Helped design IEEE 802.11b, 802.11a and 802.11g

  4. MAC Protocol • MAC stands for Medium Access Control • Used to provide the data link layer of the Ethernet LAN system • Job is to add “a 14 byte header (Protocol Control Information (PCI)) before the data and append a 4-byte Cyclic Redundancy Check (CRC) after the data

  5. Network-Level Architecture

  6. Network-Level Architecture Cont.

  7. IEEE 802.11 Standard • PHY/MAC standard for wireless LANs • First standardized in 1997 • Meet great success starting in 1999 • Several working groups • IEEE 802.11a: high speed extension to the 5GHz band • 802.11b/g: high speed extension to the 2.4GHz band • 802.11e: Quality of service (QoS) enhancement (still active) • 802.11i: Security enhancement • 802.11s: Mesh-networking support Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  8. Protocols

  9. Protocols

  10. IEEE 802.11 Group Standards

  11. IEEE 802.11 Group Standards Cont.

  12. 802.11 Protocol Entities Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  13. MAC in Detail • Channel access mechanism • Distributed Coordination Function (DCF) • Carrier sense multiple access (CSMA) with immediate MAC-level ACK • RTS/CTS(4-way handshaking) exchange (optional) • RTS stands for Request-to-Send • CTS stands for Clear-to-Send • Point Coordination Function (PCF) • Polled access through AP and distributed access • Contention-free period (CFP) and contention period (CP) • Seldom implemented in practice Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  14. CSMA/CA Explained • Reduce collision probability where mostly needed • Stations are waiting for medium to become free • Select Random Backoff after a Defer, resolving contention to avoid collisions • Efficient Backoff algorithm stable at high loads • Exponential Backoff window increases for retransmissions • Backoff timer elapses only when medium is idle • Implement different fixed priority levels • To allow immediate responses and PCF coexistence Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  15. CSMA/CA + ACK Protocol • Defer access based on Carrier Sense • Clear Channel Assessment (CCA) from PHY and Virtual Carrier Sense state • Direct access when medium is sensed free longer then DIFS, otherwise defer and backoff • Reciever of directed frames to return an ACK immediately when CRC correct • When no ACK received then retransmit frame after a random backoff (up to maximum limit) Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  16. CSMA/CD • CSMA stands for Carrier Sense Multiple Access • CD stands for Collision Detection • Allows for Retransmission Back-Off

  17. RTS/CTS Based Access • Duration field in RTS and CTS frames distribute Medium Reservation information which is stored in a Net Allocation Vector (NAV) • Defer on either NAV or “CCA” indicating Medium Busy • Use RTS/CTS is optional but must be implemented • Use is controlled by a RTS_Threshold parameter per station • To limit overhead for short frames Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  18. Frame Formats • MAC Header format differs per Type: • Control Frames (several fields are omitted) • Management Frames • Data Frames • Includes Sequence Control Field for filtering of duplicates caused by ACK mechanism Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  19. Address Field Description • Addr 1 = All stations filter on this address. • Addr 2 = Transmitter Address (TA) • Identifies transmitter to address the ACK frame to • Addr 3 = Dependent on To and From DS bits • Addr 4 = Only needed to identify the original source of WDS (Wireless Distribution System) frames Slide taken from: http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint

  20. Applications • Wireless Access Point (WAP) • Wireless Routers • Wireless Ethernet Bridge • Range Extender

  21. Gaming Applications • Nintendo DS • Sony PSP • Xbox 360 • Playstation 3 • Nintendo Wii

  22. Problems / Performance Issues • Interruption by other devices • High power consumption • When setup the network defaults to open mode • Large area coverage

  23. Security Issues • WEP encryption is breakable • Spoofing authorized MAC addresses • Wardriving and Warchalking

  24. Question Summary / Conclusion • What is WiFi MAC Protocol? • It is the underlying technology of wireless local area networks (WLAN) based on the IEEE 802.11 specifications that provides the data link layer of the Ethernet LAN system. • Who came up with WiFi MAC Protocol? • The IEEE Task Groups and created the IEEE 802.11 or IEEE 802.11x Committee, however IEEE only sets specifications. WiFi Alliance runs the certification program. • What is the difference between DCF and PCF? • DCF uses carrier sense multiple access with immediate MAC-level ACK and contains RTS/CTS optional exchange. PCF uses polled access through AP and distributed access; cotains a contention-free period and a contention period; and is seldom implemented in practice.

  25. References • http://en.wikipedia.org/wiki/Image:Wi-Fi_logo.png • http://en.wikipedia.org/wiki/Wi-Fi • http://www.tate.org.uk/liverpool/ima/rm5/images/cashier_lg.jpg • http://www.art-events.de/systeme/images/wlan3.jpeg • http://www.aria.co.uk/mainimages/wpn802a.jpg • http://media.pugetsoundsoftware.com/ask-leo.com/images/wireless2bad.png • http://www.circuitcity.com/IMAGE/product/detail/dlk/EC.DLK.DWLG710.JPG • http://www.wsdmag.com/Files/32/8624/Figure_01.gif • http://www.os2warp.be/wireless/wireless_comparison.jpg • http://ei.cs.vt.edu/~history/50th/IEEE.logo.GIF • http://tab.computer.org/tcca/images/ieee.jpg • http://upload.wikimedia.org/wikipedia/en/thumb/b/bd/Blue_infrared_light.jpg/300px-Blue_infrared_light.jpg • https://weather.unisys.com/satellite/images/sat_ir_us.gif • http://en.wikipedia.org/wiki/IEEE_802.11#802.11_legacy • http://microwave.gotovim.ru/pics/microvawe.jpg • http://reviews.cnet.com/i/qg/tvpc/power_tv.jpg • http://acni.dnsalias.net/sites/batist/Picture%20Library/1/Open-door.jpg • http://www.cse.ucsc.edu/classes/cmpe257/Spring05/lecture/lecture3.pdf#search='802.11e%20MAC%20protocol%2C%20powerpoint • http://www.ece.cmu.edu/~cpyue/PatrickWebFiles/AR5110.jpg • http://www.personaltelco.net/albums/album26/DSC00152.jpg • http://www.blurtek.com/uploaded_images/belkin-n-726511.jpg • http://www.erg.abdn.ac.uk/users/gorry/eg3561/dl-pages/crc.html • http://www.erg.abdn.ac.uk/users/gorry/eg3561/lan-pages/csma-cd.html • http://www.erg.abdn.ac.uk/users/gorry/eg3561/lan-pages/mac.html • http://ieeexplore.ieee.org/iel5/9858/31043/01443506.pdf

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