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WiFi MAC Protocol

Learn about the history and development of WiFi technology, as well as the MAC protocol used in wireless local area networks (WLANs) based on IEEE 802.11 standards.

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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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