C:\Fakepath\802 11b Wireless Lan


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C:\Fakepath\802 11b Wireless Lan

  1. 1. 802.11b Wireless LAN Presentation by Srividya Srinivasan
  2. 2. What is unique about wireless <ul><li>Interference and Noise </li></ul><ul><li>Full connectivity cannot be assumed </li></ul><ul><ul><li>Hidden terminal problem </li></ul></ul><ul><li>Battery Usage </li></ul><ul><ul><li>Requires power management </li></ul></ul><ul><li>Security </li></ul><ul><ul><li>No physical boundaries </li></ul></ul><ul><ul><li>Overlapping LANs </li></ul></ul>
  3. 3. Requirements of a wireless standard <ul><li>Single MAC to support multiple PHY mediums </li></ul><ul><li>Robust to interference </li></ul><ul><li>Need to deal with the hidden terminal problem </li></ul><ul><li>Need provision for time bounded services </li></ul><ul><li>Support for power management to save battery power </li></ul><ul><li>Ability to operate world wide </li></ul><ul><ul><li>Hence uses the 2.4GHz ISM band </li></ul></ul>
  4. 4. System Architecture <ul><li>Two basic system architectures </li></ul><ul><ul><li>Ad hoc </li></ul></ul><ul><ul><li>Infrastructure based </li></ul></ul>
  5. 5. Ad hoc wireless LANs <ul><li>A group of stations using the same radio frequency – Basic Service Set </li></ul>
  6. 6. Infrastructure based Wireless LANs <ul><li>Stations and Access Points </li></ul><ul><li>Distribution system connects cells via access points to form a single network </li></ul><ul><li>Extends wireless area coverage </li></ul>
  7. 7. Access Points <ul><li>Stations select an AP and “associate” with it </li></ul><ul><li>Support roaming </li></ul><ul><li>Provide other functions </li></ul><ul><ul><li>time synchronization (beaconing) </li></ul></ul><ul><ul><li>power management support </li></ul></ul><ul><ul><li>point coordination function </li></ul></ul><ul><li>Traffic typically (but not always) flows through AP </li></ul><ul><ul><li>direct communication possible </li></ul></ul>
  8. 8. Distribution system <ul><li>Used to interconnect multiple wireless cells </li></ul><ul><ul><li>Multiple BSS are connected to form an ESS Extended Service Set </li></ul></ul><ul><ul><li>Allows mobile stations to access fixed resources </li></ul></ul><ul><li>Not part of the 802.11 standard </li></ul>
  9. 9. MAC layer <ul><li>Controls medium access </li></ul><ul><ul><li>Distributed coordinated function </li></ul></ul><ul><ul><ul><li>Mandatory access mechanism – CSMA/CA </li></ul></ul></ul><ul><ul><ul><li>Optional RTS/CTS extension to deal with the hidden terminal problem </li></ul></ul></ul><ul><ul><li>Point Coordinated function </li></ul></ul><ul><ul><ul><li>Contention free polling method to support time bounded services </li></ul></ul></ul>
  10. 10. MAC Services <ul><li>Synchronization </li></ul><ul><ul><li>Specifies the Timing Synchronization function (TCF) </li></ul></ul><ul><ul><li>Within a BSS (quasi) periodic transmission of beacon signals – contains timestamp – all local nodes adjust their local timer according to the timestamp. </li></ul></ul>
  11. 11. MAC Services <ul><li>Infrastructure based networks – Access point relays the beacon signal </li></ul><ul><li>Ad hoc networks – each node maintains own synchronization timer and starts the transmission of a beacon signal. All stations adjust their clocks to the beacon which wins and suppress their beacons for this cycle </li></ul>
  12. 12. MAC Services <ul><li>Power management </li></ul><ul><ul><li>Two states for a station –sleep and awake </li></ul></ul><ul><ul><li>Transceiver is switched off whenever it is not needed </li></ul></ul><ul><ul><li>Transceiver wakes up periodically and checks whether there are any data frames for it buffered </li></ul></ul>
  13. 13. MAC Services <ul><li>Infrastructure based networks </li></ul><ul><ul><li>Access point buffers all frames destined for stations operating in power saving mode </li></ul></ul><ul><ul><li>With every beacon signal it transmits a TIM – traffic indication map – list of stations for which data frames are buffered in the access point </li></ul></ul>
  14. 14. MAC Services <ul><li>Ad hoc networks </li></ul><ul><ul><li>No access point to buffer the data </li></ul></ul><ul><ul><li>Each station needs to have the ability to buffer data if it wants to communicate with a power saving station </li></ul></ul><ul><ul><li>All stations now announce a list of buffered frames during a period when they are all awake – the ATIM window </li></ul></ul>
  15. 15. MAC Services <ul><li>Roaming </li></ul><ul><ul><li>Station starts scanning for a new access point if current link quality is very poor </li></ul></ul><ul><ul><li>Passive scanning and Active scanning </li></ul></ul><ul><ul><li>Selects best access point based on signal strength and sends Association request to it </li></ul></ul><ul><ul><li>Selected AP informs DS which updates its database </li></ul></ul>
  16. 16. PHY layer <ul><li>Supports 3 different physical layers </li></ul><ul><ul><li>Infrared </li></ul></ul><ul><ul><li>Radio transmission </li></ul></ul><ul><ul><ul><li>Frequency hopping spread spectrum </li></ul></ul></ul><ul><ul><ul><li>Direct sequence spread spectrum </li></ul></ul></ul><ul><ul><ul><li>These two do not interoperate with one another </li></ul></ul></ul><ul><li>The data rates achieved are 1 and 2 Mbps using radio waves </li></ul>
  17. 17. 802.11b Enhancements <ul><li>802.11b supports two new speeds 5.5 and 11 Mbps </li></ul><ul><li>DSSS is the sole physical layer technique </li></ul><ul><li>802.11b systems will interoperate with 1 Mbps and 2 Mbps 802.11 DSSS systems but not with FHSS systems </li></ul>
  18. 18. 802.11b Enhancements <ul><li>Uses Complementary Code keying ( CCK ) to encode 4 bits per career to get 5.5 Mbps and 8 bits per career to get 11 Mbps. </li></ul>