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C:\Fakepath\802 11b Wireless Lan C:\Fakepath\802 11b Wireless Lan Presentation Transcript

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