Wireless LAN IEEE 802'11, WiFi Neil Tang 09302009

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Wireless LAN (IEEE 802.11, WiFi)Neil
Tang09/30/2009
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Outline

• Basic Architecture
• Physical Properties
• MAC Protocols
• WLAN with Multiple APs

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Wireless LAN (802.11)

• Basic Architecture Infrastructure mode and ad
  hoc mode
• MAC Protocol CSMA/CA (MACA)

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Basic Architecture
AP
Infrastructure mode
Ad hoc mode
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Physical Properties

• Encoding Modulation FHSS (Frequency Hopping
  Spread Spectrum), DSSS (Direct Sequence Spread
  Spectrum), OFDM (Orthogonal Frequency Division
  Multiplexing)
• Spectrum 2.4GHz/5GHz
• Error Detection CRC
• Maximum Data Rate 11Mbps/54Mbps
• Transmission Range 100-1000m

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Hidden Terminal Problem
A
B
C
Transmission Range
Nodes A and C are hidden terminals.
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Exposed Terminal Problem
A
B
C
D
Transmission Range
Nodes A and B are exposed terminals.
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MAC Protocols

• Distributed Coordination Function (DCF) Simple
  protocol with no centralized control.
• Point Coordination Function (PCF) APs control
  medium access and provide collision-free
  communications.

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DCF

• Basic Idea CSMA/CA (MACA)
• Request-To-Send (RTS) and Clear-To-Send (CTS) are
  used to reserve space and time for transmission.
• Both physical and virtual carrier sensing
  (Network Allocation Vector (NAV)) are used to
  determine the time for transmission.
• ACK frame and stop-and-wait protocol
• The exponential backoff algorithm

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DCF
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WLAN with Multiple APs
Distribution System
AP1
AP2
A
B
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WLAN with Multiple APs

• AP Selection (Scanning)
• The node sends a Probe frame.
• All APs within reach reply with a Probe Response
  frame.
• The nodes selects one of the access points based
  on signal strength or other metrics, and sends
  that AP an Association Request frame.
• The AP replies with an Association Response
  frame.
• Active scanning Vs. passive scanning

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Frame Format

• Address Fields
• Normal Case (point-to-point) Addr1(destination),
  Addr2(source)
• Complex Case (routing via distribution system)
  Addr1(ultimate destination), Addr2(intermediate
  sender), Addr3(intermediate destination),
  Addr4(original source)