An Experimental Study on the Capture Effect in 802.11a by tjm72505

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									An Experimental Study on the
  Capture Effect in 802.11a
         Networks




                   C.Abishek


                               1
           Capture Effect

• Let us assume only 2 frames collide
• MIM mode enabled in radios
• Second frame arrives when first frame
  reception is going on
• Receiver detects the energy increase
  due to the second incoming frame



                                          2
           Capture Effect
• Two consequences for the detected
  energy increase :
• If (energy increase > capture threshold)
 Receiver receives the second frame
 Else
 Receiver continues with the first frame



                                         3
           Capture Effect
• Retraining process
  To synchronize with and demodulate the
  2nd frame
• Successful Capture determined by:
  Preamble Detection ( SYNC )
  Frame body FCS



                                       4
             Experiment
• A collision can results in:
a) Both frames getting corrupted
b) Stronger frame arrives first
c) Stronger frame arrives later but
   within the preamble time of the weaker
   frame
• Combination of timing relation, signal
   strength, and PHY bit rate
                                        5
  Testbed, Topology design, and
        Global Timeline
                  SIR at the receiver
                  = RSS from sender –
                  RSS from interferer
                   TSFT - Timestamp

• Two transmitters, One receiver, and
Two sniffers ( record timestamps)
• Atheros Card to set MAC
configuration ( Beacons disabled )
                                        6
        PHY layer Capture
Sender First Capture ( SF )
• Preamble successful but FCS suffers due
  to interferer
• Timing relation is not significant
Sender last Capture ( SLC )
• First arriving frame is decoded until
  sender’s frame arrives
• First arriving frame was garbled ( Noise
  )
                                        7
SLC with interferer’s garbled frame (
SLG )
• Interferer’s frame is garbled,
sender’s frame arrives.

                                        8
         PHY layer capture
The interferer’s frame garbled because:
• Of previous frame of the sender
• Outside the communication range




                                          9
SIR threshold for multiple bit
            rate



SF capture:
• High SIR required to decode at
higher rate because of interferer’s
frame
• The SIR threshold for SF = SIR
threshold for FCS check               10
SLC capture :
• 802.11a preamble encoded at 6 Mbps
  always
• For bit rate lower than 24 Mbps, SIR
  threshold for successful frame capture is
  determined by preamble detection stage
  and by FCS for above 24 Mbps.
SLG capture :
• Preamble detection stage determine the
  capture SIR ( has to be higher because
  the receiver is already trying to decode
  the garbled frame )

                                              11

								
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