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Simulating the 802.11a PHY Model: Making It Accurate. Jeongkeun Lee , Sung-Ju Lee, HP Labs Jiho Ryu , Taekyoung Kwon , Seoul National University. Carrier Sense. Motivation. Preamble Detection (PD) PHY holds the CS busy for the duration indicated in the PHY header Energy Detection (ED)

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Simulating the 802.11a PHY Model: MakingIt Accurate

Jeongkeun Lee,Sung-Ju Lee, HP LabsJiho Ryu, Taekyoung Kwon, Seoul National University

Carrier Sense


  • Preamble Detection (PD)
    • PHY holds the CS busy for the duration indicated in the PHY header
  • Energy Detection (ED)
    • If the preamble portion was missed, the PHY holds the CS busy for any signal above CSThresh
  • Measurement-based testbed studies show operations of frame reception, capture effect and carrier sensing in real IEEE 802.11 systems differ from those of popular research simulators
  • Different chipset vendors implement physical layer capture differently
  • Revise the 802.11a PHY model simulation model
    • SINR-based Preamble Detection
    • Second Frame Capture : preamble capture + MAC frame capture
    • Carrier Sense threshold
  • Observe the effects of the revised models on network performance
  • Selected References
    • [Wintech07Lee] J. Lee, W. Kim, S. J. Lee, D. Jo, J. Ryu, T. Kwon, and Y. Choi. “An Experimental Study on the Capture Effect in 802.11a Networks,” ACM WiNTECH, Montreal, Canada, Sept. 2007.
    • [ICNP04Kochut] A. Kochut, A. Vasan, A. Shankar, and A. Agrawala. “Sniffing out the correct physical layer capture model in 802.11b,” IEEE ICNP, Berlin, Germany, Oct. 2004.

Simulator Model Revision

802.11 Frame Receive Procedure

Frame format:

  • Frame error check
  • Detects and measure signal energy
  • Timing synchronization
  • Read bitrate, frame length info etc.
  • Parity bit check
  • Go intoReceiving state (Lock on)
  • Read payload

Receiver actions:

  • Three steps for a successful frame reception
    • 1) Preamble detection 2) PHY header reception 3) MAC CRC check

Net TCP Throughput Performance


Physical Layer Capture

CSThres = RX_sense + 20dB

PHY3: PHY2 + MAC Frame Capture

  • Preamble Capture
  • MAC Frame Capture

PHY1: 350% increase over PHY0

PHY2: PHY1 + Preamble Capture





PHY1: PHY0 + SINR-based preamble detection

PHY0: Original Qualnet 3.9.5

  • Receiver has already locked on the 1st frame (in Receiving state)
    • If Message-In-Message (MIM) mode is implemented and SINR(2nd) > CAPTURE_Thres
      • Unlock the 1st frame and begin to re-synchronize with the 2nd frame (SFC)
    • W/O MIM mode, MAC frame capture is not possible.
  • MIM mode implementation is chipset-dependent
    • Atheros chipset supports MIM mode
    • Prism chipset does not
  • If SINR(2nd) > CAPTURE_Thres
    • Second Frame Capture (SFC) : drops the 1st frame’s preamble and tries to detect the 2nd frame’s preamble
  • Otherwise,
    • First Frame Capture (FFC) : continue to detect the 1st frame’s preamble
  • Un-captured frame signal increases the interference power

Ex) A NEW frame arrives, RX_sens < rss(NEW) < RX_sens+20dB, sinr(NEW) < CAPTURE_Thres

- In real systems with preamble detection and high CSThres (PHY1), PD and ED fail -> Stay in idle state

- In original Qualnet model (PHY0), the receiver goes into the receiving state but it will fail the CRC check


  • The aggregated TCP throughput can increase up to 430% with the revised PHY model
  • Extended version will be presented at ACM MSWiM 2008