Simulating the 802.11a PHY Model: Making It Accurate

1. Simulating the 802.11a PHY Model: MakingIt Accurate Jeongkeun Lee,Sung-Ju Lee, HP LabsJiho 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) • 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 (CSThres=RX_sensetivity) 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 1st 2nd 1st 2nd 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 Conclusion • The aggregated TCP throughput can increase up to 430% with the revised PHY model • Extended version will be presented at ACM MSWiM 2008