Research Projects

Throughput improvement in 802.11 WLANs with Hidden Terminals via Local Estimation of Collision Probabilities

Avideh Zakhor, Michael N Krishnan and Ehsan Haghani

Current 802.11 networks do not typically achieve the maximum potential throughput despite link adaptation and crosslayer optimization techniques designed to alleviate many causes of packet loss. A primary contributing factor is the difficulty in distinguishing between various causes of packet loss, including collisions caused by high network use, co-channel interference from neighboring networks, and errors due to poor channel conditions. In this paper, we propose a novel method for estimating various collision type probabilities locally at a given node of an 802.11 network. We have designed a practical approach, based on combining locally observable quantities with information observed and broadcast by the access point (AP) in order to obtain partial spatial information about the network traffic. We provide a systematic assessment and definition of the different types of collision, and show how to approximate each of them using only local and AP information. Additionally, we show how to approximate the sensitivity of these probabilities to key related configuration parameters including carrier sense threshold and packet length. We verify our methods through NS-2 simulations, and characterize estimation accuracy of each of the considered collision types. Using these estimates of collision probabilities, our goal is to develop a suite of techniques to improve throughput by adapting transmission parameters such as modulation rate, packet length, forward error correction, contention window size, retransmit limit, transmit power, carrier sensing threshold, and use of RTS/CTS. [1] [2] [3].

M. Krishnan, S. Pollin and A. Zakhor, "Local Estimation of Collision Probabilities in 802.11 WLANs With Hidden Terminals," IEEE Globecom 2009 Wireless Networking Symposium (GC'09-WNS), Honolulu, HI, December 2009. [Adobe PDF]
M. Krishnan and A. Zakhor, "Throughput Improvement in 802.11 WLANs using Collision Probability Estimates in Link Adaptation," to be presented at IEEE Wireless Communications & Networking Conference, April 2010. [Adobe PDF]
W. Song, M. Krishnan, and A. Zakhor. “Adaptive Packetization for Error-Prone Transmission over 802.11 WLANs with Hidden Terminals,” IEEE International Workshop on Multimedia Signal Processing, Rio de Janeiro, Brazil, October 2009. [Adobe PDF]
•M. Christine, M. Krishnan, E. Haghani, A. Zakhor, "Local Estimation of Collision Probabilities in 802.11 WLANs: An Experimental Study," submitted to IEEE WCNC 2011, Cancun, Mexico, March 2011. [Adobe PDF]
•E. Haghani, M. Krishnan, and A. Zakhor, "Adaptive Carrier-Sensing for Throughput Improvement in IEEE 802.11 Networks," IEEE GLOBECOM 2010, Miami, FL, December 2010. [Adobe PDF]
•M. Krishnan, E. Haghani, A. Zakhor, "Packet Length Adaptation in WLANs with Hidden Nodes and Time-Varying Channels," submitted to WiOPT 2010

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