Abstract-We evaluate two approaches for estimating the proportions of frame losses at an 802.11 station due to collisions and interference. These methods use only local 802.11 measurements available in basic access mode. We implement the estimators on an experimental testbed using off-the-shelf hardware to investigate implementation requirements and to evaluate performance in real wireless environments. We find that the estimators are accurate and of potential practical utility.
I. INTRODUCTIONThe CSMA/CA medium access mechanism of 802.11 makes estimation of channel quality challenging as frame loss due to collisions is a feature of normal operation. Importantly, the level of collision-induced loss is load dependent. The problem is how to disentangle collisions and losses due to channel impairment. Most work on channel quality estimation has focused on PHY layer approaches based on SNR and RSSI, but it is known that the correlation between these and channel behaviour may be weak. Motivated by this, in [3] the authors propose a cross-layer approach. A related technique is suggested in [5] for estimating the number of active stations. While [3], [5] test their proposals by simulation, they do not carry out an experimental validation. In view of the complex nature of the radio interference environment, and the limited accuracy of available channel models, we argue that it is vital to evaluate the performance of these schemes on real hardware.The contributions of this paper are twofold. First, we demonstrate the practicality of the estimator proposed in [3] by implementing it in a hardware testbed via driver and firmware modifications. We evaluate the performance of the estimator in real radio environments. To our knowledge, these are the first reported experimental measurements for this class of estimators and they confirm the promising nature of the approach. Second, we present an alternative technique for estimating the frame error rate due to radio interference that complements the first estimator. This makes use of information on frame status which is available in off-the-shelf network cards, but which is not yet widely utilised.