Speedo: Realistic achievable bandwidth in 802.11 through passive monitoring

End to end available bandwidth is often constrained by IEEE 802.11 hops in the access network. Available bandwidth measurement is particularly challenging in the wireless environment because of adaptive data rates, a time varying channel, and CSMA/CA based contention instead of simple FIFO queueing. We present a novel estimation scheme for IEEE 802.11 networks in infrastructure mode, based on passive monitoring, without any need for access point cooperation or protocol modifications. We employ an empirical map of relationships local to the client station, and direct measurements of channel utilisation, combined with the known packet size dependent overheads. In testbed experiments we show how the method provides a reliable lower bound of the achievable IP bandwidth, which is also a good point estimate in the case where sources are not greedy. We compare against probegap, an alternative method which also measures channel utilisation, but does not take rate or PER into account.