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...it exposed terminals [24]. In our experiments, we operate CENTAUR at 40 MHz. The traffic on all the links is backlogged. We report the aggregate throughput in each case, and use Jain’s Fairness Index =-=[13]-=- to report overall fairness. Table 3 summarizes the results presented in the paper. 7.1 Gains from using client-centric widths FLUID improves the throughput by using client-centric, link quality width...

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...ecently, this problem of choosing the right frequency and width for communication has gained relevance with the onset of white-space networking where agile adaptation of these parameters is essential =-=[6]-=-. There has also been a growing attempt to explore the usefulness of flexible channels in the context of 802.11-based networks. Current 802.11 hardware can provide a limited amount of software-level f...

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...a, USA. Copyright 2011 ACM 978-1-4503-0492-4/11/09 ...$10.00. Figure 1: Example flexible channel configurations using two channel widths of 20 and 40 MHz. The total available spectrum is 40 MHz. work =-=[7, 15, 17, 23, 25, 26]-=- has examined strategies to assign flexible channels. More recently, this problem of choosing the right frequency and width for communication has gained relevance with the onset of white-space network...

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...more links to be in carrier sensing range, but also interfere over longer distances. Partial spectrum overlaps: The extent of interference between links also depends on the amount of spectral overlap =-=[19]-=-. In case of flexible channels, partial spectral overlaps can occur when links use same center frequencies but different channel widths, or if links operate at different center frequencies. Such an in...

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...ch flexible channels, e.g., a fixed number of channel widths (5, 10, 20, and 40 MHz) and a set of permissible center frequencies in the 2.4 GHz or 5 GHz band [11]. Using this flexibility, the work in =-=[8]-=- shows how a single 802.11 link can pick an efficient channel width to adequately meet its traffic demand. At a high level, [8] shows that increasing channel width for a single, isolated link potentia...

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...a, USA. Copyright 2011 ACM 978-1-4503-0492-4/11/09 ...$10.00. Figure 1: Example flexible channel configurations using two channel widths of 20 and 40 MHz. The total available spectrum is 40 MHz. work =-=[7, 15, 17, 23, 25, 26]-=- has examined strategies to assign flexible channels. More recently, this problem of choosing the right frequency and width for communication has gained relevance with the onset of white-space network...

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...olutions to this problem. Association. APs are modified to beacon at the lowest channel width of 5 MHz, which has the most range. The center frequencies for beacon transmissions are decided using RaC =-=[5]-=-, a conflict-aware fixed-width channel assignment mechanism. Client drivers are modified to perform passive scans using a width of 5 MHz. In our current implementation, we do not support active scanni...

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...een N transmissions (all on the same channel) can be generated by performing pair-wise link throughput tests [12] at each PHY rate k, which requires a total of O(N 2 ·k) measurements. Recent research =-=[4, 14]-=- has shown that this overhead can be reduced to O(N·k) using SINR based modeling. Applying such models to a variable channel width system is not straightforward, as the number of spectral overlaps (an...

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...ization. Recently researchers have explored mechanisms that assign fine grained spectrum blocks on the basis of traffic demands. However, most of these mechanisms, such as DSAP [7], Jello [15], SWIFT =-=[22]-=-, WhiteFi [6] and DIMSUMnet [17] are designed for non-802.11 systems andfocus on deriving elegant algorithms for spectrum allocation that are primarily evaluated using network simulations or small-sc...

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... can be easily modified to scale the transmitted signal with the channel width. Further, FLUID is complementary to recently proposed fine grained frequency division mechanisms like OFDMA [3] and FARA =-=[21]-=-, and can be combined with such mechanisms to provide better gains. Scheduling in enterprise WLANs. CENTAUR [24] proposes using epoch-based scheduling mechanism for enterprise WLANs. FLUID also uses s...

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...a, USA. Copyright 2011 ACM 978-1-4503-0492-4/11/09 ...$10.00. Figure 1: Example flexible channel configurations using two channel widths of 20 and 40 MHz. The total available spectrum is 40 MHz. work =-=[7, 15, 17, 23, 25, 26]-=- has examined strategies to assign flexible channels. More recently, this problem of choosing the right frequency and width for communication has gained relevance with the onset of white-space network...

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... 54 Mbps Hidden 96 141 97 74 Exposed 107 99 73 69 Table 2: Number of hidden and exposed links depend on the channel widths. The precise methodology to identify hidden and exposed links was taken from =-=[24]-=-. MHz (throughput normalized w.r.t. 20 MHz) for four different PHY rates and rate adaptation (SampleRate), across 2872 link/interferer combinations. Since the transmitter might be outside the interfer...

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...d over all its clients) and provide a single channel to each AP that is proportional to this traffic demand. The channel choices can be periodically adapted based on demand evolution. Indeed, work in =-=[20]-=- proposes and shows the benefits of such a solution through careful simulation based studies. However, in ourattempt to implement such a solution on an 802.11 testbed, we quickly uncovered new challe...

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...the rate which maximizes the throughput (data rate × delivery probability). We note that a similar SINR-based rate adaptation mechanism for fixed channel width systems was previously proposed in DIRC =-=[16]-=-. RaC-Pack: In FLUID, the central controller uses a randomized algorithm, RaC-Pack (Randomized Compaction based Packing) to derive the transmission schedules. RaC-Pack (Algorithm 1) takes the FIFO que...

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...uld reduce in future; emerging wireless cards have switching latencies of less than 100 µs [1], while prior work in solid state electronics has shown that this delay can be reduced to as low as 40 µs =-=[9]-=-. Finally, in order to maintain an accurate conflict graph that can take into account the dynamics of the environment, it is important that the signal strengths are frequently updated. Since there is ...

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...er implementation aspects and system design issues that arise when deploying FLUID. Handling Uplink Transmissions. To account for uplink (client-to-AP) transmissions, we use a two-phase TDMA approach =-=[16, 18]-=-: the first phase uses flexible channelization for downlink traffic, and the second phase is for uplink traffic using DCF . The controller adapts the time for each phase according to the downlink/upli...

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...CTS IN FLUID In a traditional WLAN that uses a fixed channel width, the conflict graph between N transmissions (all on the same channel) can be generated by performing pair-wise link throughput tests =-=[12]-=- at each PHY rate k, which requires a total of O(N 2 ·k) measurements. Recent research [4, 14] has shown that this overhead can be reduced to O(N·k) using SINR based modeling. Applying such models to ...