Collecting Broken Frames: Error Statistics in 802.11b/g Links

We present a measurement method that allows to capture the complete set of all PSDU (PLCP Service Data Unit) transmissions and receptions in live IEEE 802.11b/g links with very high timing resolution. This tool provides an in-depth view of the statistics of frame-losses as it makes it possible to distinguish between different loss types such as complete miss, partial corruption and physical-layer capture. Getting access to this low-level statistics on nodes that actively participate in transmissions themselves is a challenging task since the software-interface provided to the network layer needs to remain untouched and cannot be used for tracing. In this contribution we describe in detail how to non-intrusively circumvent these restrictions and also present initial results

The Myth of Non-Overlapping Channels: Interference Measurements in IEEE 802.11

It has become a widely accepted assumption that multiple IEEE 802.11b/g transmissions in physical proximity can coexist without interfering each other. This is claimed to be the case when using separate channels with a minimum distance of 25 MHz, e.g. channel 1 and 6, which are often referred to as non-overlapping. In contrast we show that in practice cross-channel interference can be present also between non-overlapping channels if the interfering transmitter is in the proximity of the receiver. This phenomenon is known as the "near-far effect" in wireless communications. On IEEE 802.11 this has two main effects: frame corruption due to increased interference noise and channel blocking due to spurious carrier detection. The problem can be particularly serious when using IEEE 802.11 technology to build multi-hop mesh networks. Through an extensive set of experiments with off-the-shelf certified WiFi chipsets we demonstrate the presence and the detrimental effects of cross-channel interference between non-overlapping channels. We adopt an incremental approach: we first consider the case of unacknowledged broadcast packets, then we extend to regular UDP streams, finally we provide preliminary results for multi-hop TCP flows

On the Feasibility of IEEE 802.11 Multichannel Multihop Mesh Networks

Several scientific works have considered the possibility to build Wireless Mesh Networks (WMN) using multi-channel IEEE 802.11 architectures. At the basis of these works is the notion of ‘‘non-overlapping” channels, i.e. with a frequency separation equal or greater than 25 MHz. It is now a common assumption that multiple independent transmissions over these channels can coexist without mutual interference even in physical proximity. In this work we demonstrate that this assumption does not hold in general. Through an extensive set of experiments we illustrate the presence of cross-channel interference between ‘‘non-overlapping” channels at relay nodes due to the ‘‘near-far” effect. We analyze in what manner the MAC layer reacts to such an interference and how this problem extends to higher layers, with detrimental effects on the global throughput. The central problem is that cross-channel interference is not handled adequately by the MAC layer, and in some cases single-channel multi-hop settings perform better than multi-channel. Our results highlight a serious mismatch between some routing and channel assignment schemes proposed recently by the research community, assuming full separation between non-overlapping channels, and what is achievable in practice. More generally, as the presence of cross-channel interference can not be neglected at relay nodes, our findings point to a fundamental problem in building Multi-channel Multi-hop WMN based on IEEE 802.11b/g technology