The myth of non-overlapping channels: interference measurements in IEEE 802.11

Abstract

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