Efficiency through Eavesdropping: Link-layer Packet Caching

The broadcast nature of wireless networks is the source of both their utility and much of their complexity. To turn what would otherwise be unwanted interference into an advantage, this paper examines an entirely backwardscompatible extension to the 802.11 link-layer protocol for making use of overheard packets, called RTS-id. RTS-id operates by augmenting the standard 802.11 RTS/CTS process with a packet ID check, so that if the receiver of an RTS message has already received the packet in question, it can inform the sender and bypass the data transmission entirely. We present the design, implementation, and evaluation of RTS-id on a real hardware platform that provides a DSP-programmable 802.11 radio. While limited in scale due to restricted availability of the CalRadio platform, our testbed experiments demonstrate that RTS-id can reduce air time usage by 25.2% in simple 802.11b infrastructure deployments on real hardware, even when taking into account all of the protocol overhead. Additionally, we present trace-based simulations demonstrating the potential savings on the MIT Roofnet mesh network. In particular, RTS-id provides a 12% decrease in the number of expected data transmissions for a median path, and over 25% reduction for more than 10% of Roofnet paths.