Towards Concurrent Communication in Wireless Networks

Avoiding collisions is one of the key roles of mediaaccess (MAC) protocols. Since MACAW and 802.11, carrier sense and exchange of request-to-send (RTS) and clearto-send (CTS) packets have been used to prevent concurrent communication in wireless networks. Yet these approaches have significant cost: they prevent all concurrent communication, even exchanges that might not result in loss; they reduce end-to-end throughput in a multi-hop network; and control traffic imposes control overhead on networks with small data payloads such as 802.15.4. In this paper, we show that RTS/CTS is almost never desirable in modern wireless networks that support power control and channel capture. We use four-node experiments with 802.15.4 radios to show that concurrent communication is often possible, depending on node locations and transmit powers. We validate an SINRbased propagation model against these experiments and use simulation to systematically explore how node location affects the ability to communicate. Given optimal power settings, when two sources are outside some minimal distance, they can communicate concurrently with two receivers more than 80% of the time. An optimal algorithm requires perfect knowledge of the channel and transmission state, so we then sketch Gain-Adaptive Power Control, a MAC protocol that provides significant benefit with only local and prior information. Compared to optimal, we show that this practical MAC can transmit concurrently 75% of the time, but requires a larger minimum source separation. We also show that least one sender can capture the channel 77–88% of the time, regardless of source and receiver location, so the cost of failed concurrent communication is only slightly worse than RTS/CTS. These results provide compelling evidence that future MAC protocols should exploit power control and channel capture.