2013-06-05 / OTI
On an uncharacteristically chilly May morning, members of the Commotion and Serval projects set out for the National Mall in Washington, DC to test Serval's latest piece of hardware: the Mesh Extender. Commotion Wireless is an open-source toolkit of software, documentation, and training materials that strengthens communities by allowing them to build their own local communication infrastructure. Serval is a mesh networking software designed to act as an ad hoc communications network where other infrastructure is either absent or unavailable – such as in remote areas or disaster scenarios. Some of us stood at the Washington Monument while others were at the Lincoln Memorial – a distance of nearly a mile. Using two Mesh Extenders, we successfully sent text messages and shared files between our phones (running Serval) – entirely independent of the cellular infrastructure. On the way back to the office, we hopped on the Metro – DC's subway system – to run another impromptu field test. From opposite ends of the train, we were able to send and receive messages through six subway cars (and their passengers) while the train was moving. That meant we could do something Metro riders usually can't – send and receive messages while in the subway tunnels. These results represent a significant breakthrough, as until now Serval and Commotion have been limited by the relatively short range and low power of Wi-Fi. In addition to increasing range and power, the Mesh Extender removes a major obstacle to widespread adoption of mesh for mobile phones – rooting. Normally a prerequisite for Androids to connect to a mesh network, this technically challenging process for installing a new operating system can cause problems down the road for the rooted phone. In this case, the Mesh Extender routes the messages, not the users' phones, eliminating the need for rooting. However, the Mesh Extender is still a prototype. Software issues make voice calls possible but indecipherable. Further development, including better error correction and noise cancellation, will allow for not only voice calls but potentially even longer-distance connections. Despite these remaining challenges, the Mesh Extender is a huge step towards reliable decentralized infrastructure. A device with multiple radios and a small processor, the Mesh Extender essentially acts as a relay between phones running Serval software. It is lightweight, portable, and relatively cheap and easy to build. The parts can be purchased and assembled for as little as $99. Impressively, the battery can support three to five days of continuous use. The Mesh Extender uses omni-directional antennas (as opposed to point-to-point links), which make for much easier set up and configuration, and allow for truly mobile networks. The Mesh Extender operates simultaneously on the 2.4 GHz and 900 MHz bands, both of which are unlicensed. This allows phones running Serval to tether to the closest Mesh Extender over Wi-Fi (2.4 GHz), while the Mesh Extenders themselves communicate over the 900 MHz band, which is both less congested and has better propagation characteristics.