A random mesh of radios works very differently from the wired Internet, and Morris’s group has had to figure out protocols to get the nodes to communicate. Like the Internet, the network relies on transmission control protocol, or TCP, where data is broken up into packets and both the sender and receiver work to make sure all the packets get through and are put back together in the right order. With radio, though, one has to choose the best among several possible routes between sender and receiver. “You’ve got a huge choice how you route your data through the network,” Morris says. “Unfortunately, many of those choices are much worse than others.”
Some nodes may get radio interference that drowns out part of the transmission, or may drop out entirely for all sorts of reasons. For instance, transmissions sent on the Wi-Fi frequency of 2.4 gigahertz are strongly absorbed by water, so the network operates differently in the summer, when trees can be covered in rain-laden leaves. In other cases, a passing truck can reflect radio signals and cause interference. Nodes may also be so far apart that a signal only gets through intermittently.
Roofnet tries to work around these problems. Its nodes constantly broadcast status reports that signal where they are and which nearby nodes they’re in communication with. By tracking these status reports, the network can select the best route between any two nodes at a particular moment. If a connection drops out in mid-transmission, the network sends the data along a different route.
Monitoring the connection between each pair of nodes also allows the network to decide how fast to send the data. If the rate of transmission is too low, users don’t get all the potential out of the network. But if it’s too high, the signal-to-noise ratio drops and the data can get lost. The radios use a brute-force approach, sending out test signals at all possible rates between one and 54 megabits per second, to figure out the highest rate at which data can be transmitted consistently.
The first antennas were erected last week in a test phase, and Grochow expects that widespread use by Cambridge residents will begin in late summer. “It won’t necessarily be the highest speed, but it’s going to be everywhere, just like air, and that’s pretty cool,” he says.