Researchers are creating mobile networks that can sustain communications even in the face of broken links and long delays.

The quest for such disruption-tolerant networks, or DTNs, is being driven by military, scientific and emergency-response wireless networks, which typically lack the connectivity, stability and predictability of conventional wired networks. Instead, researchers say, the hallmarks of a DTN are the very problems that quickly bring a conventional wireless network to its knees: frequent and unpredictable disconnects, changing nearby nodes and very long delays. The trade-off: it takes a lot longer to send and receive data over a DTN.

You can think of it as the “it’s better than nothing” approach to networking.

Breaking through breakdowns

Researchers at BBN Technologies, of Cambridge, Mass., have begun the second phase of a DTN project, funded by $8.7 million from the Department of Defense’s Defense Advanced Research Projects Agency (DARPA). Earlier this year, the researchers simulated a 20-node DTN. With each link available just 20% of the time, the network was able to deliver 100% of the packets transmitted.

“Using traditional [network] routing in the same scenario, and depending on the nature of the outages, there would be a very, very low percentage delivered, or none delivered,” says Stephen Polit, project manager for BBN’s DTN research, dubbed SPINDLE.

Click to see: DieselNet is a University of Massachusetts Amherst project to test a disruption-tolerant network installed on 40 buses. Network components are mounted in a compartment above the driver; the backplane of the computer is visible at right; on the left is a power converter, and behind it is an 802.11b access point. Also included: a GPS receiver.

“Conventional routing protocols assume there is an end-to-end path, and this path is eventually [and fairly quickly] stable,” says Rajesh Krishnan, senior scientist with BBN’s Internetwork Research Group and a specialist in DTN. “Based on this, you compute routes and set your [router] forwarding tables.”

But all that breaks down when the network ruptures because of repeated disconnections and long delays. BBN has developed a network protocol and code that moves information from node to node as connections become available, and can hold information in persistent storage until a connection is available.

The BBN team is now pulling together a full reference implementation of its DTN routing protocol, called Bundle, and a hardware and software platform incorporating this implementation, for use by selected Department of Defense partners. Phase 2 also includes defining a set of APIs so that third parties can substitute their own code for some parts of the DTN system, and creating code that will let the DTN software elements run over different types of underlying network transports, such as Bluetooth, 802.11 WLAN and Ethernet. The goal is to have a working demonstration network by late 2007.

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