An Australian team has trialled a mobile phone system that works even where there's no network signal.
The Serval Project uses unlicensed spectrum. Wifi-enabled 'Batphones' rebroadcast their signal to create a cellular mesh network - a cellphone swarm, except that only two are needed.
Existing phone numbers will work on a Serval network, the team says.
"It will allow people in remote or isolated townships, or farm workers in network black spots to talk to each other," says Dr Paul Gardner-Stephen of Flinders University.
"People in a disaster ravaged area will be able to contact friends and family and aid workers will be better able to coordinate relief efforts."
The researchers have now carried out a successful trial of the system. They tested the Batphones for use in disaster relief, search and rescue and in a deep gorge.
"All three were successfully conducted, with our team making all manner of phone calls in sorts of places, from hill-top to sheltered gorge, from indoors to mountainside," says Dr Gardner-Stephen.
Using just three phones, the team was able to create a network covering a square kilometer.
The system uses the team's open source Distributed Numbering Architecture (DNA) with existing mesh network technology developed by Village Telco for unlicensed wireless spectrum.
This uses the Wifi interface in the phone as a router, carrying voice over it in such a way that the signal doesn't need to go back to a tower.
Currently, the range is limited to a few hundred meters. The researchers want to extend this, and also improve sound quality. They say that with funding, they could have a fully-operational system within 18 months.
A separate project is based on small mobile phone towers that can be dropped by aircraft.
There's a video of the trial, here.