Researchers at Microsoft's labs in Cambridge, England, are developing a file-sharing technology that they say could make it easier to distribute big files such as films, television programs and other media to end users over the Internet. When used in this way, the system places much less stress on the original file servers then traditional download methods, as users are essentially also donating their upload bandwidth to the downloading of the files by other users.

This can dramatically reduce the overall cost of distributing media, as the amount of bandwidth used by the originating server is much lower that if all users were downloading from a central point. This could make it easier for smaller companies to be able to distribute media to a wider audience. The technology also includes DRM (digital rights management) to prevent it being used to share illegal files.

Code-named Avalanche, the technology is similar to existing peer-to-peer (P2P) file swapping systems such as BitTorrent, in the sense that large files can be divided into many smaller pieces to ease their distribution. End users request the file parts from other users' hard drives and reassemble them to create the original file.

Such systems can scale well to serve millions of users, and reduce the bandwidth and computing costs of sending content directly to users from central servers. Some have also irritated publishers who complain the services are used to share copyright works illegally. For example, recently Star Wars Episode III: Revenge of the Sith was made available over BitTorrrent on it day of release.

The problem with existing systems, according to Microsoft, is that people sometimes have to wait a long time to receive the last, rare pieces of a file. This is made worse when clients drop off line unexpectedly and creates bottlenecks when only a few clients have files that are in high demand. It should be noted that this is much more of a problem for illegal content, where the original 'seeder' is unlikely to always be available or continuing seeding permanently - whereas legal systems are more likely to repopulate themselves from original content servers.

Avalanche goes a long way to solving the problems of rare pieces, according to Peter Key, joint head of the systems and networking group at Microsoft Research in Cambridge, during an open day on Wednesday.

It does this by encoding the file pieces at the server with a special algorithm before they are distributed. Each encoded piece contains information about every other piece of the original file, so users don't have to collect every last piece in order to reassemble the whole, Key said.

"Each encoded piece has the 'DNA' of all pieces in the file," another Microsoft researcher wrote. "A given encoded piece can be used by any peer in place of any piece."

When PCs in the Avalanche network receive encoded files, they randomly create new encoded files from the ones they have collected, and these are sent to other peers. When a user receives enough encoded files, they assemble them to make the original.

The system differs from BitTorrent's eponymous software in a few ways, Key said. It does not depend on central servers, called 'trackers', to orchestrate the download. The Avalanche client on each PC shares the files automatically among users; they do not look at other users' hard drives to find what they want. And the system works well in smaller networks, such as a corporate intranet, he said.

Perhaps more importantly for content creators, Microsoft claims its system prevents users from redistributing copyright material, because Avalanche will only forward files that have been signed by the publisher.

Microsoft has developed a prototype of Avalanche and is testing it by using it to distribute software applications to several thousand of its software beta testers, according to a research engineer demonstrating the software in Cambridge. The company has distributed a 4GB application in as little as a day, down from about two weeks when it sends a program directly, he said.

The software may also be interesting to TV broadcasters and movie studios. Microsoft has been in talks with both groups, and Avalanche may be introduced to users in the UK as early as next year, he said.

The BBC began testing a service last month that lets people download TV and radio programs using a P-to-P system from Kontiki, a leading corporate-focussed P2P network also used by Adobe and and the GameSpy network for distributing large game demos. It is not looking at Avalanche currently, but will put the contract out for public tender before launching the service, said Chris Charlton, a BBC spokesman.

The concept behind Avalanche is impressive, according to Mike Thompson, principal research analyst with Butler Group, who saw the technology demonstrated. But it faces two problems of perception, he said.

"Firstly, Avalanche is a mirror of P-to-P models that are coming under scrutiny for allowing illegal distribution. I believe this idea of 'good' and 'bad' P-to-P for file-sharing of copyright material will create a deal of confusion.

"Secondly, despite the 'pull' nature of the model and the security that should allow only the file to be accessed, Microsoft has had issues around security in the past -- IIS [Internet Information Server] being the clearest example of a secure solution that wasn't. I think the P-to-P network would be a prime target for the dissemination of viruses, despite Microsoft's assurances that it is 'safe.'"

Still, Avalanche is "an excellent take on P-to-P," Thompson said.