At their inception in the early 1990's, USB aimed at keyboards and mice with a 1.1 spec running at less than 12 Mbits/s. Firewire targeted audio and video applications such as camcorders at 100 Mbits/s and faster.

But over time USB has seen widespread adoption and swung to speeds approaching 480 Mbits/second while Firewire has been much less broadly adopted. With the latest move announced at the Intel Developer Forum here USB aims to leapfrog Firewire.

Aiming at long term expandability, Intel engineers have tested a basic version of the new protocol in software simulations at 5 and 25 Gbits/second, said an Intel engineer. The link is media agnostic and will run over copper and optical cables.

The interconnect, also called SuperSpeed USB, aims to serve any flash-based device including USB drives, camcorders and media players. One design goal is to keep abreast of the transfer speeds of flash chips.

"We don't want to be the bottleneck in the system," said Jeff Ravencraft, an Intel executive overseeing the USB 3.0 initiative.

Hewlett-Packard, Microsoft, NEC, NXP and Texas Instruments are helping define the new spec which will be put up for a design review at a two-day event in San Jose in November. Intel said the group will issue a call for contributors to the final spec soon. It hopes to finish the spec early next year and see first silicon implementing it emerge in early 2009.

USB 3.0 will adopt a new physical layer using two channels to separate data transmissions and acknowledgements to hit its higher speed targets. In place of the polling and broadcast mechanisms used in USB today, the new spec will employ a packet-routing technique and only allow data transmissions when end devices have data to send.

The new link also will support multiple flows per device and is capable of maintaining separate priority levels for each flow. The capability could be used to end interrupts that cause jitter in video transmissions. The flow mechanism also can enable native command queuing to optimize disk drive traffic.

Proponents said USB 3.0 could supplant Firewire which they said is losing backing from companies such as Sony who have switched to USB 2.0 for products such as camcorders. "Most people see 1394 as declining," said Masami Katagiri, a senior engineering manager helping define USB 3.0 at NEC Electronics America Inc. (Santa Clara, Calif.)

"It seems like a big jump for them and such a different approach that it could be a challenge to get silicon out in early 2009," said James Snider, executive director of the 1394 Trade Association that defines Firewire.

A variety of products, including many external hard disk drives, are now using 1394 to deliver connections offering a maximum physical layer throughput of 800 Mbits/second. The group hopes to finish a spec early next year to push that to 3.2 Gbits/second, Snider said.

The new speed grade will continue to use the same cables and connectors defined for the current 1394b standard. Those cables and connectors have been rated for speeds up to 10 Gbits/s. The trade association is reviewing proposals for a 10G Firewire spec, he added.