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Group pits USB 3.0 against FireWire

Posted: 01 Nov 2007 ?? ?Print Version ?Bookmark and Share

Keywords:USB 3.0 spec? FireWire? interconnect?

Intel Corp. said it is working with a handful of companies on a spec that could push the USB's theoretical throughput beyond 4Gbps, or 10 times its current rate. USB 3.0 aims to deliver 300MBps of usable data at the applications level and add QoS capabilities that could challenge IEEE 1394, also known as FireWire.

At the interconnects' inception in the early 1990s, USB aimed at keyboards and mice with a 1.1 spec running at less than 12Mbps, while FireWire targeted audio and video apps such as camcorders at 100Mbps and faster. Over time, USB has seen widespread adoption and has swung to speeds approaching 480Mbps. With the latest move, announced at September's Intel Developer Forum, USB aims to leapfrog FireWire.

Aiming at long-term extensibility, Intel has tested a basic version of the new protocol in software simulations at 5Gbps and 25Gbps, an Intel engineer said. The link is media agnostic and will run over copper and optical cables.

Keeping up with flash
The 3.0 interconnect, also called SuperSpeed USB, is intended 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 to define the new spec. According to Intel, the group will soon issue a call for contributors to the final spec. It hopes to finish the spec early next year and see first silicon emerge in early 2009.

USB 3.0 will adopt a new PHY 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 spec will employ a packet-routing technique and will only allow data transmissions when end devices have data to send.

It will also support multiple flows per device and will be able to maintain separate priority levels for each flow. That capability could be used to end interrupts that cause jitter in video transmissions.

Proponents said USB 3.0 could supplant FireWire, which they said is losing backing. Sony, for example, has 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.

But USB 3.0 "seems like a big jump... 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, which defines Firewire.

Inside look at USB 3.0 cable: Two-channel cabling boosts throughput.

A variety of products, including many external HDDs, use 1394 to deliver connections offering a maximum PHY throughput of 800Mbps. The group hopes to finish a spec early next year to push that to 3.2Gbps, said Snider. The new speed grade will use the same cables and connectorsrated for speeds up to 10Gbpsas are defined for the current 1394b standard. Meanwhile, the trade association is reviewing proposals for 10G FireWire, Snider said.

Will Apple switch?
Apple Inc. has been a longtime supporter of FireWire, which it uses in all iMacs for media applications currently beyond the reach of USB. Katagiri said Apple could be forced to switch from FireWire to USB 3.0 since Intel is now its major silicon supplier and is expected to use USB 3.0 in its chipsets.

On the downside, he said, the 3.0 spec probably will have to reduce the 5m reach of USB 2.0, perhaps to 2m. And host controllers will have to embed more intelligence because they will manage routing of data transmissions.

But the USB 3.0 group is looking at new techniques for supporting isochronous data transmissions that could provide an edge over features found in FireWire, Katagiri added.

- Rick Merritt
EE Times

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