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Velio Accelerates to 6.25-GigVelio Accelerates to 6.25-Gig

Velio says it taps better. That means it's the first to get to a pure 6.25-Gbit/s signal for copper backplanes

February 18, 2003

4 Min Read
Velio Accelerates to 6.25-Gig

Velio Communications Inc. today announced it's managed to send a 6.25-Gbit/s signal successfully over copper -- a feat that would put Velio on a par with the highest speed offered by Accelerant Networks Inc..

But wait -- there's more. The announcement is signficant in that Velio is the first to get a pure 6.25-Gbit/s stream to work across copper lines. Accelerant's product actually doubles up the bits on a 3.125-Gbit/s signal. But more on that later.

Why are high-speed, copper-based chips needed? The goal is to allow system OEMs to increase backplane speed without having to rebuild boxes with more expensive fiber connections. Copper-based backplanes run comfortably at 2.5 Gbit/s, but if there were a way to send signals across that same backplane at 5 Gbit/s or even 6.25 Gbit/s, that would allow an OEM to increase speeds simply by replacing line cards with new versions with faster SerDes (serializer-deserializer) chips.

Accelerant has been making noise about 6.25-Gbit/s backplanes, including a deal to put the technology into Agere Systems Inc's (NYSE: AGR/A) ASICs (see Agere, Accelerant Team Up). Velio hasn't chimed in at that speed until now.

It's always been possible to send high-speed signals across copper traces, but the signal gets noisier as the speed increases, eventually dissolving into gibberish. Using various tricks of analog electronics, chip makers have reached a point where 2.5-Gbit/s backplanes are commonplace. More recently, they've made a safe bet out of 3.125-Gbit/s signals, which every high-end transceiver company is offering.

Here we come to the core difference between Accelerant and Velio. Accelerant's chips don't actually send 6.25-Gbit/s signals. Instead, they talk at the relatively safe 3.125-Gbit/s speed, using an encoding trick called PAM4 to "double up" the bits -- they do so by having every clock pulse carry two bits rather than just one. So Accelerant gets 6.25 Gbit/s worth of data out of a 3.125-Gbit/s signal.

Velio does it the hard way, and they're proud of it. In today's announcement, officials say they've managed to get a pure 6.25-Gbit/s stream to work across copper lines, something no commercial product has done to date.

Velio's trick lies in pre-emphasis -- an intentional distortion of the signal before transmitting it.

Pre-emphasis cancels out the distortion of signals, in which a particular "0" or "1" bleeds into a neighboring clock cycle. Quite a few companies use "one-tap" pre-emphasis, which means the chip corrects by looking forward and back one clock cycle. To get to 6.25 Gbit/s, Velio's upping the ante to three taps -- looking ahead and behind by three bits.

"I'm not aware of anybody else doing three taps of pre-emphasis," says Bill Woodruff, Velio vice president of marketing.

The technology will show up as a new version of Velio's GigaCore products by the end of the year, Woodruff says.

As for the PAM4 approach, Woodruff says Velio's not interested.

"PAM is cute. PAM has its good attributes. But the baggage you carry when you do PAM is die area [i.e., chip size] and power," says Woodruff. "When you start to go to 16 or 32 channels, the baggage of PAM gets in the way." Accelerant declined to comment for this article.

Accelerant has hoped to gain market strength through its recent deal with Agere, giving Agere permission to integrate the Accelerant SerDes into an ASIC, either for internal use or external sale. Agere also will be a second source for Accelerant's chips, a relationship that gives some reassurance to OEMs that might be skittish about semiconductor startups.

The deal arose as Agere's ASIC customers started asking about Accelerant's SerDes. "We have many of the same customers, and the customers did the initial matchmaking," says Cindy Genther, Agere's director of marketing.

This kind of intellectual-property licensing is commonplace in semiconductors, practiced heavily by the likes of Rambus Inc. (Nasdaq: RMBS) (see Rambus SerDes Hits 10-Gig).

But Rambus licenses to pretty much anybody, whereas Accelerant is working only with Agere. "We're not in the intellectual property licensing business," says Jim Tavacoli, Accelerant vice president of marketing. "We are a small company. We wanted to enable ASIC integration with minimal impact to our resources."

— Craig Matsumoto, Senior Editor, Light Reading

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