Accelerant Pulls Power Play

Backplane transceiver vendor Accelerant Networks Inc. is making the leap to 10 Gbit/s, claiming it's got a device that delivers the speed and keeps power low enough to fit into an installed chassis.

Accelerant is still a struggling vendor. Its revenues will be around $2 million this year, and the company just endured a 25 percent layoff (see Headcount: Give Me Liberty). But the company's 10-Gbit/s claim for the AN6425 transceiver could get some attention.

Several companies, including Accelerant, created the High Speed Backplane Initiative to look at 6.25-Gbit/s speeds (see OIF Gets Backplane-Happy and HSBI Completes Its Objective), but almost no one has mentioned 10-Gbit/s except for Rambus Inc. (Nasdaq: RMBS), with its RaSer X serializer-deserializers (SerDes) (see Rambus SerDes Hits 10-Gig).

The trick to upping a backplane's speed is to keep the signals readable for a long enough distance for them to be useful inside a system. If 10-Gbit/s of data were put across a backplane today, the signal wouldn't be readable after traveling more than an inch or two. Faster signals get jumbled quickly on a medium such as FR4, the material commonly used for boards and backplanes. To get useful distance out of a high-speed signal, you need to upgrade to a pricier board material or find a way to massage the signal into something readable.

Large chip players such as Broadcom Corp. (Nasdaq: BRCM), Intel Corp. (Nasdaq: INTC) and Marvell Technology Group Ltd. (Nasdaq: MRVL) are always looking into the latter possibility, as are SerDes specialists such as BitBlitz Communications Inc. and Velio Communications Inc.

Accelerant and Rambus use a doubling-up scheme called PAM4, which sends two bits at a time. What they've really accomplished is a 5-Gbit/s transmission, with total throughput increased to 10 Gbit/s thanks to the PAM4 encoding.

Still, even 5 Gbit/s on a vanilla FR4 backplane is a feat. "Taking a backplane that was built for 1.5-Gbit/s or 2.5-Gbit/s and cranking the speed up is non-obvious," says Jean-Marc Patenaude, Rambus marketing director. "There are a lot of issues at the signal integrity level you have to know about."

Backplane speed is impressive, but Accelerant officials say the device's low power is just as important. "We've shown that even at 10 Gbit/s, we are not increasing the power significantly," claims Bill Hoppin, Accelerant's VP of marketing. Accelerant says the low-power nature of the AN6425 is just as important as the speed. A "real world" test put the part at 1.4 W of power consumption when pushing four lanes of 10 Gbit/s each across 30-inch FR4 traces, Hoppin says.

That's a big deal, because most of the chip business these days involves upgrades to installed systems. "The amount of power that a chassis can dissipate is fixed. The cooling is already there. So any upgrade to the backplane has to stay within the thermal envelope of the SerDes that's already there," Hoppin says.

In part, the AN6425 saves power by assigning different power levels to each channel. "If you get a short link and a long link on the backplane, other transceivers will throw the same amount of power into both of them," Hoppin says.

Of course, Rambus prefers its RaSer X devices, which use adaptive equalization to keep signals clean. Equalizers on both sides of a connection make continual checks on the signal and adapt accordingly to keep the signal clean.

Many advanced SerDes can do this kind of monitoring, but Rambus is the only one doing the "continual" part, Patenaude says. Other SerDes send an occasional test pattern to check signal quality, but Rambus monitors the actual data, making for continual monitoring.

— Craig Matsumoto, Senior Editor, Light Reading

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