What could prove to be a significant milestone in the development of electrical backplanes was announced today by Gennum Corp. (see Gennum Launches Components).

The company, which makes chips for hearing aids, among other things, says it has developed an integrated circuit that makes it possible to send 10.7 Gbit/s of serial data over at least 25 inches of copper trace. That's four times more data per trace than existing backplane products can support.

Yes, that's four times greater density. (There's no need to shout! -- ed.)

It seems like only yesterday that the industry was buzzing about the prospect of being able to design backplanes that carry 2.5-Gbit/s signals. At the time 2.5 Gbit/s seemed like an enormous step up in density, from the previous capability of 622 Mbit/s.

Velio Communications Inc. introduced the first backplane transceiver chip with 2.5-Gbit/s serial interfaces six months ago almost to the day (see Velio Cleans Up). Other companies quickly followed its lead, including Broadcom Corp. (Nasdaq: BRCM), Mindspeed Technologies, PMC-Sierra Inc. (Nasdaq: PMCS), Texas Instruments Inc. (NYSE: TXN), and Vitesse Semiconductor Corp. (Nasdaq: VTSS). Systems integrators are still busy migrating to the new technology.

Now Gennum threatens to stir things up by saying it can support 10-Gbit/s backplanes. However, it does so differently than do existing 2.5 Gbit/s products.

When a signal is sent across a backplane, the different frequency components of the electrical signal attenuate at different rates, leading to "eye closure" -- the inability to distinguish between the 0s and the 1s in the data. Signals degrade more quickly at higher data rates, so they can't travel as far before becoming unreadable. Even sending signals at 2.5 Gbit/s presents quite a challenge, vendors say.

One trick that vendors use is to add "pre-emphasis." This means tweaking the signal -- boosting some parts of it compared to others -- to compensate for the transmission characteristics of the copper trace.

Gennum's chip, on the other hand, is able to unscramble the signal after it has traveled along the trace. It does this using so-called "receive-end equalization," a technique Gennum says it uses in existing chips that it sells in quantity to television and film studios.

"A TV studio is a mini-network, except in datacom there are different speeds and protocols," notes Gary Beauchamp, Gennum's director of datacom products.

Beauchamp claims Gennum's approach is better than pre-emphasis, because it's not necessary to pre-set values on the chip to take into account the transmission characteristics of the trace. (Accelerant Networks Inc. says it has plans for a device that sets pre-emphasis values automatically, but it hasn't released products yet -- see Accelerant Boosts Backplanes).

However, there's one big drawback to Gennum's chip -- it only performs part of the job that today's backplane transceivers do. The chip takes in a garbled signal at 10 Gbit/s, cleans it up, and sends it out again at 10 Gbit/s. Unlike true backplane transceivers, it does not break the data down into multiple parallel channels (deserializing), ready for processing on the line card. That means it must be used in conjunction with SerDes (serializer/deserializer) or PHYs (physical layer devices -- used for transferring data from optics to a parallel bus) that do.

Competitors say Gennum's chip sounds interesting. But they have reservations about its performance.

"25 inches [of backplane trace] is not enough," says Andrew Schmitt, a product marketing manager at Vitesse. "A normal rack width is 19 inches. That only leaves room for a total of 3 inches of trace on the line card." Traces aren't normally straight, but have to twist and turn to avoid crossing each other. That's why companies like Velio and Vitesse are talking about using 40 inches, he says.

Gennum also announced a 6.4-Gbit/s receive-end equalizer today. That device is made in bipolar silicon technology, while the 10.7-Gbit/s chip is fabricated in silicon germanium (SiGe).

The company says it is already sampling its datacom products to selected customers. One of these is sure to be Tyco International Ltd. (NYSE: TYC), with which Gennum has an alliance.

— Pauline Rigby, Senior Editor, Light Reading
http://www.lightreading.com