XLoom Communications is just one of a growing number of companies who want to bring optical connectivity inside the datacenter. On Monday, the company unveiled its new 20 Gbps optical transceiver device, the AVDAT 4X. The device is meant to be used as a drop-in optical interconnect for InfiniBand 4X and 10Gbase-CX4 connectors. With four 5 Gbps channels, the AVDAT is targeted mainly at DDR InfiniBand applications, although SDR InfiniBand, 10 GbE and Fibre Channel are also supported.

The device is based on XLoom's iFlame optical engine, which the company introduced in September. The iFlame chip uses an array of four VCSELs (Vertical-Cavity Surface-Emitting Lasers) and four photo-diodes for data rates of 1.25 to 5 Gbps per channel, for a total aggregate of up to 20 Gbps. The entire flip-chip assembly fits into an area of less than one square centimeter. Multimode optical fiber is attached via a standard 12-ribbon fiber interface.

The whole idea behind going optical is to get rid of the multitude of problems associated with copper cabling, especially as interconnect data rates get into the multi-gigabit per second range. For example, at the 20 Gbps data rates for DDR InfiniBand, standard copper cabling is limited to a reach of around 8 to 10 meters. Beyond that, signal integrity starts to suffer, which degrades performance significantly. In addition, compared to optical fiber, the weight, bulkiness, and cost disadvantages of copper start to add up as distances increase. Most optical solutions, like XLoom's, target applications where the media connection is between 15 to 100 meters. When QDR InfiniBand and 40 GbE become established over the next few years, even sub-5-meter distances are going to become challenging for conventional copper cabling.

By supporting the standard CX4 connector with the AVDAT offering, XLoom is hoping to attract the attention of InfiniBand vendors who would like to offer optical connectivity to their switches and host channel adapters. According to XLoom CEO David Sachs, one of the difficulties thus far has been getting the optical devices to fit into the CX4 electrical interface on these cards. He says the small size of their iFlame optical engine allowed them to achieve this without encroaching into the CX4 "keep out" space.

The company is looking to capitalize on optical connectivity in the datacenter as 10 GbE and 20 Gbps InfiniBand become more mainstream. "I think we're going to see a lot of adoption for this for switch-to-switch connections, because that's where the distances are required," says Sachs. "Our aim is to take 10 to 15 percent of the copper market down the road, starting at the beginning of next year."

Just considering InfiniBand, that works out to about 100,000 optical connections in 2008, based on IDC's estimate of a million IB switches and HCA ports. Perhaps more importantly, IDC is predicting that DDR InfiniBand will be about 60 percent of the total in 2008, although the number of QDR ports will be negligible that year. To be ready for the IB surge, Sachs says the AVDAT devices will be generally available in the fourth quarter of this year and will be aggressively priced at about $100 per unit, when purchased in volume. "Everyone's trying to bring down the overall pricing of the interconnect," he says. "I think this is a step in the right direction."

The XLoom offering is similar in technology and application to Intel's Connects Cables and Zarlink's ZLynx products, both announced earlier this year. Unlike the XLoom device, the Intel and Zarlink offerings are complete optical assemblies -- two transceivers with attached fiber. Those products are presumably aimed at the system integrator or end-user, rather than as a value-added component for the switch and HCA vendors. Both of these solutions are also intended to start shipping before the end of the year.

In the future, XLoom plans to support QDR (40 Gbps) InfiniBand in a four-channel device, with a maximum data rate per channel of 10 Gbps. Although QDR products will be introduced by InfiniBand vendors in 2008, it is not likely to have a significant impact on the overall market until the following year, when IDC estimates around 250,000 QDR ports will be shipped. Luxtera Inc. has already staked a claim in the QDR InfiniBand space with their "Blazar" 40 Gbps optical cable product. That company hopes to establish itself at the high end of the optical InfiniBand market in 2008, although Sachs, like IDC, believes QDR won't reach critical mass until 2009.

Not everyone is convinced that optical interconnects are about to take over the datacenter. Quellan Inc., an IC vendor that recently received $20 million in venture financing, has developed active cable technology that extends the reach of copper by a factor of three or four. Using analog silicon devices at each end of the cable, Quellan has managed to deliver enough noise reduction to allow copper cables to carry high data rates at distances well beyond 10 meters. At the same time, the increased signal quality allows thinner gauge copper to be used, which lessens some of the objections of cost and weight.

While Quellan CEO Tony Stelliga admits that in some situations optical connectivity makes sense, he says that with typical connection distances and at data rates of 10 to 20 Gbps, their active copper cable technology delivers enough capability for the majority of users. According to him, "95 percent of the connections in the datacenter are less than 25 meters and we can serve that reach now."

Since Quellan's technology uses only about 20 percent of the power of a typical optical solution at 20 Gbps (200 milliwatts versus 1 watt), their solution can claim better energy efficiency. But as distances stretch into the 20 or 25 meters range, the up-front cost of the copper cable and the weight/bulkiness issues start to assert themselves and the utility of copper becomes more problematic.

Companies like Intel, Luxtera, Zarlink and XLoom are all searching for the price-performance sweet spot for their respective optical products. With companies like Quellan pushing the envelope of copper cable technology, the nascent optical revolution in the datacenter may be a bit slower in coming than anticipated. But with QDR InfiniBand and 40 GbE in the pipeline, optical still seems like a safe bet in the long run.