Optical Cables Light Up InfiniBand

By Michael Feldman

August 17, 2007

As InfiniBand data rates rise and clusters grow in size, copper cable technology is getting stretched to its limits. The current InfiniBand Double Data Rate (DDR) of 20 Gigabits per second (Gbps) means that conventional copper cable is limited to an 8 to 10 meter reach. While this might be fine for moderate sized clusters, as systems expand, the physical layout of the cluster becomes a real challenge.

The problems of copper as an interconnect media are numerous. It’s heavy and bulky, making it difficult to physically manage in the datacenter. The bulkiness is a particular problem when it comes to maintaining airflow around the systems to maintain proper cooling. And because of the nature of electrical signals, electromagnetic interference limits copper’s performance and reliability. These problems really start to add up when you have hundreds or thousands of connections in a scaled-out high performance cluster.

Fortunately, optical fibers provide a capable alternative to copper for higher data rates and longer distances. The problem has always been cost. The optical fiber itself is less expensive than copper, but the transceivers required to convert the optical stream to electrical signals makes these solutions prohibitively expensive at shorter distances. That’s why optical is the solution of choice for long-haul networks. At the level of the computer this model is inverted. The highest data rates are on the processor, which represent the shortest communications distances. As you move away from the CPU, onto the board, the backplane, and then out of the box, distance increases and data rates drop. To get optical connections at the volumes required for computer systems, low-cost solutions will be necessary.

That process has begun. Two companies, Intel and Luxtera, have recently unveiled optical cable products aimed at HPC clusters that are using the higher data rates of InfiniBand. The cables will also support 10GbE traffic, but until 40GbE gets established, InfiniBand will be the principle driver for these products. Intel unveiled its optical cables (“Intel Connects Cables”) at the International Supercomputing Conference in June. Luxtera announced its “Blazar” optical offering earlier this week.

Intel Outside

While Intel is mainly known for its microprocessors, the company became aware several years ago that copper-based communication was going to become an issue as CPU performance outpaced communication bandwidth at all levels of the system. The company’s silicon photonics research effort is one instance of their desire to move beyond traditional electrical interconnects. While a complete silicon photonics solution is probably years away, Intel’s optical cables are a commercial product today and represent the company’s answer to the copper problem for cluster interconnects.

Intel’s optical cable offering is aimed squarely at the rapidly expanding HPC cluster computing market, which has the most pressing need for high bandwidth, low latency communication. The cables support SDR and DDR InfiniBand (and 10GbE ) and boasts bit error rates (BER) of at least 10^-15, 1000 times better than top-of-the-line copper. Due to the nature of the optical fiber, they are 84 percent lighter than their copper counterparts, take up 83 percent less space and have a 40 percent better bend radius. The cable assemblies can be built in lengths ranging from 1 to 100 meters, enabling a lot more flexibility in system layout — even allowing for configurations that extend into multiple rooms and multiple floors.

Tom Willis, general manager of Intel Connects Cables, thinks if people are going to build large (1000-plus node) clusters using InfiniBand, they’re going to need this product. He says their business model is based on the assumption that all clusters requiring DDR interconnects of 10 meters and above would go optical, while 10 meters and below would stay as copper. But Willis says they’re actually seeing some orders for sub-10-meter cable assemblies, where people are interested in the better BER and easier manageability of optical.

The product is currently in production release, but deliveries won’t be take place until the end of the year. “This is just the first product,” notes Willis. “Over time, all the high performance links out of your computer are going to be optical.”

Luxtera Raises the Stakes

Luxtera, a Carlsbad, California-based startup, announced their optical cable offering on August 13 and, like Intel, is targeting InfiniBand-enabled HPC clusters. Their optical solution has the same basic advantages over copper as Intel’s offering. Here though, there’s a twist. Luxtera’s product, named “Blazar,” can support all InfiniBand speeds — SDR, DDR and QDR — that is, up to 40 Gbps, and in lengths of up to 300 meters. Blazar will also support four lanes of 10GbE. At 40 Gbps, conventional copper cabling would be limited to 7 meters, essentially making it impractical to go beyond the rack.

“We’ve talked with a number of datacenter managers and none of them expect copper to be a viable media for QDR InfiniBand deployment,” says Luxtera’s Marketing VP, Marek Tlalka.

Although both companies used optical fiber media, the Luxtera technology is quite a bit different from that of Intel’s. Using their CMOS photonics technology, Luxtera was able to integrate most of the transceiver functionality onto a silicon chip. Only the laser and the photodetectors are discrete components (the photodetectors will be integrated in the next-generation products). Intel’s transceiver is based on Vertical-Cavity Surface-Emitting Lasers (VCSELs) and uses an unspecified collection of discrete components.

Because Luxtera catches light from the waveguide right onto the chip, the company is able to use single-mode optical fiber. By contrast, since Intel uses VCSEL technology, multimode fiber is required in their offering. Since the cost of single-mode fiber is less than that of multimode fiber, this may afford Luxtera some price advantage, especially for longer reaches.

The Blazar product will be ready for sampling by development partners in Q4 2007 and generally available by the middle of 2008. This should coincide with the first wave of QDR InfiniBand gear expected in the second half of next year. Tlalka says he’s expecting to attract DDR InfiniBand users to their solution as well. But here, they’ll be going head to head against Intel, a company known to get rather aggressive with pricing.

“Our objective right now as the first product comes to the market is to sell our QDR solutions at price points comparable to Intel’s [DDR] solution,” says Tlalka. “So you get twice as much speed, with three times the reach for the same price.”

Luxtera’s long-term goal is to get prices down to those of copper even for short reaches. Tlalka claims this is mainly a function of volume economics. He says if they were building a million of these products every month, they would probably be there today.

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