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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
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