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PCI Express over optical cabling: Performance, simplicity, efficiency

By Reginald Conley, PLX Technology, special to Network World
March 01, 2012 10:28 AM ET

Network World - This vendor-written tech primer has been edited by Network World to eliminate product promotion, but readers should note it will likely favor the submitter's approach.

Optical solutions can be a key enabler for network architects who see value in using PCI Express (PCIe) as an I/O technology for data center connectivity. Using PCIe to natively connect servers, switches and storage elements can lower overall system costs by reducing or eliminating the number of protocol conversions. Additionally, this increased system simplicity provides networks with latency, power and dollars-per-gigabit advantages.

The PCI Special Interest Group (PCI-SIG) in 2007 released an external cabling specification enabling interconnection of PCIe systems at 2.5Gbps (Gen1) and 5Gbps (Gen2), facilitating PCIe extension and expansion. Copper cabling solutions appeared in a variety of channel widths to provide connectivity, but were limited in link distance and became bulky in size and weight at higher channel counts.

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While the copper solutions were able to meet the basic needs of PCIe Gen1 and Gen2 external expansion, it is hard for copper to meet the price, performance and size/weight needs for PCIe Gen3 and its 8Gbps speeds. Fiber optic technology provides an alternate for high-channel-count PCIe Gen3 interconnects, providing increased link distances, lower size/weight and higher performance. What's more, fiber is becoming increasingly price competitive. [Also see: "Intel's Thunderbolt with fiber optics years away"]

Collaborations by silicon vendors such as PLX Technology and Avago Technologies have yielded breakthroughs in this area, such as the first PCIe Gen3 end-to-end fiber optic link to deliver a full 64Gbps (128Gbps bidirectional) performance for PCIe applications. The PCIe bus is a high-speed serial I/O providing connectivity between system peripherals (graphics cards, memory/disk drives, external I/O cards) and the central processor unit (CPU). PCIe provides full-duplex interconnect (Transmit/Receive, or Tx/Rx) that can be scaled by adding multiple lanes such as x4, x8, x16, and possibly even x32 lane links.

A number of applications exist in which PCIe may be used to connect a CPU with devices outside the box. Even at 2.5 Gbps (Gen1) and 5Gbps (Gen2), physical connections are limited to a few meters in length using the available copper cables. Physical link distances decrease at higher data rates, so PCIe Gen3's 8Gbps will further reduce the usable distance of copper cables. System designers, therefore, have expressed interest in optical cabling for PCIe applications such as data centers and enterprise systems spread out over wider areas and requiring longer cabling distances. Optical fiber-based solutions, these designers have come to understand, not only allow connections over a much longer distance but are capable of providing better bit-error-rate performance, improved immunity to electromagnetic interference, and are thinner and lighter allowing for easier placement and routing.

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