Optical Active Cables free data centers from connectivity constraints
January 01, 2008
Connectivity solutions available today present about as many drawbacks as benefits.
When selecting connectivity solutions for data centers, IT managers face trade-offs between performance, operational expenditures, and capital expenditures. The many disadvantages of today’s connectivity options, including copper cables, copper active cables, and optical modules, often outweigh the advantages of each solution. An alternative option entering the market – Optical Active Cables (OACs) – delivers the performance of optics at much lower costs. These cables are well suited for point-to-point connectivity in data centers at data rates of 10 Gbps and beyond.
The connectivity solutions available today present about as many drawbacks as benefits. Copper cables cost less but have major reach and size performance limitations. Active copper cables offer slightly increased performance over copper cables but are reach constrained and limited at higher data rates such as 10 Gbps. Fiber and optical modules perform best but can be very costly at 10 Gbps data rates.
When compared to electrical interconnects over copper cabling, OACs offer several feature advantages for high-productivity clusters and data centers minus the shortcomings of other connectivity solutions. High-performance fiber-optic technology connects servers, switches, and storage with high bandwidth. Through a slim cable, this technology banishes ground loops that often cause intermittent performance with copper cabling and eliminates EMI between adjacent cables in cable trays. Long 300-meter reach liberates cluster design from the constraints of short-reach copper links.
High-performance fiber-optic technology encourages highly interconnected cluster topologies at any size and permits clusters to become larger, noncontiguous, dispersed, and more conveniently arranged. Low power consumption simplifies thermal management, lowers cooling costs, and supports high connector density (unlike 10GBASE-T, for example, which consumes so much power that thermal dissipation rather than connector size limits the density at which transceivers can pack along the edge of a card). In addition, OACs‚Äô extremely low latency in the transceiver supports clusters, handles message-based or request-response transaction traffic such as database or automated financial trading applications, and performs intensive simulations and host supercomputer applications.
The active cable format delivers a plug-and-play solution that encapsulates and hides the optical nature of the link and encloses fibers within a rugged cable that handles like Cat 5e. Figure 1 compares the bend radius of copper versus fiber cables.
The electrical interfaces of the Quad Small Form Factor Pluggable (QSFP) cable end (four per connector) conform to the SFP+ Multisource Agreement for 10G optical modules. Now emerging as the de facto electrical standard for 10G ASICs to which essentially all products are being designed, the SFP+ interface will almost certainly be adopted as the standard electrical interface for each lane in Quad Data Rate (QDR) InfiniBand. Figure 2 shows a QDR InfiniBand QSFP HCA card.
The SFP+ interface places the high-level link functions on the switch, network interface card, or host channel adapter, making the cable end protocol and data rate agnostic. The OAC can then benefit by supporting 4 x 2.5 Gbps, 4 x 5 Gbps, and 4 x 10 Gbps data rates (corresponding to single, double, and quad data rates in InfiniBand terminology), and seamlessly transport InfiniBand, Fibre Channel, Ethernet, and custom-defined traffic. Moreover, transitions are simplified and reduce the risk of upgrading clusters and data centers through their versatile support of multiple data rates and transport protocols.
OACs provide the performance advantages of module form factors such as the QSFP. QSFP connectors functionally resemble SFP/SFP+, except they support four 10G channels instead of one per connector. QSFP is hot pluggable, funnels 40 Gbps through connector cages that occupy just 19 mm along the edge of a card, and offers the greatest bandwidth density of any standard interconnect option. QSFP enables racks to be fully populated, resulting in compact, condensed clusters with efficient rack layouts. Ultimately, this efficient layout eliminates the need to expand facilities to increase computing power. Figure 3 compares the connector width of copper CX4 versus a QSFP OAC connector.
Flexible, light, thin optical cabling simplifies cable management by allowing cables to bend and conform to racks. Alleviating the excessive stress that heavy copper cables place on floors, racks, cable trays, and connectors by 10- to 20-fold amounts to several tons for a small cluster and potentially entails structural reinforcements to upgrade to 10G+ data rates using copper interconnects. Another benefit of thin fiber cabling is that it clears obstructive cable clutter from the space around racks by reducing cable volume 5- to 10-fold, permitting air to circulate freely and cooling hot servers and switches more effectively. Moreover, fiber cable fits into existing cable trays and floor spaces designed to accommodate 1G connections (copper links upgraded to 10G+ will typically require larger conduits because copper cables that support 10G are much thicker than their predecessors) and ensures better reliability.
Compared to conventional 10G optical modules, OACs deliver a plug-and-play solution that overcomes designers‚Äô hesitancy to embrace fiber, precludes the need to cut, polish, and verify fiber interfaces on-site, and encloses the fibers hermetically within a rugged cable that handles like Cat 5e. Fiber connectors are also eliminated because fibers are permanently attached to the cable end. Closed form factors obviate interoperability issues, enabling tighter optical operating margins that further reduce cost. Optical modules are assembled from dozens of discrete components to precise tolerances for interoperability reasons. OACs, on the other hand, only need to interoperate between each end, thereby increasing production yields. Ultimately, OACs provide a lower-cost alternative to optical modules.
The best of both connectors
Luxtera‚Äôs recently announced 40 Gbps (4X10Gbps) OAC, Blazar, extends the advantages of optical modules and copper cables. The cost-effective OAC delivers a high-bandwidth interconnect that can span distances from 1-300 meters. Blazar includes two permanently attached connectors that contain optical transceivers. Residing entirely within the sealed, self-ontained product, the optics are protected from dust, scratches, moisture, and imperfect installation and do not contain separate optical modules or fibers. Blazar utilizes a single CMOS Photonic die (as opposed to multiple discrete components found in traditional optical modules) and a single continuous wave laser that integrates the functionality of four SFP+ optical modules.
Four parallel multirate optical data lanes, each carrying from 1-10.55 Gbps full duplex, share the cable and terminate at four tiny optical transceivers fabricated on a monolithic CMOS chip embedded within each connector. The connectors plug into standard QSFP ports and exchange data with the host channel adapter or switch card through four standard SFP+ 10G electrical interfaces supporting InfiniBand, Ethernet, Fibre Channel, and other transmission protocols, as illustrated by the 10/40 GbE QSFP switch pictured in Figure 4. This disruptive design offers higher reliability, lower cost, and higher performance over traditional modules and copper active cables.
While QSFP provides the highest front-panel bandwidth density of any standard data center connector format, the 40 Gbps OAC offers IT managers a better choice for data center connectivity by combining the benefits of both optical modules and copper cables and then some.
Paul Duran is a director of marketing for Luxtera, based in Carlsbad, California. Paul has more than 15 years of experience in the communications industry. He obtained his BSEE/MSEE degrees from MIT and his MBA from Arizona State University.