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The best is not always the fastest

Posted: 16 Jan 2004 ?? ?Print Version ?Bookmark and Share

Keywords:lx4? mmf? phy? xaui? smf?

Fiber optics lends itself to the rabbit and the hare analogy, and often the hare is the hands-down favorite. Physical limitations in the existing fiber plant create an environment where 10Gbps serial is at a disadvantage against the "slower" alternative, LX4. The LX4 standard defines a physical layer (PHY) that exploits data in the XAUI form, four lanes at 3.125Gbps, where each XAUI lane transmits on a slightly different wavelength at 1,310nm down one fiber.

LX4 W-WDM has risen, fallen and risen again in industry interest and activity. A couple of years ago LX4 had the potential to be the one-size-fits-all PHY to serve all 10GbE requirements. An LX4 transponder is unique in that it is suitable for all the fibers defined under 10GbE, from the 300m multimode fiber (MMF) used in legacy networks, to single-mode fiber (SMF) used in long-haul communications.

However, the market impetus for LX4 dropped by the time the IEEE 802.3ae specification was approved. The rising popularity, and availability, of 10Gb serial solutions appeared to represent the best long-term technology answer. The rabbit took the lead, and was well on its way to winning the race.

Then, legacy issues moved to the fore. New-generation systems are being deployed to replace systems installed in existing facilities using existing fiber-optic cabling. These 10GbE ports are not for long-haul applications only; they serve between platforms as well, with more than 50 percent of the 10GbE ports being consumed in local, short reach links. This legacy, installed MMF drawn in the Fiber Distributed Data Interface days, works well enough for 1GbE and at data rates through 3.125Gbps, but is practically opaque to 10GbE serial communications. Modal dispersion is the prominent issue, with different modes in the MMF spreading the received pulse.

Modal dispersion occurs at all data rates, in about the same absolute amount. The problem is that at 10Gbps serial, modal dispersion takes up most of the 100ps eye, causing eye closure after 20m or 30m of MMF. Unfortunately, MMF was used in both horizontal applications, within a floor, and in risers. Up to 300m of MMF can, and does, exist in many installations. This makes 10Gbps serial a nonstarter for these applications, which is a major problem as data centers consider upgrading to next-generation systems.

LX4 implements 10GbE with each of the four lanes operating at 3.125Gbps. Modal dispersion will still exist, however; the larger 320ps eye permits the accommodation of much longer fiber lengths. Spec-compliant LX4 modules will achieve the 802.3ae specification limit of 240m to 300m lengths with great margin.

LX4 is the only viable solution today for the installed base of MMF. This is not a short-term issue. Today 10GbE plays a role at the high end of the data pyramid, but its role will expand as time passes. Intrabuilding links that started as 10/100 and moved to 1GbE will move to 10GbE quickly enough.

One alternative to LX4 that is being developed is 10Gbps serial with electronic dispersion compensation (EDC). EDC holds the promise of enabling 10Gbps serial to operate over 300m of legacy MMF. EDC solutions at 850nm and 1,310nm are being discussed, although they may have differences in implementation. Standards activities on EDC are just starting, and will take a couple of years to complete them.

Will LX4 meet its original promise of being the one-size-fits-all solution? Maybe not. It is, however, a solid contender to be the implementation of choice to serve the existing MMF links. It's also a contender to serve new-generation systems that extensively use 10GbE links based on MSA transponders such as Xenpak, X2 and Xpak. Incumbency is a powerful force in this market. Once LX4 becomes established as the MMF solution, it will earn a place as the 10GbE MMF optical standard.

- Bill Woodruff

VP, Sales and Marketing

BitBlitz Communications Inc.

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