40- and 100GbE transmission over multimode fiber
by David Mazzarese
The explosion in demand for bandwidth and, consequently, for high-capacity computing, is driving an urgent need for higher Ethernet network speeds. Several factors contribute to this, including broadband penetration fueled by video-rich content, data-center demands, and exponential growth in supercomputer and R&D computing activities.
Different application spaces will benefit from increased transmission speeds. For this reason, the IEEE 802.3ba Task Force is working to develop 40- and 100-Gigabit Ethernet (GbE) standards simultaneously. The 40-Gbit/sec standard will support an immediate need in the server market. The 100-Gbit/sec rate is needed for core switching and routing applications, network aggregation, and high-performance computing. As in the past, the IEEE Task Force will leverage existing media and technology to balance cost and performance.
For short-reach 40- and 100GbE applications on multimode fiber, the 802.3ba Task Force will very likely define a Physical Medium Dependent (PMD) specification that will involve already-proven parallel optics technology. This will help preserve the low-cost advantage of today's VCSEL light sources. These parallel systems will transmit one 10-Gbit/sec signal on each of four or ten fibers (for 40GbE and 100GbE, respectively). Each 10-Gbit/sec signal will be aggregated in an arrayed transceiver containing four or ten VCSELs and detectors.
For these parallel systems, IEEE is striving for a minimum reach of 100 m, specifically on OM3 fiber. That's not to say 100 m will be settled upon. Within 802.3ba there are many proponents of a longer-each capability, upwards of 200 m or more. In fact, an ad-hoc group within the 802.3ba Task Force has been formed to study extended-reach parallel multimode fiber specifications. Longer reach would likely require use of an OM3+ (or OM4, currently being specified in IEC SC86A WG1 and TIA TR42.12) extended-reach laser-optimized fiber.
For parallel transmission, a parameter called "delay skew" is being discussed within the task group. Delay skew is the difference in signal arrival time from one lane—or fiber—to the next. Although skew sounds like it could be a critical parameter in these transmission systems, it is easily compensated for within the transceiver circuitry.
The 802.3ba Task Force will of course ensure proper skew compensation is written into the standard such that any industry-recognized cable design (including loose tube, tight buffer, or ribbon) can be accommodated, and the full range of current, industry-standard OM3 multimode fibers can be used
Work on the 802.3ba 40G and 100G Ethernet standards will continue over the next two years or so. Publication of the standard is expected in mid-2010.
David Mazzarese is the global manager for optical fiber technical marketing for OFS (www.ofsoptics.com), where he coordinates standards efforts for the company. He actively participates in many standards organizations, including TIA, IEC, and ITU-T. He holds a PhD in chemical engineering and an MS in electrical engineering from the University of Massachusetts at Amherst.