Are you ready for 10 Gigabit Ethernet?

Communications News, Feb, 2004 by John E. George

Increasing bandwidth demands are driving the need for 10-Gigabit Ethernet connectivity in LANs, storage area networks (SANs) and central offices/Internet data centers. Making the right choices depends on an understanding of the bandwidth requirements and system needs that ore driving these trends, as well as the optical-fiber technologies best suited for these applications.

In the LAN, bandwidths are in creasing to 100 Mbps and 1 Gbps in horizontal links, which necessitates low-cost 10-Gbps building backbones. The 10 Gigabit Ethernet Standard (IEEE 802.3ae) supports LAN campus backbone and WAN applications on single-mode fiber (SMF) for distances up to 40 km. The standard also includes short-reach multimode fiber (MMF) solutions. One of these solutions supports both equipment room and backbone connections up to 300 meters on laser-optimized multimode fiber with low-cost 850 nm lasers. The other, more expensive solution supports conventional multimode fibers to 300 meters if a mode- conditioning patch cord is used.

For SAN implementations, Fibre Channel includes a solution that uses laser-optimized multimode fiber with vertical cavity surface-emitting lasers (VCSELs) as a light source, at a wavelength of 850 nm for 10 Gbps up to 300 meters.

The need for low-cost 10-Gbps interconnects within central offices is being driven by deployment of 10-Gbps transmission, DWDM end rooting systems. Multimode fiber systems traditionally have provided the most cost-effective solutions for meeting increased bandwidth demands. Compared to single-mode fiber, these systems enable lower transceiver, connector and connector installation costs at data rates up to 1 Gbps, and provide the same cost benefits for today's 10-Gbps applications.

The network designer or end-user who specifies short-reach systems still must choose from two types of multimode fiber: 50 [micro]m or 62.5 [micro]m. While 62.5-[micro]m fiber has traditionally had a reach advantage at 10 Mbps. the 100 Mbps minimum in today's backbone applications makes that advantage irrelevant. In addition, 50-[micro]m fiber has significant bandwidth and reach advantages for the 1-Gbps and 10-Gbps applications that most customers will use, while preserving the low system cost advantages of multimode fiber. in addition. 50-[micro]m fiber uses the same connectors and installation techniques as 62.5-[micro]m fiber.

Multimode fibers that have been optimized for laser transmission enable 1-Gbps transmissions to extend the distance of 850-nm Gigabit Ethernet applications to 750 meters. These fibers will permit 850 nm 10-Gbps application reach to 150 meters, supporting 40% of typical building backbone requirements.

Also available are fibers that support 300-meter link lengths for 10-Gbps applications. Their 56-[micro]m core size couples sufficient power from light-emitting display sources to support legacy applications like Ethernet, Token Ring, FDDI and Fast Ethernet for virtually all in-building networks and most campus networks.

For ultra-long building backbones and medium-length campus backbones, choose a fiber that supports 10-Gbps Ethernet. Fibre Channel and other standards-based applications to 550 meters or more with two LC connections using low-cost 850 nm VCSELs.

The critical performance parameter needed for 10-Gbps transmission is laser bandwidth--but not all laser bandwidth measurements are equal. The laser bandwidth required for 300-meter link lengths and beyond results from careful attention to the fiber's light-propagation properties.

The best way to ensure laser bandwidth is to limit the difference in arrival times of the various modes (known as the differential mode delay, or DMD). The greater the DMD. the more pulse spreading is present, and the lower the bandwidth will be. Limiting the allowable DMD guarantees meeting the minimum bandwidth specification.

The migration to higher data rates in short-reach optical networks continues, driven by lower-cost-per-bit solutions that meet increasing bandwidth demands. Specifying and using laser-optimized 50-micron multimode fiber enables networks to reliably and cost-effectively support both 1- and 10-Gbps applications.

For more information from OFS: www.rsleads.com/402cn-256

John George is fiber offer development manager for OFS, Norcross, Ga.

David Hall is marketing manager at Corning Cable Systems, Hickory, N.C.