Laser-optimized multimode fiber an excellent choice for 10-Gigabit Ethernet

June 1, 2002
Multimode fiber is still more cost-effective than singlemode to support 10-GbE, especially given the new breed of 850-nm multimode fiber.

Multimode fiber is still more cost-effective than singlemode to support 10-GbE, especially given the new breed of 850-nm multimode fiber.

There's no doubt that 10-Gigabit Ethernet (10 GbE) is on the horizon, and that even companies not planning to install it now are considering how to best futureproof their networks to ensure a smooth migration. Unlike its predecessors, 10-GbE is the first Ethernet standard that is an optical-media-only technology. And that has raised some questions about the type of optical fiber that will be required to support the standard.

For access and extended campus applications, it is clear that singlemode fiber will be the choice. It is within building and short campus links, however, that users must make a fiber decision. Users and network planners want to know:

  • If their installed base of multimode fiber will be sufficient;
  • Whether there are new options for new installations;
  • If they need to haul out the big guns and install singlemode fiber in their networks before it's too late.

The short answers are: Yes, most legacy multimode fibers will support their migration, but at a cost; Yes, there is a new multimode fiber that has been specifically designed for 10-GbE; and No, singlemode fiber is still expensive overkill for the vast majority of applications.

Now for the details. We've tried to answer some of the most common questions here, but we also recommend that you review the recommendations of the IEEE 802.3ae Task Group, which spent more than two years developing the 10-GbE standard and expects the standard to be published this month.

Multimode to the max
Most companies that have a base of installed fiber have either 62.5/125-µm or 50/125-µm multimode fiber in their backbones. These fibers already are supporting 6 to 8 million ports running Gigabit Ethernet at distances of up to 550 meters, powered by 850-nanometer (nm) vertical-cavity surface-emitting lasers (VCSELs), without any problems.

As for 10-Gigabit Ethernet, the standard specifies that multimode fiber is capable of transmitting at 10 gigabits/second over 82 meters using standard 50-µm fiber with minimum 850-nm bandwidth of 500 MHz·km; 66 meters using 400 MHz·km 50-µm fiber; 33 meters using 200 MHz·km; and 26 meters using 160 MHz·km 62.5-µm fiber. Distances up to 300 meters are possible with the new 50-µm 850-nm laser-optimized multimode fiber described here.

Users who wish to leverage their installed base of multimode fiber for 10-GbE beyond these distances should be aware that, according to the 10-GbE standard, they will need to use two mode-conditioning patch cords per link, plus the more-expensive 10-GbE-based LX4 optics to achieve this performance.

While this will require additional investment, it may be more desirable than pulling out the installed plant and recabling. And it would be less expensive than lighting up "dark" singlemode fiber that may have been included in the initial builds. Other users may prefer to recable with the new 50-µm 850-nm laser-optimized multimode fiber, given the complexity and cost of the LX4 solution.

While conventional multimode fiber is still a more cost-effective choice than singlemode to support applications from Ethernet to 10-GbE, the best choice is a new multimode fiber. In response to industry demand for 10-GbE, fiber manufacturers have introduced a 50-µm multimode fiber that is optimized for use with 850-nm lasers. This fiber has proven performance for the 300 meters specified in the draft standard, and in trials, companies such as OFS and Corning have demonstrated performance over distances up to 1 km.

Since this fiber works with lower-cost 850-nm optoelectronics at 10 Gbits/sec, it's considerably less expensive than deploying singlemode fiber. It is completely compatible with legacy applications and offers a seamless migration path from Ethernet to 10-GbE, with no need for mode-conditioning patch cords or changes to the cabling system. And it uses existing multimode connectors and installation practices.

What's different about laser-optimized 850-nm fiber? Standard 50-µm fiber is designed with an index of refraction profile that provides good bandwidth (typically, more than 500 MHz·km) at both the 850-nm and 1300-nm wavelengths. But to support 10 Gbits/sec over 300 meters with 64B/66B encoding, you need 2,000 MHz·km bandwidth. To accomplish this, laser-optimized 50-µm fiber is manufactured with small changes to the index of refraction profile so that it provides high bandwidth at 850 nm.

Unlike standard 62.5- or 50-µm fiber, the new fiber is specified to control differential mode delay (DMD) to assure the 2,000 MHz·km bandwidth is compliant with 850-nm VCSELs. The fiber design also addresses all of the issues that have been raised concerning standard multimode fiber; there are no issues with modal dispersion or noise when this fiber is used to rated distances.

Should you install singlemode?
What has been true for multimode fiber at lower-speed protocols remains true at 10-GbE-it is simply less expensive than singlemode to deploy and operate. Currently published prices from Foundry Networks for its 10-GbE optical modules show a large price disparity-$20,000/singlemode link vs. $9,000/multimode link.

Gigabit Ethernet ports for multimode systems are also considerably less expensive than sources for singlemode fibers. Gigabit Ethernet multimode links have been about $1,000 lower in cost than the singlemode equivalents for more than two years. So, if you're planning to deploy singlemode for short-reach 1-Gbit/sec applications and later upgrading to 10 Gbits/sec, you'll have to pay the singlemode premium twice.

Singlemode fiber solutions also are not compatible with legacy applications. At this point, there are not standard-supported ports available for singlemode fiber for Ethernet or Fast Ethernet, yet the new 850-nm laser-optimized 50-µm multimode fiber can be used from 10 Mbits/sec all the way up to 10 Gbits/sec.

While the 10-GbE standard does reference singlemode fiber, it is in the context of access and regional applications-wide area networks-with distances ranging from 2 to 10 km at 1310 nm and up to 40 km at 1550 nm. For distances up to 300 meters, the standard references a multimode fiber solution.

As 10-GbE becomes established in the marketplace, prices will come down. The Fiber Optics LAN Section (FOLS) of the TIA anticipates that 10-GbE will become a cost-effective migration for the backbone, and will be used by many companies as speeds to the desktop increase to 100 Mbits/sec and 1 Gbit/sec.

To keep this strategy cost-effective, FOLS members believe that for premises network applications, 850-nm laser-optimized 50-µm fiber is the best choice for short-reach 10-GbE applications.

Elizabeth Goldsmith writes frequently on the premises cabling industry. She wrote this article in conjunction with members of the Fiber Optics LAN Section of the TIA, which was formed to promote the adoption of optical communications technology in LANs. The FOLS Web site is

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