Gig-E stretches, pushes, and speeds toward the future

Communications News, August, 1999 by Jim Hayes

Will your fiber-optic cable plant support Gigabit Ethernet?

GBE, as the name says, is Ethernet scaled up to gigabit speeds, providing a migration path from Ethernet at 10 Mbps to Fast Ethernet at 100 Mbps and then on to gigabit speeds for network backbones. Few users will use GBE to the desktop; it will be mainly a backbone network--and that for only "power users."

Since the majority of network back-bones now use multimode optical fiber, GBE migration using the installed cable plant is highly desirable. While standards are being developed for UTP cabling, GBE will stretch the limits of copper cable capability. In fact, it begins to push multimode fiber bandwidth limits.

GBE has a data rate of one gigabit per second (Gbps, a billion bits per second) and a baud rate of 1.250 Gbps including the data encoding. These speeds require laser transmitters since LEDs run out of steam at a few hundred Mbps (millions of bits per second).

Two fiber-optic standards are approved: 1000Base-SX for short-wave-length lasers over multimode fiber and 1000Base-LX for long-wavelength lasers over multimode or single-mode fiber. The short-wavelength standard will typically be met with a 850 nm VCSEL (vertical-cavity surface-emitting laser), a new type of device that offers high performance and very low cost. A CD laser may also be used. Long-wave-length lasers will be standard 1,300 nm single-mode transmitters.

FIBER DOES NOT HAVE INFINITE BANDWIDTH

But the desire to run GBE over installed multimode fiber cable plants in premises applications, not single-mode fiber like the telco and CATV networks, creates a problem. Multimode fiber works well with LEDs but somewhat unpredictably with coherent laser sources. Coherent sources emit light in phase, and interaction of various modes in multi-mode fiber can create modal noise, causing bit error rate problems.

Furthermore, LEDs have a wide angle of light output which must be focused into the fiber, while lasers have a much narrower beam that is more easily coupled to the fiber (Figure 1). The LED will fill the higher order modes in the fiber (higher modes = wider angles), lasers will concentrate the light in the center of the fiber, especially the long-wave-length lasers with single-mode pigtails.

[Figure 1 ILLUSTRATION OMITTED]

While this could theoretically allow higher fiber bandwidth to the laser, the laser can have severe problems with some fibers. To minimize the problems, transceiver manufacturers are looking at changing the source output to create off-set or even "doughnut" launches to minimize modal problems.

Most "legacy" multimode fibers have their bandwidth optimized for 1,300 nm, not 850 nm, adding to the problems of short-wavelength lasers. Fiber manufacturers have begun making fiber with high bandwidth capability at 850 nm, as well as 1,300 nm. Led by Plasma in the Netherlands, SpecTran, Alcatel, and Corning have announced high bandwidth 62.5/125 fiber that will support GBE to lengths much longer than currently proposed in the standard.

However, many users already have 62.5/125 fiber installed in their backbone, and most of that fiber is "FDDI spec" with bandwidths of 160 MHz-km at 850 nm and 500 MHz-km at 1,300 nm. The big question is, how far can you go on this fiber before encountering problems?

For multimode fiber, it's a function of the fiber bandwidth and transceiver design (Figure 2). This is theoretical data calculated for worst-case conditions by engineers working on the GBE spec and published in the addenda.

[Figure 2 ILLUSTRATION OMITTED]

If you are planning on running on fiber already installed within the last 10 years, and you do not have the actual fiber-performance data, you must assume the worst-case lengths and/or look for modal-conditioning methods to enhance the application. The adventuresome user will just try it, as successful links running more than twice the worst-case lengths have been reported. It appears to be a very conservative standard.

You cannot easily test the fiber you already have installed for bandwidth. There are no portable testers for multimode fiber bandwidth, and there haven't been any built in 15 years. Fiber manufacturers routinely test bandwidth, but the testers are enormous and cost hundreds of thousands of dollars.

The only hope for testing fiber for GBE is to use a link-BER test. If the link is over the maximum length, you can set up two workstations and simply see if it works on all the fibers.

CABLE-PLANT LOSS

Like FDDI and ESCON, GBE link length will be limited by the bandwidth of the cable plant, not the attenuation. While these earlier LED-based networks were limited equally by modal and chromatic dispersion, GBE will be affected almost entirely by the modal dispersion since it uses narrow-spectral-width lasers as sources.

Without dispersion effects, GBE VCSEL transceivers have absolute loss margins of 7.5 dB minimum to about 13 dB maximum. Worst-case loss margin, with all the power penalties caused by dispersion and modal noise and other noise contributions, can be as low as about 3-4 dB, depending on the fiber performance and length.