Cable About Fiber Optic Cable

There are two kinds of fiber optic cable in use here on the UW - Madison Campus: multimode fiber (with a 62 micron core) and single mode fiber (with a 9 micron core). The main differences between them are the cost of interface devices and bandwidth capacity.

The first fiber optic cable manufactured was multimode, but it was only called multimode after single-mode fiber was invented. Light is transmitted within a fiber by total internal reflection. Mode refers to a path along the inside of the fiber. In multimode fiber, there can be multiple beams at slightly different incidence angles within the fiber. Since these paths have slightly different lengths, the pulses of light which travel these paths arrive at the end at slightly different times. This causes some fuzziness of the signal recieved at the far end. As cable lengths get longer or bandwidth transmission speed increases, transmission errors increase.

For a more elaborate and colorful explanation, check this slideshow from Corning (requires Adobe Acrobat Viewer) Optical Fiber Characteristics by Peter Pondillo.

After studying the limiting factors of fiber transmission, engineers discovered that making the fiber core more narrow in the manufacturing phase reduces the number of modes exhibited by a particular fiber. In fact, if core size is in the area of 9 microns, there is effectively only one mode - hence the name single-mode.

At this time, there were almost no analog uses of fiber optics, and the primary applications of fiber optic cable were done by telecommunications carriers, which could realize significant reductions in cost and improvements in reliability with long-distance cables. Some people drew the general conclusion that single-mode fiber was always better than multimode fiber.

In fact, by careful examination, it becomes clear that there are good, cost-effective applications for multimode fiber optic cables.

The highest data speed useable on multimode fiber is about 155 megabits per second. On singlemode fiber, data transmission speed has gone up to 40,000 megabits per second (on an MCI circuit between Chicago and St. Louis), and has gone hundreds of times faster in lab tests.

Because the core diameter of multimode fiber is larger, it is easier to get light into and and out of. This makes it possible to use light emmitting diodes as light sources in multimode fiber. Further, connection alighment is less critical than with singlemode fiber. This means that it is less expensive to build interface electronics for multimode fiber, and is simpler to install connectors.

In a campus environment like ours, we have a lot of interfaces and a limited area covered. This results in a high proportion of interfaces per unit length (foot, mile, meter) of fiber cable. Because interfaces for multimodemode fiber cost less than for single-mode fiber, and because the error rate on relatively short, campus-scale multimode cable is low, campuses use multimode fiber extensively.

We use multimode fiber for almost all of our standard campus data connections. Video can be carried on multimode fiber (either as an analog signal or as a digital signal), however, the number of channels is limited.

The Residential Television Network (the cable TV system built by Cyberstar which DoIT now operates with signals purchased from Charter) uses single mode fiber. We recommend that both multimode and single mode fiber be installed in new projects or remodeling projects Even if you don't immediately plan to use the Residential Television Network, we strongly recommend that you install single mode fiber at the same time as the multimode fiber. Contractors can order a special fiber cable which contains both multimode and single mode fibers, so only a single cable pull is required. This will cost less than separate multimode and single mode fiber cables.

One of the benefits of using ATM (Asynchronous Transfer Mode) is that it can carry multiple "virtual" circuits - and those circuits can carry voice, data and video - on a single fiber pair. Most fiber connections carry far less than their full capacity, so carrying multiple virtual circuits on a single pair of fibers costs less and takes less patch panel space than individual fiber pairs.

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Comments to: Dave Devereaux-Weber
Last Updated:25-February-1999