How Many Fibers Are Enough?

Our local newspaper reported that the local phone company had completed installing fiber optic cables between their central offices in the Boston area. That didn’t surprise me, but what did was the description of the cables they were installing. Each cable had 864 fibers in it! Why so many fibers, you might ask? That’s 432 links! Think about the way the telephone company personnel have to install cables. Usually, they must dig up the roads, install conduit and innerduct, and send out special trucks to complete splicing all the fibers together. If you have ever had to install cabling under a roadway or sidewalk, you know how expensive it is to dig up the road. You certainly do not want to do it again until you absolutely have to. If you are installing fiber optic cable, you want to install a cable that will last a long time, and that means lots of fibers. Colleges once installed cables with six to 12 multimode fibers for most of their campus local area network (LAN) backbones. As their networks grew, they discovered that was too few fibers, so they increased the fiber count to 48 multimode fibers or more. Then they realized that they were installing applications that required single-mode fibers and added about a dozen single-mode fibers. Today, campus backbones may include 144 to 288 fibers, with half the fibers multimode and half single-mode. Colleges have learned that installing cable is expensive, time-consuming, and disruptive. It’s better to install more capacity at once than it is to add cables later. Most companies have learned the same lesson: fiber is cheap and cabling is expensive. Installing extra fibers in the beginning simplifies and lowers the cost of adding services and upgrades. Another big advantage is timing. If the fibers are available, the installation can be done as soon as networking equipment is available, instead of waiting for a new cable installation. We recommend the same philosophy for all fiber optic installations, even those in buildings. The cost of the cable installation generally exceeds that of the cable itself and fiber is cheap, so it’s easy to justify more fibers. The question is how many fibers do you need and of what kinds? The answer depends on where the cable is being installed, its intended use, and budget limitations. Future-proofing is a myth. Today’s technology is changing so fast that new generations of equipment are coming out several times a year. Installing extra fibers is like buying insurance—you may never need it, but if you do, it can really save you a lot of money! Choosing the number and types of fibers you put in a cable requires knowing what you plan for future communications needs. Consider first your timetable. If your plans include moving to a larger facility in a few years, invest in more fibers conservatively. If you plan to stay a long time, consider installing more fibers with a higher performance potential. Likewise, what will the fiber be used for? Data entry has low bandwidth needs, while CAD/CAM, graphics, or “prepress” in the printing industry requires massive data transfer bandwidth. Low-bandwidth applications will need only basic multimode fibers for Ethernet, while high-bandwidth applications will start at Fast Ethernet and soon move to Gigabit Ethernet. This will require high performance multimode fiber now and perhaps single-mode fiber in the near future. If you are doing fiber to the desktop, two fibers may be all you need. If it’s just one desk, expansion is not an issue. Desks move often, so adding more fibers for higher performance later may not make sense either. Concentrating on the backbone makes more sense. Once you decide how many fibers and what kinds you need, then you can choose cable. For indoor applications, distribution cable is most popular for fiber counts over a dozen, due to its smaller size, lighter weight, and lower cost. Outdoors, you need water-blocking, but you can now choose between the traditional loose-tube, gel-filled cable and dry, water-blocked distribution cable. Either works well, but dry cable is much less messy to install and requires less complicated hardware. Mixing fiber types creates another potential problem. You must identify different fibers (multimode or single-mode, 62.5/125 or 50/125) properly to prevent confusion, because they are incompatible. You cannot mix and match them! Cable plant documentation must include color codes for the cables and identifications (IDs) for the terminations. I know you want me to quit addressing the real question by giving you a “number.” That’s harder than it seems, but here are some approaches: * Count the links you have to support, add some spares, and double that number. * If it’s all multimode, add a pair of single-mode fibers to each major termination point in a backbone, with at least six to 12 fibers. * The more difficult the installation, the more fibers you should install. * Few installations justify more than 144 fiber cables. * How much can you afford? HAYES is president of Fotec/Fiber U, Medford, Mass. He can be reached at

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