Choosing the right fiber optic cable is extremely important for any installation. The purpose of the cable is to protect the fibers during installation and the service lifetime. Several types of cable are available. Your choice will affect the level of fiber protection, ease of installation, splicing or termination, and, most importantly, cost.
All cables share some common characteristics. For example, they all include various plastic coatings to protect the fiber, from the buffer coating on the fiber itself to the outside jacket. All also include some strength members for pulling the cable without harming the fibers. Outdoor cables have moisture protection, either a gel filling or a dry powder or tape. Direct-buried cables may have a layer of metal armor to prevent damage from rodents.
With all these variations, it’s understandable why there are so many fiber optic cable designs and why cable of more than 1 km may be made to order. This is an advantage if you need larger quantities because you can customize your cable to make it better and more cost effective.
How many fibers?
Unless you are making patchcords or hooking up a simple link with two fibers, it’s highly recommended that you include a number of spare fibers. Corporate network backbones are often 48 fibers or more. Most backbone cables are hybrids—a mix of 62.5/125 multimode fiber for today’s networks and single-mode fiber for future networks. If the slowest network planned today is at gigabit speeds, it might even be better to use the new 50/125 multimode fiber optimized for the laser sources used in gigabit networks.
So how many fibers do you want to install? As many as you can afford. Fiber is really inexpensive, compared to the cost of installation, so install a lot of them.
Indoor or outdoor—or both?
The major difference between indoor and outdoor cables is water blocking. Any conduit is someday likely to get moisture in it. Outdoor cables are designed to protect the fibers from years of exposure to moisture. Until recently, your only choice for outdoor cables was loose-tube, gel-filled cables. But now you can buy dry water-blocked cables similar to indoor designs that are easy to terminate without breakout kits, saving incredible amounts of time. In a campus environment, you can even get cables with two jackets: an outer PE jacket that withstands moisture and an inner PVC jacket that is UL-rated for fire retardancy. You can bring the cable into a building, strip off the PE jacket and run it anywhere, while normal outdoor cables are limited to 50 feet inside the building.
The fibers in outdoor cables only have their primary coating, so they are tiny—250 microns or ¼ mm in diameter. Such small fibers are hard to work with, so you need to put them into a breakout kit of tiny plastic tubes to provide enough protection for direct termination.
Indoor cables are what we call “tight-buffered” cables, where the glass fiber has a primary coating and secondary buffer coatings that enlarge each fiber to 900 microns—about 1 mm or 1/25-inch—to make the fiber easier to work with. All these cables can be directly terminated.
Simplex or duplex cables are generally used for patchcords, although some duplex cables and zipcords are installed permanently for desktop installations, where spare fibers for future expansion are deemed unnecessary.
The most popular cable for indoor use is distribution cable, which has a number of 900-micron buffered, color-coded fibers inside a single jacket. It’s the smallest and lightest cable, and each fiber is sturdy enough for direct termination. Since the fibers are terminated directly and have little protection, the terminations need to be placed inside a patch panel or box.
Another choice for indoor use is the breakout cable, which is just a bunch of simplex cables inside a common jacket for convenience in pulling and ruggedness. Each individual cable can be terminated directly without a patch panel. It’s ideal for industrial applications where ruggedness is important or in a location where only one or two pieces of equipment (such as local hubs) need to be connected.
Cable ratings and markings
Remember all indoor cables must carry identification and ratings per National Electrical Code (NEC), paragraph 770. Cables without markings will not pass inspections! Indoor cables should also be bright colors, typically orange for multimode and yellow for single-mode, to distinguish them from other cables and help protect them. EC
HAYES is a VDV writer and trainer and the president of the Fiber Optics Association. Find him at www.JimHayes.com.