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Building reliable fiber networks - TvTechnology

Building reliable fiber networks

Learn the tips and tricks to installing a fiber-optic network for broadcast.
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If you want to build reliable fiber-optic networks for computer and video, there are a few basics you should get right. First, you must use the proper cabling and hardware. Second, you should use good techniques, and have a relatively clean work environment. Finally, nothing is perfect — you should have a backup plan that suits the criticality of the service you are carrying.

Proper cabling and hardware

It is very important to choose the correct cable for your application. The equipment you use will determine the diameter and mode of the cable you need. And the application environment will determine whether you use indoor or outdoor cabling. But there are many instances when you need to terminate cables or splice them together. For these procedures, you must use the proper hardware.

There are two types of indoor cables: single-fiber and multifiber. Single-fiber cables contain a single fiber strand with a tight buffer surrounding it. These cables are also available as a “Siamese” zip-cord pair. You can terminate this robust cable directly without any special considerations. Multifiber cables have a common outer jacket that contains two or more tight-buffer cables, each of which contain a single fiber. You can strip the outer jacket away to reveal fibers that are ready for termination. It does not require a breakout kit for termination. (More on breakout kits later.)

Fiber-optic cables designed for outdoor use differ significantly from their indoor counterparts. The outer jacket of the outdoor cable is much thicker because it is treated to resist water and ultraviolet radiation. It contains several loose buffer tubes, each of which contains several fibers. Each tube slides over its set of fibers, which are lubricated by a gel. Thus, no individual fiber strand is surrounded by its own protective jacket. This saves space in the cable, but it means that you must use a breakout kit to terminate the cable.

Usually when connecting outdoor cable to indoor cable, you need some sort of transition. This involves breaking out the loose-tube individual fiber strands and connecting them to a panel. You connect the other side of the panel to indoor cable.

Breakout kits allow you to convert the loose-tube multifiber outdoor cable to a number of tight-buffer individual fiber cables that you can terminate for indoor use with the appropriate connector. The tight-buffer cables are much more rugged and will withstand the environment of a broadcast facility much better than the individual strands of a loose-tube cable. The fibers in the breakout kit are fusion-welded to the individual fibers in the outdoor cable. This precludes terminating the cable in the field.

It is important to use the proper hardware to keep the fibers from becoming deformed or broken. Typically, a fiber must not be bent at a radius that is less than ten times the fiber core diameter. This is not only to prevent cable breakage, but to prevent light leakage and cable losses as well. Special fiber raceways are available to protect the fiber cables.

Good technique and a clean environment

If you have a large number of fibers, or if your cable runs are such that pre-terminated cables are not suitable, it might make more sense for you to terminate the cables yourself. But, if you don't have previous experience terminating fiber, you might be better off purchasing terminated cables. Fluke has an on-line training course at www.cableu.net. This course is a good starting point, but it's no substitute for hands-on training.

If you do it yourself, and if your connections are critical, you should use an optical-cable test kit to be sure that your terminations are working properly. If your technique is good, you will only lose between one-half and one dB per connector. (Check the specifications of the connector to be sure.) That figure can go up substantially if the connectors are covered with dust, or if you shatter the end of the fiber by using the wrong tool for the job. Use a microscope to check the end of the fiber for a clean surface that is devoid of scratches. Once the cable is terminated, the end is protected and there is no risk of shattering of the fiber or scratching of the end of the fiber. From that point on, dust becomes the major concern. Always use dust caps on fiber connectors when they are not connected to a device.

It is extremely difficult to field-terminate angle-polished connectors. For this reason, fusion splicing has become very popular. To perform a fusion splice, cleave the fiber and fusion-splice it to a short “pig-tail” consisting of a connector and a short piece of fiber.

Appropriate backup/redundancy plan

If you are running fiber from one building to another, it would be prudent to pull more than one fiber. The cost of a multifiber cable is almost exactly the same as a single-fiber cable, but the cost of the installation can easily outweigh the cost of the cable itself. And if you have to pull a cable in an emergency (after your only fiber fails), the costs can be very high indeed.

Also, since fiber is more sensitive than copper to being grabbed by a backhoe bucket, you should consider running multiple cables over separate paths between buildings. Terminate all spare fibers, even if you are not using them. It will take less time to terminate the spares while you are doing the initial installation than it will take to go find all the tools, clear out the cobwebs, and terminate the cable months or years later when you have a failure.

There are all sorts of fail-over switches, automatic rerouting technologies and other things available to help you build redundant networks. Also, you might want to consider RF networking technologies or optical networking using free-space optics as a backup for your fiber system.

Brad Gilmer is president of Gilmer & Associates, executive director of the AAF Association, and executive director of the Video Services Forum.

Send questions and comments to:brad_gilmer@primediabusiness.com

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