Proper selection and installation of connectors and splices can make a big difference in the successful completion of your next datacom cabling job.
Last month, we covered the roughing in and trimming up of cables for a voice-/data-/video-type project. This month, we'll discuss the installation of connectors and splices.
Connectors and splices are an important component of any datacom cabling system. In fact, these components are where problems are most likely to occur. Since manufacturers precisely engineer these systems, anything you do in the field is a potential problem. For example, just altering the twist pattern of twisted pair copper conductors in preparation for termination can make a cable fail its test.
UTP connectors. Almost all copper connectors in use today are of the RJ45 type—that is, they're 8-pin modular connectors. They are made up by stripping the cable (to the correct length of free conductors) and then inserting the conductors (no insulation need be removed) into the connector, and crimping it with an appropriate tool. The process of crimping the connector pushes the contact pins through the insulation, thus making proper contact. This is typical of many data connections. The new Cat. 5e and Cat. 6 connectors fall into this category.
In the next year or two, larger connectors for Cat. 7 (or some other variation) may come into use. With larger connectors, the design places the conductors farther apart from each other, thus reducing crosstalk (induction between adjacent conductors). From a consistency standpoint, manufacturers are trying hard to stay with the RJ45 modular connector design. This is due to compatibility issues. There are millions of RJ45 jacks in use today, so settling on a new connector type and size would require you to replace all of the old jacks after the new cabling installation. Needless to say, this can be an expensive process.
Fiber optic connectors. Since fiber optic cable has such a small diameter, you must keep it rigidly in place and accurately aligned, to mate with other fibers, light sources, or light detectors. While terminating a fiber is not yet as easy as installing a coaxial cable connector, it has become far easier than several years ago. Now, you can terminate fibers (termination involves installing a connector and polishing the face of the fiber) in half the time—and the process continues to get easier.
There are basically four types of fiber optic connectors in use today. Let's take a look at each in detail.
ST connector. The ST connector has long been the most popular type. This is due to its low cost and widespread popularity among installers.
FC connector. We use FC connectors for single-mode fibers. Rounded, they look similar to ST connectors.
SC connector. We can find SC connectors mentioned in a number of standards for multimode fiber. However, many installers prefer to use ST connectors instead. The SC connectors offer better performance, but are much more expensive.
FDDI and ESCON connectors. FDDI connectors are duplexed SC connectors (two connectors attached together), designed especially for FDDI (Fiber Distributed Data Interface) systems. We use ESCON connectors almost exclusively with the IBM ESCON system.
UTP splices? Installers must run this cable with no splices. Spliced copper data cable will not transfer the high-speed data signals required with data networks within the proper design requirements.
Fiber optic splices. Splices permanently join the ends of the fiber. There are two primary ways you can do this: by fusion (melting the pieces of glass together); or by mechanical means. With splicing, the fiber joint must pass light without loss, and the joint must be mechanically secure. That is, it should not come apart easily or break free.
Most single-mode fiber is fusion-spliced because of its lower loss and better return loss performance. Multimode fiber, with its complicated core structure, does not always fusion splice easily;so mechanical splices can give equal performance at a lower overall cost.
Fusion splices. Fusion splicing uses an electric arc to ionize the space between prepared fibers to eliminate air and heat the fibers to proper temperature (2000 degreess F). You must feed the fibers in as semi liquids, and meld them together. You must also replace the previously removed plastic coating with a plastic sleeve or other protective device.
The perfect fusion splice results in a single fiber rather than two joined fibers. One drawback to fusion splicing is you usually must do this work in a controlled environment, such as a splicing van or trailer. You should never perform fusion splicing in open spaces because of dust and other contamination.
In the case of fusion splicing, the initial capital investment in equipment is generally much greater than the cost for mechanical splicing. A fusion splice machine is a $3000 to $30,000 investment. However, a fusion splice offers low loss (sometimes 0 dB) and shows almost no back reflection. An additional benefit of fusion splicing is that the mechanical tensile strength of the fiber remains near that of the original fiber (on the order of 50 psi to 75,000 psi).
Fusion splicing in manholes is prohibited because of explosive gasses in such locations and because of the electric arc generated during this process.
Mechanical splices. On the other hand, mechanical splicing is quick and easy. It is a good choice for restoration and new construction. It does not require a controlled environment (other than a reasonable level of dust control), and its strength is adequate, though fusion splices are stronger. Unfortunately, back reflection and loss vary dramatically from one type of splice to another.
Mechanical splicing kits are less expensive than a fusion splice unit. Splices are glued, crimped, or faced. Mechanical splices use index matching gel or liquid, which is subject to contamination and aging.
Mechanical splices use either a V groove or tube-type design to obtain fiber alignment. The V groove is probably the oldest and most popular method—especially for multi-fiber splicing of ribbon cable. You must either crimp or snap this type of splice to hold the fibers in place. However, tubular splices may rely on glue or crimping to hold the fibers together, while a small tube inside (where the fibers are inserted) forces the fibers to align themselves.
After a splice is complete, you place it into a splicing tray. The trays then fit into splice organizers and in turn into a splice closure for protection from outside forces (i.e. weather, physical damage, etc.).
Next month, we'll wrap up this series with a discussion on how to test datacom cabling systems.