Information Technology Reference
In-Depth Information
The scheme for backbone interconnections should be determined before any
work begins, and should be consistently applied. Remember that all fiber cable con-
nections must reverse end-to-end for this scheme to work.
A couple of typical fiber-optic connection patterns are shown in Fig. 7.16. Note
that the scheme must have an odd number of reversals to work, but that this is auto-
matically accomplished by the fact that the coupling devices at the end of each run
do reversals, too. Because cables always have the same type of male connectors, each
patch cord, horizontal cable, or backbone cable always provides one reversal. The
adapter couplings at either end provide two additional reversals, restoring the signal
to the original polarity. All that you (or your installer) must remember is where to
clip each SC connectorized fiber into its duplex housing to provide the proper AB/BA
orientation. SFF connectors must provide a means to maintain fiber AB polarity, and
cable polarity is not easily reversed, so the installation must be checked carefully.
If you do fiber to the desktop, you will want to install a fiber patch panel in
your TR. This panel will provide a point of termination for each station drop and
will also provide labeling and identification for the horizontal run. It will also pro-
vide a handy point for testing and troubleshooting of fiber runs. An all-fiber instal-
lation will usually involve locating network hubs with fiber-optic interfaces in the
TR. You will connect a duplex fiber optic patch cord between the station location
on the patch panel and the hub port. You may have to use an adapter cable or
hybrid adapter if your equipment does not have the new SC connectors. All of the
standard cable management trays and panels may be used with fiber patch cords, in
general, although you should pay particular attention that the cords are not bent
sharply or otherwise stressed. The tiny glass fibers in these cables are subject to
stress fractures when excessively bent, and this may cause signal loss that prevents
a good connection. A sharp bend may actually break the fiber entirely. Suspected
cords should be tested and scrapped if bad.
Fiber-Optic Termination Fixtures
You will want to carefully plan the location of your fiber-optic terminations in the
TR. Because of the nature of fiber-optic cable, it is difficult to re-terminate when you
move the termination location. Fiber-optic cable is often terminated in a special fix-
ture in the TR. Fiber-optic termination fixtures, as shown in Fig. 7.17, often have
space for extra fiber to be wound. However, the extra fiber is really intended to
allow some slack in case a damaged connection needs to be replaced, and to assist
in the termination process by allowing greater access to the fiber end. Usually, the
amount of extra cable is only 1 m or so, and may have already had the outer jacket
removed to expose the fibers, so it would not be useful in a relocation.