Alternatives To Fiber Optic Termination In The Field: Prefabricated Cabling Systems
Is it necessary to field terminate fiber optic cabling systems?
Not necessarily. If one has a good layout design and can accurately
determine cable lengths between termination points, it's possible to
have a complete cabling system factory made and delivered to the worksite ready to install and plug together.
Prefabricated cable assemblies have been around for decades. In the
early days of fiber optics for premises links, making a good field
termination was more difficult than today. Many connectors used metal
ferrules to hold the fiber that were hard to glue to the glass fiber.
These metal connectors were difficult to properly polish because the
metal contaminated the polishing films, sometimes requiring wet
polishing. And the tolerances on the machined ferrules make low loss
connectors a hit-or-miss proposition. Many cable
and patchcord manufacturers offer a cable termination service. You
specified what kind and number of fibers you needed, the cable and
connector type and the length and the manufacturer would supply a
completed assembly. And they guaranteed the connector and cable losses.
There was no field termination needed. However,
it was very important to accurately specify the cable length. That
could be difficult in a premises environment, where cables go up and
down walls, over ceilings and around obstacles. The usual method was to
have plenty of extra length and store the excess in service loops in
out of the way places like above the ceiling.
Another problem was how to protect the connectors during the
installation process. Several manufacturers designed custom boots that
fit over the connectors for protection and included a pulling loop
attached to the strength members of the cable. (Photos shown courtesy
of Nexans) This worked fairly well, especially
for small fiber count cables of the breakout type where the jacketed
simplex fibers provided protection to each connector. Distribution
cables required more care because the connectors directly attached to
the tight-buffered fibers had less protection. But under any
circumstances, the total fiber count was limited because of the size of
the fiber optic connectors common to the era made bunches of them
inside a pulling boot bulky and hard to pull.
The next development was creating a modular system that used
multifiber cables with mass termination connectors (generally MTPs) and
plug-in modules at the patch panel. Ribbon cable has become a standard item,
widely used in many large fiber count cables. Ribbon connectors,
especially the MTP, have improved in performance enough to allow them
to be used in mass terminations of pre-terminated cables. With the MTP,
12 fibers can be terminated in a connector hardly more bulky than a
typical SC, which is easily pulled and installed.
Using 12 fiber ribbon cables and MTP connectors, 12 connector modules
could be easily prefabricated and installed. Complete modular systems
included a module at each end and prefabricated ribbon cables for
installation between termination points. (Photo courtesy Corning Cable
Systems) Verizon’s desire to reduce
installation time and cost in their fiber to the home programs (FiOS)
spurred the development of outside plant prefab fiber optic cabling
systems. The Verizon need was a result of volume. When you need to
install connections to several million homes per year, small savings in
time and labor cost become very important. In addition, finding enough
experienced installation techs or training them is not easy.
Manufacturers like Corning Cable Systems actually came up with a
simple, elegant solution. Create a rugged, weather-sealed plastic shell
around the MTP connector that provides protection before, during and
after installation. Multifiber cables are installed by regular fiber
techs from the central offices to near the homes, where mating drop
boxes with 4, 6, 8 or 12 terminations are available. The FTTH tech
plugs a cable into the drop box on one end an a network interface box
on the home and the fiber installation is completed. (Photo courtesy Corning Cable Systems)
But is a cabling system based on a concept developed for applications
numbering in the millions cost effective for a single custom system?
The only generalization that can be made is a factory-made product will
probably be higher in component cost and lower in installation labor
cost. For any given installation, it’s really necessary to price it
both ways (being realistic on installation costs) for a cost
comparison. Then the customer should consider the intangibles.
Prefabricated systems are faster to install so if the cabling is
going into a building currently occupied, say an airport, it may cause
less work disruption. If the location is in a big city where labor
costs are high, the factory-made system may be lower cost. If good
design drawings are available, like in new construction, the
engineering time to develop the cable plant design may be much lower.
More fiber users are considering pre-terminated fiber optic
cabling systems, so it is to your advantage to learn more about them.
Companies like Corning Cable Systems
have information on their websites that can provide detailed
information on their prefabricated cable system products and even offer
planning guides to designers.