When designing and installing optical fiber cables, one must
forecast the future. How many fibers and what types will be needed?
Starting with today's needs, one should add a few fibers as spares in
case we underestimate the number needed or some are damaged in
installation. For the future, what new systems will be installed and
what fibers will they need? Will routes for the fibers change?
Generally, big changes would require installing more cables.
Back in the early 1980s, when everything in fiber optics was changing
rapidly - even more rapidly than today - British Telecom came up with a
different idea. Why not install tubes along cable routes and use air
pressure to blow fibers down the tubes. One could add extra tubes for
future use and even blow out unused fibers and replace them with new
Today, air blown fiber (ABF) systems are well
developed, available from multiple vendors and some installers are
trained and experienced in their installation. The hardware and
installation is somwhat different from installing conventional fiber
optic cables. One has the tubing into which the fiber will be blown,
special coated fiber or bundles of fibers which can be blown into the
tubes, special hardware for termination and splicing and the blowing
apparatus which provides the controlled air stream (or dry nitrogen)
necessary for blowing the fibers into the tubes up top several km.
Sumitomo Tube Cables
The tubes come in many designs with single or some number of
tubes inside a single jacket. Cables of tubes are installed like
regular cables, meeting usual building codes. Couplings provide
tubes for longer runs or branching. Boxes of various designs allow
routing tubes or terminating fibers. ABF has been used indoors, on
ships, outdoors including FTTH or practically anywhere conventional
cables are used.
Since ABF can be used almost anywhere
conventional cables can be used, the tubes must be designed for the
same environmental requirements as regular cable. Indoor cables must
meet appropriate fire codes and outdoor cables must be designed to
prevent moisture damage. And since air pressure is being used to
install fibers, the tubes require connections that seal properly to
maintain air pressure along the path.
coated with a foamed plastic (polyethylene generally) are used for ABF
systems. Single fibers up to bundles of 24 fibers can be used. Fibers
are available from several vendors.
The advantages of
ABF are predicated on the installation of an adequate number of tubes
initially. Generally, an ABF tube bundle will be larger than a
equivalent conventional fiber optic cable and have a larger bend
radius, so the cable plant design must take this into account.
economics of ABF vs. conventional fiber cables depends on the
application and plans for the future. It may be more expensive in the
beginning but less expensive to expand the number of fibers or change
the fiber types. The tradeoff is to install conventional fiber cables with more fibers, even hybrid SM/MM cables, initially when extra fibers are relatively inexpensive.
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