Air
Blown Fiber Installation
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
ones.
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
connections between
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.
Special fibers
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.
The
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.
Air-blown fiber should not be confused with "Blown
Cable" where special cable is floated on air and
pushed into a duct. See
this FOA Guide section for Blowing and Jetting
Cables.
More
Topics On Fiber Optic Installation
Table of Contents: The FOA
Reference Guide To Fiber Optics
|