Article by Dwayne Crawford
Ever wonder what the difference is between loose-tube (mini distribution) 250 µm fiber cables and tight buffered 900 µm fiber cables?
Loose-tube 250 µm and tight-buffered 900 µm fiber cables actually start with the same 250 µm bare fibers that feature the same size fiber core (i.e., 50 µm for multimode and 9 µm for singlemode), 125 µm cladding and soft 250 µm coating. The difference between the two is all in the cable construction.
An added layer
Tight-buffered 900um fiber includes an additional 900um layer of hard plastic over the 250um fibers for protection. Within the cable, several of these color-coded 900um tight buffered fibers are situated around a central strength member and then covered with Kevlar or aramid yarn for protection, a rip cord and then the jacket.
Tight-buffered fiber cables come in
various fiber counts that typically range from 2
to 144 fibers, with larger fiber counts featuring
fiber subunits of 6 or 12 fibers within the cable.
For example, a 144-fiber cable usually has
twelve 12-fiber subunits while a 36-fiber cable
could have six 6-fiber subunits or three 12-fiber
sub units.or-coded 900 µm tight buffered fibers
are situated around a central strength member
and then covered with Kevlar or aramid yarn
for protection, a rip cord and then the jacket.
Down the Tubes
Loose-tube 250 µm fiber places up to 12 bare 250 µm fibers inside a flexible plastic tube, which are also color coded and situated around a central strength member with Kevlar or aramid yarn for protection. Buffered loose-tube cables feature an outer water-blocking tape around the tubes, beneath the outer jacket.
The tubes themselves are gel-filled to prevent water migration, or they are available with a dry water-blocking technology—sometimes referred to as gel-free cable. Both of these materials are vital to prevent water from migrating into the tubes and potentially freezing, expanding and breaking the fiber. Dry water-blocking technology significantly reduces installation time by eliminating the
need to clean off the gel prior to termination.
Loose-tube 250 µm fiber cable comes in various fiber counts that typically range from 6 to 144, with some manufacturers offering higher counts of up to 216 fibers. With the exception of a 6-fiber cable, the fibers are grouped into sets of 12 for maximum density.
Speaking of density, without the 900 µm plastic coating, loose-tube 250 µm fiber cables are less than half the size of 900 µm fiber cables—1.4 in (35.6 mm) for a 144-fiber tight buffer cable and only .67 in(17 mm) for an outdoor 144-fiber loose-tube cable.
From outdoor to indoor
Generally speaking, tight-buffered 900 µm fiber cables are used for indoor applications, including intra-building riser and plenum applications and in the data center. Loose-tube 250 µm fiber cables are typically used in outside plant (OSP) applications, such as inter-building duct, aerial and direct buried installations.
While indoor/outdoor cables are popular for eliminating the need for service entrance splicing to in-building cable, OSP loose-tube 250 µm cabling must be terminated within 50 feet of entering a facility. To accomplish this, breakout kits are used to build the 250 µm cable up for protection and termination to 900 µm connector boots.
The problem with breakout kits is that they add additional material costs and a significant amount of labor. One option is to terminate the 250 µm fiber directly to 250 µm connector boots. This can speed network deployment in the data center and fiber-to-the-home applications.
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