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Fiber Optic Cabling
Loose Tube, Tight Buffered, Hybrid Cables, Composite Cables
Today's communications networks generally use only two types of optical fibers, singlemode
and multimode fiber. To accommodate a wide variety of environments, these optical fibers are encased in a variety of protective sheaths. However, basic cable construction can be
categorized into two solutions, loose tube and tight buffered construction
Basic Coatings
As fiber is manufacturer, a base coat of ultraviolet acrylate is applied which provides a moisture and mechanical protective layer. The standard diameter at this point is 125um.
From this basic configuration, additional coatings are applied as per the cable specification.
Loose tube fiber consists of multiple strands of fiber encased within a protective outer
jacket, generally know as a buffer tube. Depending on the manufacturers specifications,
the cable can consist of a series of buffer tubes wrapped together around a central strength member or a central tube can house the fibers with strength members being wrapped
around this fiber optic tube. In either case, loose tube fiber provides rugged construction with high fiber counts. This cable generally comes with gel filled buffer tubes and either water
blocking tapes and or gels to protect the fiber from water penetration. These cables are used for underground, direct burial, and aerial installations. These cables are especially suitable
for areas with high temperature gradients. As with all materials, fiber within the cable is subjected to expansion and contraction during extreme temperature changes. Loose tube
construction allows the internal fibers to physically minimize the stress on the fibers during periods of change.
When installing loose tube fiber, some additional criteria must be taken into consideration.
The standard working diameter of loose tube fiber is 250um. If this fiber is to be fusion or mechanically spliced, the exposed fiber must be protected in specially designed splice tray
and enclosure to protect the fiber from damage. If the fiber is to be connectorized,
each buffer tube will require the installation of a fan-out kit, which will increase the working
diameter to 900UM and provide additional protection to the exposed fiber. WARNING, the diameter of the buffer tube can vary with the manufacturer. Consult with the manufacturer for the recommended fan-out kit.
Most forms of loose tube fiber are coated with sheaths that are considered to be outdoor
rated by NEC (USA National Electric Code). Because of this, these cables are required to be terminated within 50 feet of the building's entrance. This distance can be extended by
placing the cable in metallic conduit. This could add considerable cost to the installation. Also, because of the fluid nature of the gel, the use of loose tube fiber within multi-story
buildings risers should be avoided. The forces of gravity may cause the gel to settle down at the bottom of the cable run which could lead to the binding of the de-gelled fibers on the
upper floors. This will increase the fiber's susceptibility to damage and possible changes in attenuation. Also, if an outside splice point is in close proximity to the building, the
additional pressure gradient could force the gel to back feed to the splice point and escape into the enclosure.
Generally loose tube fiber construction provides a cost effective solution for environmentally
challenging installations while providing low fiber attenuation, high cable pulling strength and durability, and high fiber strand counts.
Tight Buffered
Fiber that consists of single strands of fiber buffered with an additional layer of protective coating and placed directly into the construction of the cable. For most manufacturers the
outer working diameter for this outer coating is 900um. Wrapped with Kevlar for added protection, the fibers are then coated with a variety of sheaths. These cables generally
exhibit greater flexibility than loose tube construction and with the proper sheath, they are suitable for indoor installation. The tight buffer constructed cable is the cable of choice for
interior and sometime indoor/outdoor installations. The 900um buffered coating allows for the direct connectorization of the fiber. This eliminates the need for fan-out kits and the
associated installation consumables.
Breakout fiber is a more robust version of the tight buffered fiber. In addition to the 900um
protective coating, break-out fibers are wrapped in additional protective layers of sheathing and or Kevlar to build their outside diameter to 2 to 3 mm. These cables are generally used
in fiber to the workstation applications, industrial environments or anywhere else that the cable maybe exposed to abrasion.
Hybrid Cables
Cables that contain elements with similar media but with different performance standards. For example an IBM type II cable can be considered a hybrid cable, it has a four conductor
22 AWG shielded component for data transmission and an 8 conductor 24 AWG UTP component for voice transmissions. Likewise a cable containing a multimode fiber
component and a singlemode component within the same sheath is considered to be a hybrid cable. The cables can be found in multiple fiber densities and in loose tube and tight buffered configurations
Composite Cables
Cables that contain elements with dissimilar transmission media. For example, these cables would have a copper element as well as a fiber element. Though most manufacturers
shy away from placing high speed UTP cabling (category 5e and category 6) in the composite configuration, there are cables with voice and fiber components. Composite
construction is increasingly being utilized for security cameras, fire alarm control panels and wireless voice and data networks where a low voltage current must accompany a fiber optic data component.
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