The FOA Reference Guide To Fiber Optics
Cable Types: (L>R): Zipcord, Distribution, Loose Tube, Breakout
Cable provides protection for the optical fiber or fibers within it appropriate for the environment in which it is installed.
Fiber optic "cable" refers to the complete assembly of fibers, other internal parts like buffer tubes, ripcords, stiffeners, strength members all included inside an outer protective covering called the jacket. Fiber optic cables come in lots of different types, depending on the number of fibers and how and where it will be installed. It is important to choose cable carefully as the choice will affect how easy the cable is to install, splice or terminate and what it will cost.
Cable's job is to protect the fibers from the environment encountered in an installation. Will the cable become wet or moist? Will it have to withstand high pulling tension for installation in conduit or continual tension as in aerial installations? Does it have to be flame-retardant? Ultra flexible? Will the cable be exposed to chemicals or have to withstand a wide temperature range? What about being gnawed on by a woodchuck or prairie dog? Inside buildings, cables don't have to be so strong to protect the fibers, but they have to meet all fire code provisions. Outside the building, it depends on whether the cable is buried directly, pulled in conduit, strung aerially or whatever.
All cables are comprised of layers of protection for the fibers. Most all start with standard fiber with a primary buffer coating (250 microns) and add:
Tight buffer coating (tight buffer cables like simplex, zipcord, distribution and breakout types): A soft protective coating applied directly to the 250 micron coated fiber to provide additional protection for the fiber, allowing easier handling, even direct termination on the fiber.
Loose Tubes (loose tube cables): Small, thin plastic tubes containing as many as a dozen 250 micron buffered fibers used to protect fibers in cables rated for outside plant use. They allow the fibers to be isolated from high pulling tension and can be filled with water-blocking materials to prevent moisture entry.
Strength members and stiffeners: Usually aramid yarn, the same used in bulletproof vests, often called by the duPont trade name Kevlar, which absorbs the tension needed to pull the cable and provides cushioning for the fibers. Aramid fibers are used not only because they are strong, but they do not stretch. If pulled hard, they will not stretch but eventually break when tension exceeds their limits. This ensures that the strength members will not stretch and then relax, binding the fibers in the cable. The proper method of pulling fiber optic cables is always to attach the pull rope, wire or tape to the strength members.
Some cables also include a central fiberglass rod used for additional strength and to stiffen the cable to prevent kinking and damaging the fibers. When included, these rods should be attached to swivel pulling eyes.
Jacket: The outermost layer of protection for the fibers that is chosen to withstand the environment in which the cable is installed. Outside cables will generally be black polyethelene (PE) which resists moisture and sunlight exposure. Indoor cables use flame-retardant jackets that can be color-coded to identify the fibers inside the cable. Some outdoor cables may have double jackets with a metallic armor between them to protect from chewing by rodents or kevlar for strength to allow pulling by the jackets. Indoor-outdoor cables have a PE outer jacket that can be removed to expose a flame-retardant inner jacket for use within buildings.
Protection Against Water and Moisture: Cables installed outdoors require protecting the fibers from water. Either a gel or absorbent tape or powder is used to prevent water from entering the cable and causing harm to the fibers. Generally, this applies to loose tube or ribbon cables, but dry water-blocking is used on some tight buffer cables used in short outdoor runs, such as building to building on a campus or to an outdoor wireless antenna or CCTV camera.
Look at the pictures below to see how each type of cable incorporates these components.
Tight buffer cables (Simplex, Zipcord, Distribution and Breakout) are used where cable flexibility and ease of termination are important, more so than ruggedness and pulling strength which characterize loose tube and ribbon types of cable. Generally, tight buffer cables are used indoors and loose tube/ribbon cables outdoors.
Simplex and zip cord
These types are used mostly for patch cord and backplane applications, but zipcord can also be used for desktop connections. Simplex cables are one fiber, tight-buffered (coated with a 900 micron buffer over the primary buffer coating) with Kevlar (aramid fiber) strength members and jacketed for indoor use. The jacket is usually 3mm (1/8 in.) diameter. Zipcord is simply two of these joined with a thin web.
Distribution cable is the most popular indoor cable, as it is small in size and light in weight. They contain several tight-buffered fibers bundled under the same jacket with Kevlar strength members and sometimes fiberglass rod reinforcement to stiffen the cable and prevent kinking. These cables are small in size, and used for short, dry conduit runs, riser and plenum applications. The fibers are double buffered and can be directly terminated, but because their fibers are not individually reinforced, these cables need to be broken out with a "breakout box" or terminated inside a patch panel or junction box to protect individual fibers.
Breakout cable is a favorite where rugged cables are desirable or direct termination without junction boxes, patch panels or other hardware is needed. They are made of several simplex cables bundled together insdie a common jacket. This is a strong, rugged design, but is larger and more expensive than the distribution cables. It is suitable for conduit runs, riser and plenum applications. It's perfect for industrial applications where ruggedness is needed. Because each fiber is individually reinforced, this design allows for quick termination to connectors and does not require patch panels or boxes. Breakout cable can be more economic where fiber count isn't too large and distances too long, because is requires so much less labor to terminate.
Loose tube cables
Loose tube cables are the most widely used cables for outside plant trunks because it offers the best protection for the fibers under high pulling tensions and can be easily protected from moisture with water-blocking gel or tapes.These cables are composed of several fibers together inside a small plastic tube, which are in turn wound around a central strength member, surrounded by aramid strength members and jacketed, providing a small, high fiber count cable. This type of cable is ideal for outside plant trunking applications, as it can be made with the loose tubes filled with gel or water absorbent powder to prevent harm to the fibers from water. It can be used in conduits, strung overhead or buried directly into the ground. Some outdoor cables may have double jackets with a metallic armor between them to protect from chewing by rodents or kevlar for strength to allow pulling by the jackets. Since the fibers have only a thin buffer coating, they must be carefully handled and protected to prevent damage. Loose tube cables with singlemode fibers are generally terminated by spicing pigtails onto the fibers and protecting them in a splice closure. Multimode loose tube cables can be terminated directly by installing a breakout kit, also called a furcation or fan-out kit, which sleeves each fiber for protection.
Fiber Optic Microcable
Microcable is a term applied to a new class of cables that are very high density cables. Two fiber developments make a microcable feasible. Bend insensitive fiber allows fibers to be packed into cables with much higher density since the fibers are not as sensitive to the stress caused by the crowded fibers. In addition, the bend insensitive fibers can be coated with smaller diameter primary buffer coatings, 200 microns or less compared to 250 microns for conventional fibers, allowing more fibers to be packed into a smaller space.
The differences between conventional and micro cables are substantial. A 144 fiber loose tube cable is typically 15-16mm diameter while a comparable micro cable is only about 8 mm diameter - half the size and about one-third the weight. The smaller size allows for much larger fiber counts, over 3,000 fibers in some designs.
Microcables are available for both premises and outside plant installations. Their small size allows a different installation technique where the cable is "blown" into micro ducts, plastic tubes much smaller than conventional fiber innerducts or conduits. The cable is not really blown into the duct but floated on air to reduce friction then pushed into the duct.
Ribbon cable is preferred where high fiber counts and small diameter cables are needed.This cable has the most fibers in the smallest cable, since all the fibers are laid out in rows in ribbons, typically of 12 fibers, and the ribbons are laid on top of each other. Not only is this the smallest cable for the most number of fibers, it's usually the lowest cost. Typically 144 fibers only has a cross section of about 1/4 inch or 6 mm and the jacket is only 13 mm or 1/2 inch diameter! Some cable designs use a "slotted core" with up to 6 of these 144 fiber ribbon assemblies for 864 fibers in one cable! Since it's outside plant cable, it's gel-filled for water blocking or dry water-blocked. Another advantage of ribbon cable is Mass Fusion Splicers can join a ribbon (12 fibers) at once, making installation fast and easy. Ribbon pigtails are spliced onto the cable for quick termination.
Armored cable is used in direct buried outside plant applications where a rugged cable is needed and/or rodent resistance. Armored cable withstands crush loads well, needed for direct burial applications. Cable installed by direct burial in areas where rodents are a problem usually have metal armoring between two jackets to prevent rodent penetration. Another application for armored cable is in data centers, where cables are installed underfloor and one worries about the fiber cable being crushed. Armored cable is conductive, so it must be grounded properly.
aerial - messenger
Aerial cables are for outside installation on poles. They can be lashed to a messenger or another cable (common in CATV) or have metal or aramid strength members to make them self supporting. The cable shown has a steel messenger for support. It must be grounded properly.
A widely used aerial cable is optical power ground wire (OPGW) which is a high voltage distribution cable with fiber in the center. The fiber is not affected by the electrical fields and the utility installing it gets fibers for grid management and communications. This cable is usually installed on the top of high voltage towers but brought to ground level for splicing or termination.
Even More Types Of Cable Are Available: There's double-jacketed indoor/outdoor, dry water-blocked, simple jacketed POF, etc. Every manufacturer has it's own specialties and sometimes their own names for common cable types, so it's a good idea to get literature from as many cable makers as possible. And check out the smaller cable companies; often they can save you a bundle by making special cable just for you, even in relative small quantities.
Another "cable" type is not really cable at all. By installing a "cable" which is just a bundle of empty plastic tubes, you can "blow" fibers into the tubes using compressed gas as needed. If you need to upgrade, blow out the old fibers and blow in new ones. Both indoor and outdoor versions of air-blown fiber cables are available and its even been used for FTTH. Special fibers are required that have been coated for easier blowing through the tubes, but any singlemode or multimode fiber is available. It's more expensive to install since the tubes must be installed, special equipment and trained installers are needed but can be cost effective for upgrades.
Hybrid and Composite Cables
These two types of cables are often confused, but almost everybody and the NEC defines them as:
Hybrid cables: Cables that contain two types of fibers, usually multimode and singlemode. These cables are often used in campus and premises backbones where the singlemode fibers may be used in the future.
Composite cables: Cables that contain both fibers and electrical conductors. Underwater tethered vehicles use cables like this, as do some cables used for remoting wireless antennas or CCTV cameras. These cables must be properly grounded and bonded for safety.
Cable Design Criteria
Choosing a cable requires consideration of all the environmental factors involved during installation and during the cable's lifetime. Here are some of the most important factors.
Pulling Strength: Some cable is simply laid into cable trays or ditches, so pull strength is not too important. But other cable may be pulled thorough 2-5 km or more of conduit. Even with lots of cable lubricant, pulling tension can be high. Most cables get their strength from an aramid fiber (Kevlar is the duPont trade name), a unique polymer thread that is very strong but does not stretch - so pulling on it will not stress the other components in the cable. The simplest simplex cable has a pull strength of 100-200 pounds, while outside plant cable may have a specification of over 800 pounds.
Bending Limits: The normal recommendation for fiber optic cable bend radius is the minimum bend radius under tension during pulling is 20 times the diameter of the cable. When not under tension, the minimum recommended long term bend radius is 10 times the cable diameter.
Water Protection: Outdoors, every cable must be protected from water or moisture. It starts with a moisture resistant jacket, usually PE (polyethylene), and a filling of water-blocking material. The usual way is to flood the cable with a water-blocking gel. It's effective but messy - requiring a gel remover (use the commercial stuff - it's best- -but bottled lemon juice works in a pinch!). A newer alternative is dry water blocking using a miracle powder - the stuff developed to absorb moisture in disposable diapers. Check with your cable supplier to see if they offer it.
Crush Loads or Rodent Penetration: Armored cables are used because their strong jackets withstand crushing and rodent penetration. Direct burial OSP cables are usually armored or installed in conduit. Armored indoor cables are available with NEC rated jackets for placement with other cables under false floors, as in data centers.
Fire Code Ratings: Every cable installed indoors must meet fire codes. That means the jacket must be rated for fire resistance, with ratings for general use, riser (a vertical cable feeds flames more than horizontal) and plenum (for installation in air-handling areas. Most indoor cables use PVC (polyvinyl chloride) jacketing for fire retardance. In the United States, all premises cables must carry identification and flammability ratings per the NEC (National Electrical Code) paragraph 770.
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