Fiber Optic Cable
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Fiber Optic Cable Construction
Fiber Optic Cable Types
Cable Design Criteria
Choosing Cables
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, strength
members and 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. Outdoors, it depends on whether the cable is buried
directly, pulled in conduit, strung aerially or even placed underwater.
Will the cable become wet or moist? Will it have to withstand high
pulling tension for installation in conduit or continual tension in an
aerial installation? Will the cable be exposed to chemicals or have to
withstand a wide temperature range? What about being gnawed on by a
squirrel, woodchuck or prairie dog? Indoors, cables don't have to be so
strong to protect the fibers, but they have to meet all fire code
provisions so the cable jacket has to be flame-retardant,
Outside
plant cable designs are optimized for the application type. In OSP
installations, cables may be underground, direct buried, aerial or
submarine (or simply underwater.)
Underground Cables
Underground
cables are generally installed in conduit which is usually a 4 inch (10
cm) conduit with several innerducts for pulling cables. Here cables are
designed for high pulling tension and lubricants are used to reduce
friction on longer pulls. Automated pulling equipment that limits
pulling tension protects the cables. Very long runs or those with more
bends in the conduit may need intermediate pulls where cable is pulled,
figure-8ed and then pulled to the next stage or intermediate pulling
equipment is used. An alternative is to install duct lines and blow
special lightweight cables into the ducts which can be faster and less
stressful on the cable.
Splices on underground cables are
generally stored above ground in a pedestal or in a vault underground.
Sufficient excess cable is needed to allow splicing in a controlled
environment, usually a splicing trailer, and the storage of excess
cable must be considered in the planning stage.
Direct Buried Cable
Direct
buried cable is placed underground without conduit. Here the cable must
be designed to withstand the rigors of being buried in dirt, so it is
generally a more rugged cable, armored to prevent harm from rodent
chewing or the pressures of dirt and rocks in which it is buried.
Direct burial is generally limited to areas where the ground is mostly
soil with few rocks down to the depth required so trenching or plowing
in cable is easily accomplished. Splices on direct buried cables can be
stored above ground in a pedestal or buried underground. Sufficient
excess cable is needed to allow splicing in a controlled environment,
usually a splicing trailer, and the storage of excess cable must be
considered. Aerial Cable
Aerial
installations go from pole to pole, but the method of securing cables
can vary depending on the situation. Some cables are lashed to
messengers or other cables, such as CATV where light fiber cables are
often lashed to the heavy coax already in place. Cables are available
in a “8” configuration with an attached steel messenger that provides
the strength to withstand tension on the cable. Some cables are made to
directly be supported without a messenger, called all-dielectric
self-supporting cables that use special hardware on poles to hold the
cables. Optical ground wire is used by utilities for high voltage
distribution lines. This cable is an electrical cable with fibers in
the middle in a hermetically-sealed metal tube. It is installed just
like standard electrical conductors. Splices on aerial cables can be
supported on the cables or placed on poles or towers, Most splices are
done on the ground, although it is sometimes done in a bucket or even
on a tent supported on the pole or tower. Hardware is available for
coiling and storing excess cable. Submarine/Underwater Cables
Sometime
OSP installations involve running cables across rivers or lakes where
other routes are not possible. Special cables are available for this
that are more rugged and sealed. Even underwater splice hardware is
available. Landings on the shore need to be planned to prevent damage,
generally by burying the cable close to shore and marking the landing.
Transoceanic links are similar but much more complex, requiring special
ships designed for cable laying. Since
OSP applications often use significant lengths of cables, the cables
can be made to order, allowing optimization for that particular
installation. This usually allows saving costs but requires more
knowledge on the part of the user and more time to negotiate with
several cable manufacturers.
- Fiber Optic Cable Construction
- 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:
Strength members are 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 for pulling and clamps in splice closures or patch panels when
spliced or terminated. Few cables today use metallic strength members
since it complicates installation by requiring the cable to be properly
grounded and bonded.
Cable Jacket: The
outermost layer of protection for the fibers which is chosen to
withstand the environment in which the cable is installed. Outdoor
cables will generally be black polyethylene (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 tough layer of
armor between them to protect from chewing by rodents or Kevlar for
strength to allow pulling by the jackets.
Indoor cables usually
have a flame-retardant PVC (polyvinyl chloride) jacket for general or
riser use and some other special plastic for plenum use in air handling
areas. Indoor-outdoor cables usually 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 as is becoming more common, 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.
Protection Against Crushing or Rodent Penetration
Some
cables have armor, usually metallic but sometimes hard plastic, under
the outer jacket resist crushing loads, such as cables installed under
floors in data centers or in rocky soil, as well as to prevent rodent
penetration. Metallic armor requires that the cable be properly
grounded and bonded.
Look at the pictures below to see how each type of cable incorporates these components.
- Fiber Optic Cable Types
Tight Buffer Cable Types
There
are two basic types of cables, generally defined as tight buffer and
loose tube. Tight buffer cables (simplex, zipcord, distribution and
breakout) are primarily used in premises applications where cable
flexibility and ease of termination are important, more so than
ruggedness, weather sealing 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, but some tight
buffer cables with moisture protection are used in short runs like on a
campus or between buildings.
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,
but some 2 mm cable is sometimes used with small form factor
connectors. Zipcord is simply two of these joined with a thin web.
Distribution cables:
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 cables:
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.
Ribbon Cable:
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 in ribbons 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. Premises cabling also
uses some ribbon cables, often in preterminated cabling systems where
12 fibers are terminated in one MTP connector. These cables are common
in LAN backbones and data centers. Armored Cable:
Armored
cable is used in direct buried outside plant applications where a
rugged cable is needed and/or for rodent resistance. Armored cable
withstands crush loads well, for example in rocky soil, often necessary
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 under the
floor and one worries about the fiber cable being crushed. Indoor
armored cables may have nonmetallic armor. Metallic armored cable is
conductive, so it must be grounded properly.
As with other fiber
optic components, there are different names or meanings used. “Armor”
in some companies’ jargon denotes a twisted heavy wire rope type cable
surrounding the entire poly cable sheath/jacket. Single or double armor
(two opposite ply layers of the steel wire) is typically used
underwater near shore and shoals. Inner metallic sheath members
of aluminum and/or copper are used for strength and for buried cable
locating with a tone set.
Aerial Cables
Aerial
cables are for outside installation on poles where consideration must
be given to continual tension from the cable weight as well as wind and
ice loads. Regular outdoor loose tube cables can be helically lashed to
a messenger or another cable (common in CATV.) Some cables have heavier
jackets and stronger metal or aramid strength members to make them self
supporting (called all-dielectric self-supporting or ADSS cable if the
strength members are nonconductive.) Self-supporting cables use
special hardware to handle the installed tension on the cables caused
by the weight of the cables and environmental factors like wind. Aerial
hardware to store slack cable can be mounted on the cable itself or on
utility poles.
The
cable known as a figure 8 cable has a cable bonded to an
insulated steel or all dielectric messenger for support. The
messenger is supported at each pole. If the messenger is steel, it must
be grounded properly. You can also get figure 8 conduit which can be
installed then cable pulled into the aerial conduit.
All
dielectric, self-supporting cable (ADSS) uses special hardware on the
poles and at the ends to distribute the tension on the cable to prevent
damage.
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.
Underwater and Submarine Cables
It
is often necessary to install fibers under water, such as crossing a
river or lake where a bridge other above water location is not
possible. For simple applications a rugged direct burial cable may be
adequate. For true undersea applications, cables are extremely rugged,
with fibers in the middle of the cable inside stainless steel tubes and
the outside coated with many layers of steel strength members and
conductors for powering repeaters. Submarine cables are completed on
shore, then loaded on ships and laid from the ship, often while
operational to ensure proper operation.
Even More Types Are Available:
There's double-jacketed indoor/outdoor, dry water-blocked, 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.
Air-Blown Fiber:
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.
Protection starts on the outside with a moisture resistant jacket,
usually PE (polyethylene), and a filling of water-blocking material.
The usual way has been 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 water–absorbent tape or
powder – similar to the material developed to absorb moisture in
disposable diapers. Most cable manufacturers now offer dry
water-blocked cables. 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 For Premises Cables
Every
cable installed indoors must meet fire codes. OSP cables can only be
run 50 feet (about 15 meters) inside a building without conduit or
connecting to a rated premises cable. 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. In
Canada, it’s CEC and other countries have similar cable ratings.
These ratings
are:
| NEC Rating | Description | | OFN | optical fiber
non-conductive | | OFC | optical fiber
conductive | | OFNG or OFCG | general purpose | | OFNR or OFCR | riser rated
cable for vertical runs | | OFNP or OFCP | plenum rated
cables for use in indoor air-handling spaces or plenums | | OFN-LS | low smoke
density |
Cables without markings should
never be installed indoors as they will not pass building inspections! Outdoor
cables are not fire-rated and can only be used up to 50 feet
indoors. If you need to bring an outdoor cable indoors, consider
a double-jacketed cable with PE jacket over a PVC UL-rated indoor
jacket. Simply remove the outdoor jacket when you come indoors
and you will not have to terminate at the entry point.
Grounding and Bonding
Any
cable that includes any conductive metal must be properly grounded and
bonded per the NEC for safety. Indoor cables rated OFC, OFCG, OFCR or
OFCP and outdoor cables with metallic strength members or armor must be
grounded and bonded. All composite cables must be properly grounded and
bonded also.
Cable Color Codes
Outdoor cables are generally black but premises cables are color-coded. De facto standard color
codes for cable jackets have been yellow jackets for singlemode and
orange jackets for multimode. With two multimode fibers now in common
use, 62.5/125 and 50/125, and three versions of 50/125 fiber, a more
comprehensive industry standard for color codes was required. It's
important to follow the TIA-598 standard to prevent mixing up cables.
Fiber Color Codes
Inside
the cable or inside each tube in a loose tube cable, individual fibers
will be color coded for identification. Fibers generally follow the
convention created for telephone wires except fibers are identified
individually, not in pairs. Since most loose tube cables have 12 fibers
per tube, colors are specified for fibers 1-12, then tubes are color
coded in the same manner, up to 144 fiber cables. In ribbon cables,
each ribbon is color coded in this format then ribbons are
stacked. For splicing long cable runs from similar cables
(called concatenation), like color fibers are spliced to ensure
continuity of color codes throughout a cable run.
| Fiber Number | Color | | 1 | Blue | | 2 | Orange | | 3 | Green | | 4 | Brown | | 5 | Slate | | 6 | White | | 7 | Red | | 8 | Black | | 9 | Yellow | | 10 | Violet | | 11 | Rose | | 12 | Aqua |
Choosing Cables
Choosing
a fiber optic cable for any given application requires considering
installation and environmental requirements plus long-term fiber
requirements to cover expansion to newer communications networks.
Installation requirements include where and how the cable will be
installed, such as pulled in conduit outdoors or placed in cable trays
in a building. Long term requirements need to consider moisture or
water exposure, expected temperature range, tension (aerial cables), or
other environmental factors.
You should contact several cable
manufacturers and give them detailed specifications for the
installation. They will want to know where the cable is going to be
installed, how many fibers you need and what kind of fibers
(singlemode, multimode or both in what we call "hybrid" cables.) You
can also have a "composite" cable that includes copper conductors for
signals or power. The cable companies will evaluate your requirements
and make suggestions. Then you can get competitive bids.
Since the
plan will call for a certain number of fibers, consider adding spare
fibers to the cable - fibers are cheap compared to the cost of
installing additional cables. Then you won't be in trouble if you break
a fiber or two when splicing, breaking-out or terminating fibers. And
consider future expansion needs. Most users install many more fibers
than needed, especially adding singlemode fiber to multimode fiber
cables for campus or premises backbone applications.
Additional information on choosing cables is included in the chapter on fiber optic network design.
More
information on cables
General Guidelines For Installing Fiber Optic Cable
Here are some general guidelines for installing fiber optic cables
that should be read by everyone before installing any cable.
More
information on installation. Videos on cable design, pulling and preparation on the FOA Channel on 
References
The FOA Reference Guide to Outside Plant Fiber Optics
The FOA Online Reference Guide to Fiber Optics 
You can buy the printed version of the The FOA Reference Guide to Outside Plant Fiber Optics from the FOA eStore or Amazon.
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