LuxInside a fiber optic cable
Cables
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The outer layers: jacket, armor and rip cord
A fiber optic cable is mostly protection. The glass that carries the light is about as thin as a hair, and everything around it keeps it straight, dry and free of strain for decades in the ground or on a pole. This guide strips a typical outdoor loose-tube cable layer by layer, then shows how to find any fiber in it by color.
The cable in the figure is an armored loose-tube cable with six tubes of twelve fibers, 72 fibers in all. From the outside in:
Jacket
The jacket is the outer skin, usually black polyethylene. It keeps out water and sunlight and protects the cable as it is pulled into place. It is printed at regular intervals with the maker, the fiber count and a running length mark, which crews use to measure how much cable went in.
Armor
Under the jacket of an armored cable is a corrugated steel tape wrapped around the core. It resists rodent bites and crushing. Because steel conducts electricity, armor must be bonded and grounded where the cable enters a closure or a building. All-dielectric cables have no metal at all, so there is nothing to ground; they are common on aerial routes and in conduit.
Rip cord
A rip cord is a strong thread laid along the cable under the jacket. A technician cuts into the jacket, catches the cord and pulls it, and it slits the jacket cleanly without a blade getting near the tubes. Many cables have two, on opposite sides.
The core: water blocking, aramid, tubes and strength member
Water blocking
Water that gets into a cable can travel along it, and if it freezes it can squeeze the tubes and the fibers. Older cables fill the gaps with gel. Many newer outdoor cables use dry water-blocking tape and yarn instead, which swell into a gel when wet and seal the gap. A dry core is faster and cleaner to prepare for splicing.
Aramid yarn
Aramid is a strong synthetic yarn laid around the core in many cable designs. It carries the pulling tension during installation so the fibers do not.
Buffer tubes
The fibers live in buffer tubes, plastic tubes a few millimeters across, commonly holding 12 fibers each. The fibers lie loose inside and are slightly longer than the tube, so when the cable stretches under a pull or shrinks in the cold, the tube takes the strain and the glass does not. That is why the design is called loose tube. The tubes wind around the center in a back-and-forth lay, so a technician can open one tube in the middle of a span without cutting the cable.
Strength member and fibers
At the center is a rod, usually fiber-reinforced plastic (FRP) and sometimes steel. It keeps the cable from buckling in the cold, gives the tubes a core to wind around, and is what a closure clamps to.
Each fiber is glass 125 microns across, with a colored coating that brings it to 250 microns, or 200 microns in some newer cables. Single-mode fiber, the kind used for fiber to the home, has a core about 9 microns across. Only the core carries the light.
The 12-color code
Every buffer tube, and every fiber inside a tube, is colored in the same order, set by the TIA-598 standard:
1 blue
2 orange
3 green
4 brown
5 slate
6 white
7 red
8 black
9 yellow
10 violet
11 rose
12 aqua
The tube color tells you which group of twelve you are in. The fiber color tells you the position inside that group. Tube 1 is blue and holds fibers 1 to 12; tube 2 is orange and holds 13 to 24; and so on. The fiber colors start over in every tube, so every tube has a blue fiber. That is why you always name the tube first.
Cables with more than twelve tubes repeat the twelve colors on the second set, marked with a stripe or dash. The way you read them does not change.
Worked example, key terms and questions
Worked example: find fiber 30
A splice plan calls for fiber 30 of a 72-fiber cable with six tubes of twelve.
Find the tube: 30 divided by 12 is 2.5, which rounds up to 3. Tube 3 is green.
Find the fiber: subtract the 24 fibers in tubes 1 and 2. 30 minus 24 is 6, and fiber 6 is white.
Fiber 30 is the white fiber in the green tube. Going the other way, green tube and white fiber give (3 - 1) x 12 + 6 = 30. Point at a layer in the figure to name it.
Key terms
Loose-tube cable: A cable whose fibers lie free inside buffer tubes, so the tubes take the strain.
Buffer tube: A plastic tube inside the cable that holds a group of fibers, commonly 12.
Rip cord: A thread under the jacket that slits it open when pulled.
Water-blocking tape: A dry tape that swells into a gel when wet and keeps water from running along the cable.
Aramid yarn: A strong synthetic yarn that carries pulling tension during installation.
Strength member: The central rod, usually FRP, that stops the cable buckling and anchors it in a closure.
Armor: A corrugated steel tape under the jacket that resists rodents and crushing.
Single-mode fiber: Fiber with a core about 9 microns across, used for long links and fiber to the home.
Common questions
What is the difference between loose-tube and tight-buffered cable?
In loose-tube cable the 250-micron fibers lie free inside tubes, which suits outdoor temperature swings. In tight-buffered cable each fiber has its own 900-micron plastic coat and no tube, which makes it easier to handle and terminate indoors.
Do I need armored fiber cable?
Armor helps where rodents, rocks or crushing are a risk, such as direct-buried routes. Many conduit and aerial routes use all-dielectric cable instead, which is lighter and needs no grounding. Your network construction standard decides.
What comes after fiber 12 in the color code?
Fiber 13 is the first fiber of the second tube: the blue fiber in the orange tube. In a single tube or unit with more than twelve fibers, fibers 13 to 24 repeat the colors with a stripe or dash mark.
How Lux does this work
The Lux field app gives crews the splice plan joint by joint, hands free, and checks each photo against the design. See the field app.
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