Fibre optic "cable" refers to the complete assembly of fibres, strength members and jacket. Fibre optic cables come in lots of different types, depending on the number of fibres and how and where it will be installed. Choose cable carefully as the choice will affect how easy it is to install, splice or terminate and, most important, what it will cost!
Choosing a cable - what hazards will it face?
Cable's job is to protect the fibres from the hazards encountered in an installation. Will the cables be exposed to chemicals or have to withstand a wide temperature range? What about being gnawed on by a rodent or some animal? Inside buildings, cables don't have to be so strong to protect the fibres, 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.
Since the plan will call for a certain number of fibres, consider adding spare fibres to the cable - fibres are cheap! That way, you won't be in trouble if you break a fibre or two when splicing, breaking-out or terminating fibres. And request the end user consider their future expansion needs. Most users install lots more fibres than needed, especially adding singlemode fibre to multimode fibre cables for campus or backbone applications.
Simplex and zip cord
Simplex cable is one fibre, tight-buffered (coated with a 900 micron buffer over the primary buffer coating) with Kevlar (aramid yarn) 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. It's used mostly for patch cord and backplane applications, but zipcord can also be used for desktop connections.
This contains several tight-buffered fibres bundled under the same jacket with Kevlar strength members and sometimes fibreglass rod reinforcement to stiffen the cable and prevent kinking. The cable is small in size, and used for short, dry conduit runs, riser and plenum applications. The fibres are double buffered and can be directly terminated, but because their fibres are not individually reinforced, these cables need to be broken out with a "breakout box" or terminated inside a patch panel or junction box.
This cable is made of several simplex cables bundled together. 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. Because each fibre 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 Fibre count isn't too large and distances too long, because is requires so much less labour to terminate.
Loose tube cable
This cable is composed of several fibres together inside a small polymeric buffer tube or tubes, which are in turn wound around a central strength member and jacketed, providing a small, high fibre 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 fibres from water. It can be used in conduits, strung overhead or buried directly into the ground. Since the fibres have only a thin buffer coating, they must be carefully handled and protected to prevent damage.
This cable offers the highest packing density, since all the fibres are laid out in rows, typically of 12 fibres, and laid on top of each other. This way 144 fibres only has a cross section of about 1/4 inch or 6 mm! Some cable designs use a "slotted core" with up to 6 of these 144 fibre ribbon assemblies for 864 fibres in one cable! Since it's outside plant cable, it's gel-filled for water blocking.
Cable installed by direct burial in areas where rodents are a problem usually has metal armouring between two jackets to prevent rodent penetration. This means the cable is conductive, so it must be grounded properly.
Aerial cable is for outside installation on poles. It can be lashed to a messenger or another cable (common in CATV) or have metal or aramid strength members to make it self-supporting.
Cable design criteria
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 km or more of conduit. Even with lots of cable lubricant, pulling tension can be high. Most cables get their strength from an aramid fibre (Kevlar is the duPont trade name), a unique polymer fibre 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.
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.
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 us 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. These ratings are:
|OFN||Optical Fibre Non-conductive|
|OFC||Optical Fibre Conductive|
|OFNG or OFCG||General purpose|
|OFNR or OFCR||Riser rated cable for vertical runs|
|OFNP or OFCP||Plenum rated cables for use in air-handling plenums|
|OFN-LS||Low smoke density|
Cables without markings should never be installed as they will not pass 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. You can use an Indoor/Outdoor rated product with a Low Smoke Zero Halogen jacket.
Choosing a cable
With so much choice in cables, it is hard to find the right one. The table below summarises the choices, applications and advantages of each.
|Tight Buffer||Premises||Makes rugged patchcords|
|Distribution||Premises||Small size for lots of fibres, inexpensive|
|Breakout||Premises||Rugged, easy to terminate, no hardware needed|
|Loose Tube||Outside Plant||Rugged, gel or dry water-blocking|
|Armoured||Outside Plant||Prevents rodent damage|
|Ribbon||Outside Plant||Highest fibre count for small size|
Pulling fibre optic cable
Installation methods for both wire cables and optical fibre cables are similar. Fibre cable can be pulled with much greater force than copper wire if you pull it correctly. Just remember these rules:
Do not pull on the fibres, pull on the strength members only! The cable manufacturer gives you the perfect solution to pulling the cables, they install special strength members, usually duPont Kevlar aramid yarn or a fibreglass rod to pull on. Use it! Any other method may put stress on the fibres and harm them. Most cables cannot be pulled by the jacket. Do not pull on the jacket unless it is specifically approved by the cable manufacturers and you use an approved cable grip.
Do not exceed the maximum pulling load rating. On long runs, use proper lubricants and make sure they are compatible with the cable jacket. On really long runs, pull from the middle out to both ends. If possible, use an automated puller with tension control or at least a breakaway pulling eye.
Do not exceed the cable bend radius. Fibre is stronger than steel when you pull it straight, but it breaks easily when bent too tightly. These will harm the fibres, maybe immediately, maybe not for a few years, but you will harm them and the cable must be removed and thrown away!
Do not twist the cable. Putting a twist in the cable can stress the fibres too. Always roll the cable off the spool instead of spinning it off the spool end. This will put a twist in the cable for every turn on the spool! If you are laying cable out for a long pull, use a "figure 8" on the ground to prevent twisting (the figure 8 puts a half twist in on one side of the 8 and takes it out on the other, preventing twists.) And always use a swivel pulling eye because pulling tension will cause twisting forces on the cable.
Check the length. Make sure the cable is long enough for the run. It's not easy or cheap to splice fibre and it needs special protection. Try to make it in one pull, possible up to about 2-3 miles.
Conduit and Innerduct: Outside plant cables are either installed in conduit or innerduct or direct buried, depending on the cable type. Building cables can be installed directly, but you might consider putting them inside plenum-rated innerduct. This innerduct is bright orange and will provide a good way to identify fibre optic cable and protect it from damage, generally a result of someone cutting it by mistake! The innerduct can speed installation and maybe even cut costs. It can be installed quickly by unskilled labour, then the fibre cable can be pulled through in seconds. You can even get the innerduct with pulling tape already installed.
Cable plant hardware
Various enclosures, cabinets, racks and panels are used to protect and organise splice and termination points. The network designer should know the type of network, support systems and the routes to be taken. Then the connection/splice locations can be determined and the hardware planned.
There are lots of rules to follow, of course (the EIA/TIA 569 has something to say about all this).
Here are some examples of fibre optic hardware:
- Breakout kits - allow you to separate and protect individual fibres in a loose tube cable so it can be terminated
- Splice enclosures - for long cable runs outside, the point where cables are spliced, sealed up and buried in the ground, put in a vault of some kind or hung off a pole
- Splice panels - connect individual fibres from cables to pigtails
- Patch panels - provides a centralised location for patching fibres, testing, monitoring and restoring cables
- Racks and cabinets - enclosures for patch panels and splice panels. Usually these also include cable management - without this the cables start looking like spaghetti!
There is a huge selection of hardware and manufacturers to choose from. Be sure to choose panels that have the connections behind locked doors, since the biggest problem we see is connectors broken by people messing around in communications closets! Fibre doesn't need maintenance or inspection. Lock 'em up and only unlock it when you have to move something!