Thin strands of glass or plastic that efficiently carry light signals over long distances without amplification and is not affected by RF interference. Replacing traditional copper everywhere because they carry so much more information. One drawback to fiber optic cabling is that it takes a long time to repair if it is cut.

Part of the reason that fiber takes so long to repair is the fact that the glass strand often is insulated with Teflon(tm). Whilst providing very good electrical, chemical and mechanical insulation, this also lets the fiber slide inside the insulation. So when Mr. Backhoe goes to work on the cable, the fiber might snap more than a mile away from the spot where Mr. Backhoe is.
This then requires the entire length of cable to be excavated and/or replaced, a time-consuming work done by ... that's right: Mr. Backhoe!

Mr. Backhoe is familiar with the concept of job security.
A type of cable that transmits data as light through strands of glass instead of electricity through copper. Fiber-optic cable is a wonderful thing; it can transmit almost insane amounts of data per second, and it is completely impervious to surges, magnetic fields, lightning, and all the other EM nasties that can affect copper cable. Unfortunately it's not impervious to hicks bulldozing big holes in the ground for their swimming pools, but I doubt anything ever will be.

A few layer-1 protocols that are used over fiber include OC-3, OC-12, OC-48, 100base-FL, 100base-FX, and many, many more.
Additional information concerning fiber-optic cabling:

Copper-based cabling transmits electrical signals, fiber-optic cables transmit light pulses. Fiber-optics are much more efficient. There is little attenuation (assuming the cable is installed correctly), it is immune to electromagnetic interference (EMI), and it has no signal leakage, which means nobody can eavesdrop.

Optical fibers used in the cable include:

  • Single-mode, 8.3 micron core fiber, 125 micron cladding.

  • Multi-mode, 62.5 micron core fiber, 125 micron cladding.

  • Multi-mode, 50 micron core fiber, 125 micron cladding.

  • Multi-mode, 100micron core fiber, 140 micron cladding.

    
                          Sheath
                             |    Cladding
                             |       |     Optic Fiber
    --------------------     |       |          |
                        |--------    |          |
                        |        |-----         |
    Outer Jacket        |        |     |-------------
                        |        |-----
                        |--------
    --------------------
    
    
    The center fiber is made of either glass or optically pure plastic, about the thickness of a human hair. This fiber is covered by an additional layer of glass that reflects the light back into the center fiber. A plastic sheath covers the cladding for protection. An outer jacket covers the assembly, and this jacket is sometimes filled with a gel or thin wires to strengthen the cable assembly.

    Most fiber-optic cables include several strands of optic fiber (with the associated cladding and sheath).

    Single-Mode Fiber-Optic Cable

    Single-mode cables are normally used with lasers, and are therefore more expensive to set up due to the initial cost of equipment. The lasers and the cable allow for a larger bandwidth, and can be set up with a longer cable run than multi-mode because the fiber only allows for a single light path.

    Multi-Mode Fiber-Optic Cable

    Multi-mode cables are cheaper because they can use Light Emitting Diodes (LEDs). The multi-mode fiber-optic cabling allows for multiple paths within the fiber, but is designed to have all of these paths converge and appear as one pulse at the end of the cable.

    
    ______________________________________________
    Cladding
    ----------------------------------------------
             /\    /\    /\    /\    /\    /\    /\
    Fiber   /  \  /  \  /  \  /  \  /  \  /  \  /  \
           /    \/    \/    \/    \/    \/    \/    Signal
    ----------------------------------------------
    ______________________________________________
    
    
    The light is reflected back into the core of the fiber by the cladding. The angle of the light hitting the wall between these two medium must be below a particular angle, called the critical angle. As long as there are no physical breaks, the lightwaves bounce down the fiber until they reach the end. (Note: There is a lot more to how the light travels down the fiber, but it requires a lot of physics and math to explain. An example of the technical aspects includes the theory that light travels as both packets and waves.)

    To send a signal down a single-mode cable, an Injection Laser Diode (ILD) is used. For multi-mode, an LED is used. At the end of the fiber, a photodiode converts the received light pulses back into an electronic signal for the computer. The network cards/devices used in fiber-optic networks have the photodiodes built in.

    Installation requires a skilled technician. Fiber-optic cable splicing is very critical, and requires training. The cables have a limited bending radius, and the cable runs must be carefully planned out in advance. Fiber-optic lines are only installed between two devices, just like the Ethernet unshielded twisted-pair cables.

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