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Fibre

Guidelines for choosing and testing fibre optic cable.


Fibre optic cable has become more affordable. It's now used for many applications that require complete immunity to electrical interference. Fibre is ideal for high data-rate systems such as FDDI, multimedia, ATM, or any other network that requires the transfer of large, time-consuming data files.


Other advantages of fibre optic cable over copper include:

  • Greater distance - You can run fibre as far as several kilometres.
  • Low attenuation - The light signals meet little resistance, so data can travel farther.

  • Security - Taps in fibre optic cable are easy to detect. If tapped, the cable leaks light, causing the entire system to fail.
  • Greater bandwidth - Fibre can carry more data than copper.
  • Installation - Fibre optic cable is easier to install than copper cable.


Single-mode or multimode?
Single-mode fibre gives you a higher transmission rate and up to 50 times more distance than multimode, but it also costs more.

Multimode fibre gives you high bandwidth at high speeds over long distances. Lightwaves are dispersed into numerous paths, or modes, as it travels through the cable's core. Typical multimode fibre core diameters are 50, 62.5, and 100 micrometres.


Testing and certifying fibre optic cable.
If you're used to certifying Category 5 cable, you'll be pleasantly surprised at how easy it is to certify fibre optic cable.


Attenuation (or decibel loss) - The decrease of signal strength as it travels through the fibre optic cable is measured in dB/km.


Return Loss - The amount of light reflected from the far end of the cable back to the source. The lower the number, the better.


Graded Refractive Index - Measures how much light is sent down the fibre. This is commonly measured at wavelengths of 850 and 1300 nanometres.


Propagation Delay - This is the time it takes a signal to travel from one point to another over a transmission channel.


Time-Domain Reflectometry (TDR) - Enables you to isolate cable faults by transmitting high-frequency pulses onto a cable and examining their reflections.


There are many fibre optic testers on the market today. Common basic fibre optic testers function by shining a light down one end of the cable. At the other end there's a receiver calibrated to the strength of the light source. With this test, you can measure how much light is going to the
other end of the cable. Generally, these testers give you the results in dB lost, which you then compare to the loss budget. If the measured loss is less than the number calculated by your loss budget, your installation is good.


When to choose fibre optic.
While fibre optic cable is still more expensive than other types of cable, it eliminates electrical interference and is widely favoured for today's high-speed data communications.


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