Fibre Cabling Pros and Cons

Unlike copper cabling which carries an electrical signal, fibre uses light pulses to transmit data from point to point. The losses, or attenuation, of the signal is much less than that of electrical signals and therefore the distance covered before amplification is required can be much greater.

Fibre makes use of a phenomenon known as Total Internal Reflection. The fibre acts as a waveguide where any light that hits the side of the cable reflects back in and so travels down the fibre, even round corners, and comes out at the end. This effect is commonly seen in novelty applications such fibre optic lighting, and in medical applications such as endoscopes.

A basic fibre optic system consists of a transmitter to generate a light signal, an optical cable to contain the light, and a receiving device. The fibre itself is a completely passive component in the system.

Two main technologies are used to produce the light. Light-emitting diodes (LED), and  Injection-Laser diodes (ILD). LED light sources are less precise in their frequency than ILD sources, but cost a lot less. The choice is dependent on the bandwidth required, the transmission distance and the type of fibre being used.

Data is encoded as a series of light pulses and injected into the fibre by the transmitter. The receiver picks up the pulses and converts them back to the original data stream.

In theory all the signal stays inside the fibre. Of course in the real world some signal does leak out of the cable and losses are caused by internal reflections and dispersion in the cable. However these losses are very low and so fibre can be used over extremely long distances. For very long haul applications repeaters can be used to regenerate the signal.

Advantages of Fibre

Fibre has many advantages over copper.

• The carrying capacity of fibre is much greater than copper (though advancing technology has allowed copper speeds to be vastly increased in recent years).
• Due to the low signal losses very long distance transmissions are possible.
• Light is not electrical so it is not affected by electrical interference, a major benefit when running cables near lift motors or lighting ballasts, or anywhere subject to electrical interference or 'noise'.
• On long fibre runs using a technique known as Wavelength Division Multiplexing (WDM) it is possible to send multiple signals at slightly differing frequencies down a single fibre at the same time, increasing the carrying capacity even further.
• Fibre is difficult to tap into making it a very secure form of data transmission, though it is possible and you would still be advised to encrypt any sensitive data.
• Fibre is thin and light and flexible so you can fit more into a given size of cable duct.

Disadvantages of Fibre

• Cost. Fibre is generally more expensive than copper. Network interface cards, hubs, fibre switches, routers and patch cables all cost a lot more than their copper equivalents. Oddly the cable itself isn't much more expensive, just all the peripherals.
• More difficult installation. It may not be a disadvantage for you as you can charge more for your time, but it is indisputably harder to make good connections in fibre. For copper connections an electrical connection is sufficient, for fibre you need a good mechanical connection and a good optical contact. Not only must the ends of the join be a good close fit, but the alignment must be very accurate too.
• Specialist Tools. You will need specialist cleaving (cutting) polishing and joining equipment, in fact you may need more than one set of tools depending on the types of connectors that you are using.
• Two fibres per link. You will need two fibres for every connection, one for sending, one for receiving. It's very important that you label things correctly as connecting them the wrong way round will cause the connection to fail. This problem is further compounded by the range of connector types available, some are easier to mix up than others. A label printer is a wise investment.
• Individual fibre diameters are very small and potentially dangerous. Tiny shards of fibre left over from splicing can easily get into your skin, or worse into your blood stream, where they can be extremely hazardous. It is particularly important to protect your eyes from small fragments of fibre.
• Laser light is dangerous. Never look down a fibre, you won't see anything and it may damage your eyesight. There is never a good reason to look down a fibre. Remember that the frequencies used are infrared and therefore not visible to the eye. Take particular care when using a microscope to make sure that there is no power on the fibre. If you are using a Visible Fault Locator (VFL) red light will spill out of any breaks or faults in the fibre, but even here you don't need to look down the fibre to see the light.