Fibre optic cable has gained in popularity in recent years and is becoming commonplace in networks. It’s easy to assume that all fibre cables are the same, but this is not the case. There are, in fact, several different types and it’s important to understand these and what they mean for installations.
Fibre cable is composed of a filament, usually made of glass. This is thinner than a human hair and has a reflective outer coating that allows the light signal to be bounced from side to side so it’s able to go around corners. Outside that is a sheath that protects the cable from damage and moisture. Most cables are made up of a bundle of fibres rather than just one, allowing them to carry more traffic.
There are three types of fibre optic cable in common use and these have key differences that affect how and where they are used. They are:
There are differences between each and each type suits a different need.
Single mode optical fibre is the thinnest, with a diameter of between 8.3 and 10 microns. It’s used in applications where data is sent at multi-frequency – known as WDM Wave-Division-Multiplexing. This means only one cable is needed.
Using single mode fibre means you get a higher transmission rate that can be used over longer distances. The combination of a small fibre and a single light wave helps to eliminate distortion.
Multi mode fibre has a larger diameter, between 50 and 100 microns. Multi mode fibre allows you to have high bandwidth at high speeds, typically between 10 and 100Mbps, over medium distances of up to two kilometres.
In a multi mode fibre, light waves are dispersed into several paths, or modes, as they travel through the cable’s core. The downside of this is that in cable runs over 914 metres, the multiple paths of light can lead to some distortion of the signal by the time it reaches the receiving end. This can compromise the integrity of data transmission. For gigabit networks, therefore, it’s now more common to use single mode fibre to ensure reliability.
Plastic optical fibre uses polymer rather than glass to carry the light signal down the cable. Plastic fibre has a much larger diameter of around one millimetre. It is a lot cheaper and more robust than silica-based fibre optic but is only suitable for use over very short distances.
Each type of fibre optic cable has different advantages over the other, but there are some advantages to fibre which are common to all types. These include:
There are some additional considerations to be taken into account too. Fibre can suffer a loss of signal quality over longer distances, which may mean that boosters need to be installed. It also needs care in installation as fibre cables can’t turn sharp corners in the same way as their copper counterparts.
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Michael Turner, ICT manager, Downend School