Choosing the right transmission medium for any application

February 2003 IT infrastructure

It used to be when you were designing a security or surveillance system choosing the right cabling was the easy part. You just planned on installing enough coaxial cable to cover the distance. If control was needed, just plan on installing the same amount of twisted pair to cover the distance. Factor in amplifiers for distances over 304,8 m, lightning and ground loop protection and all you had to do was settle on the camera, control and monitor manufacturer.

Easy, you just lived with the downside - the possibility of poor quality video, interruptions and electrical interference. It did not matter how near or far the transmission distance was, just keep adding cable and amplifiers and you could get the signal where it needed to be.

Then, about 15 years ago, a slender glass fibre clouded the entire picture. Fibre-optic transmission was gaining acceptance as a transmission medium. Everyone who tried fibre-optic transmission grew to be in favour of the inherent benefits brought by fibre-optic transmission.

By now, it has come to be generally accepted that fibre-optic transmission is a practical method of transmitting video, audio and data signals for the following reasons:

* Radio frequency interference (RFI) and electromagnetic interference (EMI) are eliminated and protection from lightning and ground loops is gained.

* No signal radiation, optical fibre eliminates cross talk or electrical shorts.

* Fibre-optic transmission offers very high bandwidth that is ideal for moving large volumes of audio, video and data signals simultaneously.

* Fibre-optic cables are difficult to break into, making very secure communications possible.

* Optical fibre is lightweight and small.

* Installation flexibility: optical fibre offers flexible installation options relevant to electrical codes that need to be abided by.

* System upgrades and changes are easily accomplished: upgrading the channel-carrying capacity and type of signal is as easy as changing the fibre-optic transmitter and receiver on each end. No additional cables to be pulled.

But, as it was initially gaining acceptance, there was a downside. Ten years ago, optical fibre itself was more expensive. Plus, fibre termination was labour and cost intensive. And on top of that, the electronic equipment that converted the video or data signal so it could be passed over the optical cable was an additional cost to be considered.

Although, even when the extra costs were considered, the economics of larger security projects generally started swinging more and more of these projects toward fibre-optic transmission. When extended distances had to be covered, the inherent benefit of long distance signal transmission capabilities that fibre offered was clear to see.

But finding this break-even distance of when to use fibre versus coaxial was hard to predict. Now on to today. With prices of fibre and the transmission equipment becoming more competitive and fibre termination easier than ever, the break-even distances are becoming shorter, and, it used to be that anything over a kilometre required fibre. But now those distances have been reduced substantially.

Flexibility and growth

And two more important elements have been added to the equation that can really direct the value toward fibre-optic transmission. Those elements are system life span and flexibility for system growth.

The life span of coaxial cable is generally considered 7-10 years before the elements it is exposed to, begin to have an effect on the video performance. Although there are certain types of cables available that have manufactured-in resistances to such things as moisture, ultraviolet light, and solvents, they generally add to the cost of the cable. Fibre-optic cables can be subjected to similar environmental conditions but being non-metallic are not subject to the corrosion that can happen with coaxial cable and have considerably longer life cycles.

Individual optical fibres utilise a buffered coating that protects them within a larger protective jacket. In terms of size relationship, a fibre-optic cable the same diameter of a common coaxial cable can contain six optical fibres adding to the flexibility and another of the cost advantages that can be gained from using optical fibre.

As reviewed earlier, there is no medium available at this time that has the bandwidth capacity fibre-optic transmission offers. While a single coaxial cable has the capacity to transmit only a single channel of base-band video, a single strand of fibre in combination with the different types of fibre-optic transmission equipment available can carry as many as 32 individual video signals a distance of 64,36 km. The cost-efficiencies gained by this flexibility are virtually incalculable.

Fibre-optic transmission product companies such as International Fiber Systems, (USA) have been at the forefront of making fibre-optic transmission the medium of choice for virtually any application in the security industry. Utilising that same single fibre, depending on the need, video, audio and data signals can be transmitted on that single optical fibre.

In the short term and for a very simple installation, for example a four-camera CCTV system, the initial transmission medium, equipment and installation costs are very similar between coaxial cable and fibre. In many cases, even distances as short as 500 m, selecting fibre as the medium can be a very cost-efficient choice.

In addition, if the system needs to be expanded to an eight-camera system the process is very simple to perform. Simply replace the existing transmission equipment with an eight-channel fibre-optic multiplexer, and connect the camera feeds.

How do you decide which, coax or fibre, is best for your next application? As outlined, for shorter distances and very small systems coax may still be the way to go. But as soon as the distance increases or the installation moves outdoors, the value fibre-optic transmission offers becomes very evident. For most new installations, choosing fibre for all the inherent benefits and future flexibility might be the right choice.

For more information contact Brian Hill, Tactron Technologies, 011 791 7550.





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