Fibre optic cabling is generally chosen over copper for its greater speed and reliability. But can speed loss still occur on fibre networks? The answer is yes it can, although the problem is far less severe than with other cabling methods.
Degradation of speed on a fibre cable is generally caused by either distance or by a problem with the transmission of the light source. Fluctuation in the LED or laser light can lead to scattering, which affects the integrity of data transmission. This problem gets worse over longer distances.
Similarly, the longer the cable, the more chance there is of the light signal degrading over time. Both of these issues can be overcome by adding signal repeaters at intervals to boost the signal through the fibre optic cable.
Other variables come into play too. For example, the number of splices or joins in the cable and the types of connectors used can all play a part in the performance of a fibre optic link. The networking equipment itself and the transmitter power are important too. Optical power is measured in dBm (decibel milliwatts) while signal loss is measured in decibels (dB) – the same measure that’s used for sound. Manufacturers of fibre hubs and other kit will tell you the transmission power of their devices.
Armed with this information you can work out the likely level of loss. Just to recap, losses can be a function of:
There are ways in which you can determine the amount of loss likely in a fibre optic circuit. An experienced installer will be able to carry out a test known as an Optical Loss Self Test (OLTS). This involves using a light source at one end of the circuit and an optical meter to measure the signal at the other.
The loss rate for fibre optic cable is a function of its length and the manufacturer’s spec sheets should tell you this. In addition, each connector also introduces an element of loss. Typically connectors on fibre optic cables, whether single mode or multimode, should have loss levels somewhere between 0.3dB and 0.75dB. You can, therefore, work out a ‘loss budget’ based upon the number of connectors and the length of the cable.
On a short point-to-point cable this might be around 1.5dB. On longer cables, with multiple connection nodes, it could be considerably more. Your installer should be able to advise you as to an acceptable level. Losses higher than this and you need to look at taking steps to boost the signal via installing repeaters.
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Steve Morris, Network Manager, Oldfield School