Introduction to Data Communications
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2. Comparison of Optical Fibres

The Wavelength of the light sources is measured in nanometers or 1 billionth of a meter. We don't use frequency to talk about speed any more, we use wavelengths instead.

Indoor cable specifications:

• LED (Light Emitting Diode) Light Source
• 3.5 dB/Km Attenuation (loses 3.5 dB of signal per kilometre)
• 850 nM - wavelength of light source
• Typically 62.5/125 (core dia/cladding dia)
• Multimode - can run many light sources.

Outdoor Cable specifications:

• Laser Light Source
• 1 dB/Km Attenuation (loses 1 dB of signal per kilometre)
• 1170 nM - wavelength of light source
• Monomode (Single Mode)

3. Advantages of Optical Fibre:

• Noise immunity: RFI and EMI immune (RFI - Radio Frequency Interference, EMI -ElectroMagnetic Interference)
• Security: cannot tap into cable.
• Large Capacity due to BW (bandwidth)
• No corrosion
• Longer distances than copper wire
• Smaller and lighter than copper wire
• Faster transmission rate

4. Disadvantages of Optical Fibre:

• Physical vibration will show up as signal noise!
• Limited physical arc of cable. Bend it too much and it will break!
• Difficult to splice

The cost of optical fibre is a trade-off between capacity and cost. At higher transmission capacity, it is cheaper than copper. At lower transmission capacity, it is more expensive.

Introduction to Data Communications
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