Long distance communication
Optical fibres may be used for long distance communication. You may be asked to compare the use of optical fibres with satellites for long distance communication.
It is possible to calculate the time delay for both methods of communication.
Question
It is possible to communicate between Cardiff and Brussels using satellite communication or optical fibres. Use the data below in order to calculate a time delay for both methods.
Speed of infrared in glass = 2 × 108 m/s
Speed of microwaves in air = 3 × 108 m/s
Height of satellite above earth = 36,000 km
Distance from Cardiff to Brussels = 530 km
Using optical fibres
Optical fibres communicate point to point, so the distance travelled by the signal is 530 km.
\(\text{time}=\frac{\text{distance}}{\text{speed}}\)
\(\text{time}=\frac{{530\times10}^{3}}{{2\times10}^{8}}={2.65\times10}^{-3}~{\text{s}}\)
Using satellite communication
We need to convert the 36,000 km into metres and multiply this distance by 2 (because the signal must reach the satellite and return to Earth). These steps are sometimes forgotten in an exam.
\(\text{time}=\frac{\text{distance}}{\text{speed}}\)
\(\text{time}=\frac{{36,000\times10}^{3}\times2}{{3\times10}^{8}}={0.24~{\text{s}}}\)
Question
Compare the use of optical fibres and satellites for long distance communication.
Optical fibres are point to point communication. Infrared signals sent by optical fibres do not suffer interference (for example, from a storm), and are more difficult to tap into. The distance travelled via optical fibres is shorter than that travelled by a microwave signal via satellite, but the speed of the infrared signal through the fibre is lower than the speed of the microwave through the air.