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.

Diagram showing the transmission of a microwave signal from Cardiff to a satellite and then down Brussels. The diagram is not to scale.

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.