(a)[2]
State two changes that happen to a signal as it is sent through an optic fibre.
(b(i))[1]
For the optic fibre, the attenuation per unit length is $1.6\,\text{dB km}^{-1}$. The least allowable signal-to-noise power ratio in the fibre is $25\,\text{dB}$. The mean noise power in the optic fibre is $6.1 \times 10^{-19}\,\text{W}$. Suggest one reason why power ratios are expressed in dB.
(b(ii))[5]
The input signal power to the optic fibre is set at $6.5\,\text{mW}$. Determine whether repeater amplifiers are needed in the optic fibre between the two towns.