A telephone link between two towns is to be provided using an optic fibre. The length of the optic fibre between the two towns is 
.
State two changes that occur in a signal as it is transmitted along an optic fibre.
The optic fibre has an attenuation per unit length of 
. The minimum permissible signal-to-noise power ratio in the fibre is 
. The average noise power in the optic fibre is 
.
Suggest one reason why power ratios are expressed in 
.
The signal input power to the optic fibre is designed to be 
. Determine whether repeater amplifiers are necessary in the optic fibre between the two towns.
. So, repeater amplifiers are not necessary in the optic fibre between the two towns.
