State three advantages that an optic fibre has over a metal wire when carrying a signal.
As shown in Fig. 5.1, a transmitter and a receiver are joined by an optic fibre of length $57\ \text{km}$.\n\nFor the fibre, the attenuation per unit length is $0.50\ \text{dB km}^{-1}$. The transmitter sends an input signal with power $15 \times 10^{-3}\ \text{W}$ into the fibre. At the receiver, the noise power is $9.0 \times 10^{-7}\ \text{W}$.
Show that the power $P$ of the signal arriving at the receiver from the optic fibre is $2.1 \times 10^{-5}\ \text{W}$.
A minimum signal-to-noise ratio of $24\ \text{dB}$ is required at the receiver so that it can distinguish the signal from the noise. Determine whether the receiver can distinguish the signal from the noise.