Describe the role played by glassy fibres in optical communications and Iist some of the advantages associated with their use. Calculate the maximum permissible totals loss in dB/km in order not to exceed an error rate of 10- 9 (200 photons per bit); assume an operating wavelength λ = 900 nm, transmission rate of 100 Mbit/s, fibre length of 10 km, Iaser input power of 1m W and operating margins of 8 dB.