Q$3)$ A SM fiber is used to connect between two cities as shown below. A LD is used as Tx and has output power of $0$dBm$.$ A PN PD is used as $x and has is $0\mathrm{.}8$dB$.$ Find the safety margin in dB to obtain a data rate of $2\mathrm{.}5$Gb$//$s$.$If we need to send $10$Gb$//$s by using WDM$,$ how many wavelengths are needed?$(10$ Marks$)$