$1.$ In FDM$,$ if there are $8$ input signals each requiring a bandwidth of $3$ MHz$,$ what is the minimum bandwidth required for the combined signal?

$2.$ Compute the $12-$bit internet checksum for the following data.

E$3$D $3$F$8$ AD$2$ F$75$

$3.$ Let there be $4$ bits reserved in the header for the frame sequence number. If the sliding window protocol is used for flow control,

a$)$ How many different frame sequence numbers are there?

b$)$ What is the maximum window size?

c$)$ Can you use a window of size $20?$