$1)$ In Go$-$Back$-$N ARQ protocol which uses a window size of $1000,$

the transmitter receives a negative acknowledgment $($REJ$900)$ after it

sent frame $950.$ In this case the transmitter must retransmit frames:

$($a$)900$ to $950$

$($b$)1$ to $900$

$($C$)900$ to $1000$

$($d$)900$ only

$1\mathrm{.}1$

$($a$)$

$($b$)$

$(($c$)$

$($d$)$

Frame $900$ is lost and the receiver gets frames $901$ to $1000.$ In this

case the receiver should:

$($a$)$ wait for frame $900($b$)$ send RR$1($c$)$ send NARR$900($d$)$ time out

and send a special acknowledgment

$1\mathrm{.}2$

$($a$)$

$($b$)$

$($c$)$

$($d$)$