Suppose that a sender and a receiver are using ARQ to perform reliable data delivery.

$1\mathrm{.}5$ points

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$($a$)$ In a Go$-$Back$-$NARQ protocol, the window size is $6.$ Frames with sequence numbers $1,2,3,4$ and $5$ have been sent. The sender just received an AcK for frame $1$ and $2.$ Frames $6,7,8,9$ and $10$ are waiting to be sent.

$($b$)$ Which frame$($s$)$ can the sender send before it must wait for the next ACK from the receiver? Explain.

$($c$)$ Some time later, the sender transmitted frames $11,12,13,14,15,$ and $17$; however, frame $13$ got lost. If Go$-$Back$-$N is used, what frame$($s$)$ would the sender have to retransmit? Explain.

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