In the data link layer of the communication between machines A and B$,$ the window width is $7$ frames.

Go$-$back$-$N protocol is used. The states of the windows of machines A and B before sending Frame $2($a$0$ and b$0)$ and the frames sent and positive$/$negative received $($ACK$/$NAK$).$

The receipts are shown below.

$($a$)$ Drawing which frames the windows in a$1,$ b$1,$ a$2,$ b$2,$ a$3,$ b$3,$ a$4,$ b$4$ should contain

show.

$($b$)$ Between the window width $($N$)$ and the field length $($k$)$ in bits of the frame sequence number

Write the relationship.