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Station X needs to send a message consisting of 12 packets to station Y using a sliding window protocol (Window Size 4) and Go back n error control strategy. All packets are ready and immediately available for transmission. If every 7th packet that X transmits gets lost (but no acks from Y ever gets lost), then the number of packets that X will transmit for sending the message to Y are _____.
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Window size  = 4 and every 7th packet that X transmits gets lost 

so at 7th transmission lost i.e., in GBN entire window(4 packets 7 8 9 10  is discarded) and again sent.

the next 7th transmission is 10th packet so 10 11 12 discarded and again sent.

Total packets that X transmits is 19.

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1 2 3 4 5 6 7 8 9 10 7 8 9 10 11 12 10 11 12

packet 7 is lost

packet 10 is lost

19 packets are transmittted

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