On a wireless link, the probability of packet error is 0.2. A stop-and-wait…
2006
On a wireless link, the probability of packet error is 0.2. A stop-and-wait protocol is used to transfer data across the link. The channel condition is assumed to be independent from transmission to transmission. What is the average number of transmission attempts required to transfer 100 packets?
- A.
100
- B.
125
- C.
150
- D.
200
Attempted by 153 students.
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Correct answer: B
Key idea: each packet is retransmitted until it is received correctly; with independent transmissions and packet error probability 0.2, the number of attempts per packet follows a geometric distribution.
Success probability for a single transmission = 1 − error probability = 1 − 0.2 = 0.8.
Expected number of transmission attempts for one packet = 1 / success probability = 1 / 0.8 = 1.25.
For 100 packets, expected total attempts = 100 × 1.25 = 125.
Answer: The average number of transmission attempts required to transfer 100 packets is 125.
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