Calculate the minimum transmission time for a 100 MB file over a 100 Mbps link…
2025
Calculate the minimum transmission time for a 100 MB file over a 100 Mbps link with 20 ms latency. Add a 1% packet loss and calculate expected retransmissions using TCP.
Attempted by 11 students.
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Transmission Time with TCP Effects (Computer Networks)
Given:
File size = 100 MB ≈ 800 Mb (decimal assumption)
Bandwidth = 100 Mbps
RTT = 20 ms = 0.02 sec
Packet loss = 1%
1. Ideal Transmission Time
Transmission time = File Size / Bandwidth
= 800 / 100 = 8 seconds
Including propagation delay (≈ 1 RTT):
Total ideal time = 8 + 0.02 = 8.02 seconds
2. Effect of Packet Loss (Retransmissions)
Packet loss = 1%
Extra data to be transmitted = 1% of 800 Mb = 8 Mb
Time required for retransmission:
= 8 / 100 = 0.08 seconds
3. TCP Congestion Control Impact (Important Point)
TCP follows AIMD (Additive Increase, Multiplicative Decrease)
On packet loss:
Congestion window (cwnd) reduces (half or 1 MSS)
Sending rate decreases temporarily
Total packets ≈ 66,000 (MTU ≈ 1500 Bytes)
Lost packets ≈ 667
Each loss may introduce ≈ 1 RTT delay due to fast retransmit
This causes additional delay and reduces effective throughput
4. Final Conclusion
Minimum transmission time ≈ 8.1 seconds
Actual TCP transmission time greater than 8.1 seconds
Conclusion: Packet loss not only increases retransmission time but also reduces throughput due to congestion control, significantly affecting overall performance.