ITPEC FE Morning October 2020 Question 30
Topic: Networks
Approximately how many seconds is required to transmit a file of 106 bytes via a 64-kbit/s communication line? Here, the transmission efficiency of the line is 80%.
Explanation
About 156 seconds — divide the file size in bits by the line speed actually achieved at 80% efficiency.
- •File size: 106 bytes × 8 = 8,000,000 bits
- •Effective speed: 64,000 × 0.8 = 51,200 bit/s
- •Time: 8,000,000 ÷ 51,200 = 156.25, about 156 seconds
Answer (d)
Why not others:
- •(a) 19.6 — about 106 ÷ 51,200 ≈ 19.5: the byte count is used as if it were bits, skipping the multiplication by 8
- •(b) 100 — divides the line speed by the efficiency instead of multiplying, 64,000 ÷ 0.8 = 80,000 bit/s, which makes the line faster than its rating
- •(c) 125 — ignores the efficiency, 8,000,000 ÷ 64,000 = 125
Key rule: Convert bytes to bits first, and multiply the line speed by the efficiency: the usable rate is always below the nominal one, so the time goes up.
Networks · AI-written, may contain errors
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