問95
頻出度★★★
伝送速度が20 Mbps（ビット／秒），伝送効率が80％である通信回線において，1 Gバイトのデータを伝送するのに掛かる時間は何秒か。ここで，1 Gバイト＝10³ Mバイトとする。
令和2年度 ITパスポート試験 問95
テクノロジ系ネットワーク › ネットワーク方式
This question's explanation and your answers so far.
解説
問 95
Incorrectア
“0.625” differs from the correct answer 500 by many orders of magnitude; it is a value produced by mishandling the magnitude of the data amount (10⁹ bytes) or the unit conversion, and is clearly far too small.
Incorrectイ
“50” comes from forgetting both the byte-to-bit conversion (×8) and the 80% transmission efficiency, simply computing 10⁹ ÷ (20×10⁶) = 50. Both corrections are missing.
Incorrectウ
“62.5” comes from forgetting the byte-to-bit conversion (×8) and dividing the byte amount directly by the bit rate: 10⁹ ÷ (16×10⁶) = 62.5. The ×8 factor is actually needed, so the correct result is 500 seconds.
- エ
正解 The effective transmission speed is the nominal 20 Mbps multiplied by the 80% transmission efficiency: 20×10⁶×0.8 = 16×10⁶ bits/second. The amount of data to transfer is 1 GB = 10³ MB = 10⁹ bytes, and since 1 byte = 8 bits, 10⁹×8 = 8×10⁹ bits. The required time = data amount ÷ effective speed = 8×10⁹ ÷ (16×10⁶) = 500 seconds. Therefore the correct answer is エ, “500”.
ここがポイント
- Effective transmission speed = nominal speed × transmission efficiency; required time = data amount ÷ effective speed.
- A data amount given in bytes must be converted to bits by multiplying by 8 before dividing by a rate in bps (bits per second), so that the units are consistent.
用語