問77
頻出度★★★
次の条件で, インターネットに接続されたサーバから5MバイトのファイルをPCにダウンロードするときに掛かる時間は何秒か。 〔条件〕
| 通信速度 | 100Mビット/秒 |
|---|---|
| 実効通信速度 | 通信速度の20% |
平成31年度 春期 ITパスポート試験 問77
テクノロジ系ネットワーク › ネットワーク応用
This question's explanation and your answers so far.
解説
問 77
Incorrectア
0.05 is a double mistake that forgets both the byte-to-bit conversion (×8) and the 20% effective-speed factor, simply computing 5 ÷ 100 = 0.05. The data must be converted to bits, and the effective speed must be used.
Incorrectイ
0.25 uses the effective speed 20 Mbit/s correctly but forgets the byte-to-bit conversion (×8), computing 5 ÷ 20 = 0.25. 5 Mbytes must be converted to 40 Mbits.
Incorrectウ
0.5 mistakes “20% of the communication speed” for “a 20% reduction = an effective 80 Mbit/s”, computing 40 Mbits ÷ 80 = 0.5. The effective speed is 100 × 0.2 = 20 Mbit/s, not 80 Mbit/s.
- エ
正解 Transfer time is found as data size [bits] ÷ effective communication speed [bits/s]. First convert the file size to bits: 5 Mbytes × 8 = 40 Mbits (1 byte = 8 bits). The effective communication speed is 20% of the 100 Mbit/s communication speed, so 100 × 0.2 = 20 Mbit/s. Therefore the required time = 40 Mbits ÷ 20 Mbit/s = 2 seconds, and the correct answer is エ (2).
ここがポイント
- Transfer time [s] = data size [bits] ÷ effective communication speed [bits/s]. File capacity is given in bytes, so always convert it to bits with ×8 before dividing.
- Effective communication speed = nominal communication speed × utilization rate. In this problem, 100 Mbit/s × 20% = 20 Mbit/s is used as the denominator.
用語