問82
頻出度★★★
100Mビット/秒の伝送速度のLANを使用して，1Gバイトのファイルを転送するのに必要な時間はおおよそ何秒か。ここで，1Gバイト＝10^9バイトとする。また，LANの伝送効率は20%とする。
平成21年度 秋期 ITパスポート試験 問82
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 82
Incorrectア
4 seconds is exactly one hundredth of the correct 400 seconds; it is the value you get when the order of magnitude is mistaken by a factor of 100 somewhere in the calculation. For example, if you multiply by the 20% efficiency as 20 instead of 0.2, the effective speed becomes 10^8 × 20 = 2×10^9 bit/s, and you get 8×10^9 ÷ 2×10^9 = 4 seconds. Converting the efficiency to a decimal first (20% = 0.2) and only then multiplying the speed by it prevents this mistake.
Incorrectイ
50 seconds is the value you get when you forget the conversion 1 byte = 8 bits. If you leave the file size in bytes as 10^9 and divide by the effective speed 2×10^7, you get 10^9 ÷ 2×10^7 = 50 seconds. Since the transmission speed is given in bit/s, the file size must also be converted into bits; forgetting the conversion makes the answer one eighth of the correct value.
- ウ
正解 The procedure has three steps: unify the units into bits, find the effective speed, then divide. First, convert the file size into bits. 1 Gbyte = 10^9 bytes and 1 byte = 8 bits, so 10^9 × 8 = 8×10^9 bits. Next, find the effective speed of the line. The nominal speed is 100 Mbit/s = 10^8 bit/s, but since the transmission efficiency is 20%, what can actually be carried is 10^8 × 0.2 = 2×10^7 bit/s. Finally, 8×10^9 ÷ 2×10^7 = 400 seconds. So the time is approximately 400 seconds, and ウ is correct.
Incorrectエ
5,000 seconds is more than an order of magnitude larger than the correct 400 seconds; it is the value you get when the effective speed is underestimated. For example, if you skip the bit conversion and divide the file size 10^9 bytes by the 20% efficiency to get 5×10^9, and also mistake the speed as 10^6 bit/s instead of 10^8, you get 5×10^9 ÷ 10^6 = 5,000 seconds. Dividing the file size by the efficiency gives the same result as multiplying the speed by it (10^9 ÷ 0.2 ÷ 10^6 = 10^9 ÷ (10^6 × 0.2)), so the causes of this value are the missing bit conversion and taking the speed as 10^6 instead of 10^8.
ここがポイント
- Transfer time = amount of data (bits) ÷ effective transmission speed (bit/s). When a transmission efficiency is given, apply it to the speed side first: effective speed = nominal speed × efficiency.
- bps is measured in bits, while file sizes are often given in bytes. Always unify the units with 1 byte = 8 bits before calculating.
用語