問62
頻出度★★★
ADSL 回線に関する記述として, 適切なものはどれか。
平成22年度 春期 ITパスポート試験 問62
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 62
Incorrectア
ADSL is a technology that sends high-frequency signals over the metal (copper twisted-pair) telephone line running from the subscriber home to the central office. If there is an optical fiber section along the way, the electrical signal cannot pass through as it is, so the ADSL signal does not get there. In practice there is a restriction that ADSL cannot be provided on lines that have been converted to optical fiber partway from the central office, so saying that optical fiber may be used in part of the section is wrong.
Incorrectイ
The speed of ADSL is greatly affected by distance. On a metal line the higher the frequency the greater the attenuation, so the longer the line from the ADSL modem to the central office, the less usable bandwidth there is and the lower the communication speed becomes. Once the distance exceeds a few kilometers the speed drops sharply, and at still longer distances the connection itself cannot be established. The statement that a constant speed is always maintained regardless of distance is the opposite of the fact.
Incorrectウ
On one telephone line, ADSL assigns a low frequency band to voice calls and a higher frequency band to data communication so that the two coexist. A splitter separates them, so even when an analog telephone and a PC are used at the same time they do not take bandwidth from each other. Therefore simultaneous use does not lower the data communication speed, and being able to use the Internet during a call is one of the advantages of ADSL.
- エ
正解 The correct answer is エ. ADSL stands for Asymmetric Digital Subscriber Line, and as the word “Asymmetric” in its name shows, its greatest characteristic is that the downstream speed (from the central office to the user, i.e. downloading) and the upstream speed (from the user to the central office, i.e. uploading) are different. It sends data using the high-frequency band of the existing metal (copper) telephone line that voice calls do not use, and it allocates more of that band to the downstream direction, so downstream is far faster than upstream. This design matches how the line is actually used: in home use such as web browsing, watching video, and downloading files, the amount of data received is overwhelmingly larger than the amount of data sent. Therefore the statement that “the communication speed when downloading is faster than the communication speed when uploading” is appropriate.
ここがポイント
- ADSL (Asymmetric Digital Subscriber Line) is an asymmetric broadband line that uses the high-frequency band of the existing metal telephone line. As the word “asymmetric” in the name shows, downstream (downloading) is faster than upstream, which comes from a bandwidth allocation matched to ordinary use, where the amount received is large.
- As the constraints of ADSL, keep three points in mind: (1) the transmission medium is a metal line, and it cannot be used if there is an optical fiber section along the way; (2) the longer the distance to the central office, the more the high frequencies attenuate and the lower the speed becomes; (3) voice and data are divided by frequency band and separated by a splitter, so a telephone and a PC can be used at the same time.
用語