問57
頻出度★★★
ADSL に関する記述のうち, 適切なものはどれか。
平成24年度 春期 ITパスポート試験 問57
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 57
Incorrectア
It is FTTH (Fiber To The Home) that requires an optical fiber cable to be drawn into the home, not ADSL. On the contrary, ADSL is a method aimed at achieving high speed using only the existing telephone line without newly laying optical fiber; if drawing in an optical cable were a prerequisite, the very point of using ADSL would disappear. This choice mistakes the type of access line (metal or optical fiber).
Incorrectイ
ISDN is a method that uses a metal line with digital signals from the start, and it occupies frequency bands that overlap those ADSL needs, so the two cannot be used at the same time on one subscriber line. To introduce ADSL on a line contracted for ISDN, you must first go through the procedure of returning it to an analog telephone line. In addition, neighboring ISDN lines running through the same cable bundle can cause crosstalk (interference) that lowers the ADSL speed, so the two coexist poorly.
- ウ
正解 ADSL (Asymmetric Digital Subscriber Line) is a high-speed digital communication method that does not use optical fiber; instead it reuses, as it is, the ordinary analog telephone line (metal, i.e. copper twisted pair) already installed in each home. Voice calls use only the low frequency band below about 4 kHz, so ADSL carries data on the empty higher frequency bands above it. The two are separated by a device called a splitter, so the telephone and the Internet can be used at the same time. Its biggest advantage is that no new line has to be laid and existing equipment can be reused, which is also why it spread so rapidly in the early days of broadband. Therefore ウ, which says “It uses an ordinary analog telephone line, not an optical fiber cable,” is appropriate. Note also that, as the name “Asymmetric” indicates, the downstream speed from the exchange to the home is set faster than the upstream speed from the home to the exchange, a design that matches usage centered on web browsing and downloading.
Incorrectエ
Distance is the biggest factor governing ADSL speed, so saying it has “no relation” is wrong. The high-frequency signals ADSL uses attenuate as they travel along the metal line, and the longer the transmission distance, the greater the attenuation, so the signal is more easily buried in noise. As a result, the farther the home is from the nearest telephone exchange (the accommodating station), the lower the effective speed becomes; beyond a few kilometers the contracted maximum speed may not be reached at all, or the service may not be available at all. It is worth keeping this in mind in contrast with optical fiber, where the speed drop caused by distance is small.
ここがポイント
- ADSL is one type of xDSL: it reuses the existing ordinary analog telephone line (metal line) and sends data on frequency bands above the voice band. A splitter separates voice from data, so it can be used at the same time as the telephone, and the fact that the downstream direction (exchange to home) is faster than the upstream direction (home to exchange) — asymmetric communication — is the origin of its name.
- The communication speed of ADSL depends greatly on the distance to the telephone exchange and on the quality of the line: the farther away it is, the more the signal attenuates and the slower it becomes. FTTH, by contrast, draws optical fiber into the home, so degradation caused by distance is small. It is good to learn these as a contrast between the characteristics of the different access-line methods.
用語