問77
頻出度★★★
収容局から家庭までの加入者線が光ファイバケーブルであるものはどれか。
平成21年度 秋期 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
ADSL (Asymmetric Digital Subscriber Line) reuses the existing metal (copper) telephone subscriber line as it is and carries data on a frequency band higher than voice. Its subscriber line is copper wire, not optical fiber, so this choice is wrong. Another characteristic is that it is asymmetric, with the downstream faster than the upstream.
- イ
正解 Choice イ, FTTH, stands for “Fiber To The Home.” It is an access line method in which the subscriber line from the central office to the user's home is drawn in with optical fiber cable over the entire section. Light is more resistant to attenuation and noise than metal lines (copper wires) that carry electrical signals, so it allows high-speed, large-capacity communication even over long distances. For this reason it spread into homes as a broadband line that is fast both upstream and downstream. FTTH is the only option that directly matches the question's condition that “the subscriber line from the central office to the home is an optical fiber cable.”
Incorrectウ
HDSL (High bit rate Digital Subscriber Line) also belongs to the DSL family; it is a method for businesses that transmits at the same speed upstream and downstream over a metal subscriber line. Its transmission medium is copper wire, not an optical fiber subscriber line, so this choice is wrong.
Incorrectエ
ISDN (Integrated Services Digital Network) is a service that digitizes the metal telephone subscriber line and handles voice and data in an integrated way. The subscriber line remains copper wire, and the speed is only about 64k bits per second × 2 channels on the basic interface, which is slow, so this choice is wrong.
ここがポイント
- FTTH is an access line method that makes the subscriber line optical fiber over the entire section from the central office into the home; its features are high speed, large capacity, and low attenuation.
- xDSL technologies such as ADSL and HDSL, as well as ISDN, all use the existing metal (copper) telephone subscriber line, so they can be distinguished from FTTH by the transmission medium.
用語