問84
頻出度★★★
PLC アダプタの役割として, 適切なものはどれか。
平成25年度 秋期 ITパスポート試験 問84
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 84
Incorrectア
The device that performs signal conversion so that PCs, analog telephones, and similar equipment can be connected to an ISDN line is a terminal adapter (TA). Because ISDN is a digital line, telephones and fax machines that handle analog signals, or PCs without an ISDN interface, cannot be connected as they are; the TA converts between the signal formats to make the connection possible. This has nothing to do with power wiring, so it is not a description of a PLC adapter.
Incorrectイ
The mechanism that converts between host names (domain names) and IP addresses when connecting to a website or a file server is DNS (Domain Name System), and the DNS server is what performs the conversion. This is name resolution, a service closer to the application layer, operating at a completely different layer from a PLC adapter, which deals with the physical transmission medium. Hence this choice is wrong.
Incorrectウ
It is ADSL (xDSL) that performs high-speed communication over an analog telephone line by using the higher frequency bands not used by voice calls, and the devices that handle its transmission and reception are the ADSL modem plus the splitter, which separates the telephone signal from the communication signal. The decisive difference is that the medium used is telephone line, not power wiring. The idea of using vacant frequency bands resembles PLC and is easy to confuse, but this is not a description of a PLC adapter.
- エ
正解 The correct answer is エ. PLC stands for Power Line Communication (high-speed power line communication), a technology that builds a LAN by using the electrical power wiring already installed inside a building in place of LAN cables. Power lines normally carry low-frequency electricity such as 100V at 50/60Hz, while PLC sends signals in a much higher frequency band (several MHz and above) superimposed on the power waveform. A PLC adapter is the device that performs this superimposing and extraction: the adapter on the sending side converts data received from a LAN cable or the like into a high-frequency communication signal and superimposes it on the power line, and the adapter on the receiving side separates the electric power coming in from the outlet from the communication signal, turning only the communication signal back into data and passing it to a PC or other device. In other words, simply plugging two or more adapters into outlets lets you build a wired LAN across rooms without construction work to run new LAN cables through the walls. Choice エ, ‘When building a LAN using indoor electrical power wiring, superimposes or separates electric power and communication signals,’ describes exactly this behavior and is therefore the appropriate role of a PLC adapter.
ここがポイント
- PLC (power line communication) is a technology that repurposes existing electrical power wiring as a communication medium, and the role of a PLC adapter is to superimpose the communication signal onto the electric power (sending side) and to separate the power from the communication signal (receiving side). Its advantage is that a LAN can be extended through wall outlets without any LAN cabling work.
- Memorize similar-looking connection devices by distinguishing them by medium: power line → PLC adapter; high-speed communication over an (analog) telephone line → ADSL modem + splitter; ISDN (digital line) → terminal adapter (TA); conversion between host names and IP addresses → DNS server.
用語