digital service unit

Japanese: digital service unit（英語表記）digitalserviceunit

…The CCU is connected to the terminal device through various communication lines. When the communication line is an analog leased line, the connection to the communication line is made through a modulator/demodulator (modem), and when it is a digital leased line, the connection is made through a digital service unit (DSU). Furthermore, when using a telephone exchange line, a network control unit (NCU) is provided to know when dialing is being done, when responding to a call signal, etc. …

From [Modem]

…Modems are also used for sending and receiving facsimiles, sending and receiving videotext, etc. When the line is digital, the device that connects the line to the terminal is not called a modem, but is generally called a digital service unit (DSU). [Tadao Saito] …

*Some of the terminology that mentions "digital service unit" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…CCUは各種の通信回線を通して端末装置に接続されている。通信回線がアナログ専用回線であるときには変復調器(モデム)を通して通信回線への接続が行われ，ディジタル専用回線であるときにはディジタルサービス装置DSU(digital service unitの略)を通して接続が行われる。さらに電話交換回線を使用するときには，ダイヤルの発信，呼出し信号に対する応答などを知る目的でネットワーク制御ユニットNCU(network control unitの略)が設けられる。…