ISDN is an older way of sending computer information along a special kind of telephone line, faster than the old noisy modems of the 1980s and early 1990s. The letters stand for Integrated Services Digital Network. Phone companies sold it so a home or office could use the same line for calls and for data at the same time. It felt “very high speed” then; it does not feel fast now. Ordinary broadband and fibre later replaced it in most places. Though actually, you will still meet the letters in old textbooks, in office history, and in some specialist phone gear. Students today may never have used it, and that is fine — the word is a fossil of a real technology, not a made-up exam term. In speech people say the four letters. It is a technical word, not an everyday one any more.

ISDN（早期用電話線傳電腦資料的系統；教科書作整合服務數位網路）