Stands for "Integrated Services Digital Network." Nope, it's not the same thing as the ISBN you see in books. ISDN is a data transfer technology, created in 1984, that allows for faster transfer rates than modems do. Basically, ISDN enables wide-bandwidth digital transmission over the public telephone network. It's just that easy. Well, there's a lot more that goes into it, but it's too complex to explain. Using ISDN, you can transfer 64K or 128K of data per second. However, the newer DSL technology and cable modems seem to be making ISDN obsolete.
ISDN
Meaning of ISDN in English
English glossary of computer and Internet terms. Английский глоссарий компьютерных и Интернет терминов. 2012