ISDN

ISDN is an abbreviation for Integrated Services Digital Network and is a type of telephone line.

ISDN offers advantages over the standard type of telephone line, as it is possible to make two phone calls at once and to have up to 10 numbers, although normally only 3 are assigned.

In the days before internet telephones and cheap mobile phones, this meant that a family could have a number for each person and even if someone made a long phone call the other line would remain available. Small businesses often use ISDN lines to separate telephone and fax calls.

ISDN also offers additional services such as conference calls and call re-direction (“follow me”).

Normal telephones require an adapter to work on an ISDN line, known as an “A/B-Wandler”. The line itself is a digital connection to the telephone exchange and the numbers dialled are transferred digitally as opposed to using DTMF tones or pulses.

To hear a simple explanation and a short discussion in German, listen to the podcast:

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