I know about ISDN and its elements, but unable to apprehend ISDN card. What exactly is ISDN card and how does it connects to PSTN when it is installed in PC.
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Voting to close because question not about programming – Sami Kuhmonen Mar 14 '17 at 05:47
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ISDN is digital protocol working via wire similar to ethernet one.
ISDN card for asterisk is just dumb interface card which read signal level over that wire and send it to software level - dahdi module. Dahdi analyze protocol and do all other magic, give asterisk rtp and isdn messages.
ISDN card connect with other "ISDN card" at provider side. After that provider connect it to other equipment in network, including PSTN gates.
ISDN is DIGITAL line, not analog like PSTN.

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