Telecom::VoIP::FXS and FXO Interfaces
What is an FXS interface vs FXO interface?
FXS = Foreign Exchange Subscriber = "client" - controls "off hook"
FXO = Foreign Exchange Office = "server" - provides power, controls everything else.
When making a connection from a telephone system to another device this distinction is important.
http://www.patton.com/technotes/fxs_fxo.pdf has a very good discussion about this.
It boils down to the same thing as a similar interface and pinout discussions - consider null-modem cables and crossover ethernet cables
This category of topic came way back when I wanted to setup my "normal" PC devices to print to my DEC laser printers that I used with the PDP-11. (Until "progress" required we replace it with the slow, piddly Laserjet-III......) Had to have a serial cable custom built so that all the right pins on one side would talk to all the right pins on the other.
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