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(Public Switched Telephone Network) The worldwide voice telephone network. Also called the "plain old telephone system" (POTS) and originally analog only, the heart of most telephone networks today is digital. However, the lines from the home and office to the digital loop carrier (DLC) junction box in the neighborhood typically remain analog. At that point, analog signals are converted to digital.

IP Backbones
Increasingly, the digital infrastructure of the PSTN is migrating from the TDM-based circuit-switched architecture to IP-based packet switching (see IP telephony). See digital loop carrier and AIN.




Switched, But Not Too Public
In 1886, this 50-line magneto switchboard, made by Bell Telephone of Canada, was used to switch voice calls in small localities. These instruments were the beginning of the worldwide PSTN. (Image courtesy of Nortel Networks.)