The 56K modems of 20 years ago were near the theoretical limit of bandwidth on a conventional phone line. They required the far end (the ISP) to have a digital

The 56K modems of 20 years ago were near the theoretical limit of bandwidth on a conventional phone line. They required the far end (the ISP) to have a digital connection to the telco; i.e. you can't have 2 56K modems call each other and connect at 56k. Depending on the telco setup; your analog telephone call is encoded into 64k digital, and some signaling schemes used one of the 8 bits for signaling, leaving only 56K for digitizing the audio. For analog-to-analog calls the best rate was 33.6K. https://en.wikipedia.org/wiki/Modem#Using_digital_lines_and_PCM_.28V.90.2F92.29 https://en.wikipedia.org/wiki/Modem#Using_digital_lines_and_...