56K really was the maximum for telephone modems. This is because the telephone network, internally, transmitted 8 bit samples at 8K samples/sec - 64Kbps - except the occasional low bit got stolen for other signaling, so the net data rate was a bit less (and the result was inaudible to a telephone customer). Instead of trying to synchronize the modem to the stolen bits, it was simpler to just use the 7 that were reliable - thus 56Kbps. https://en.wikipedia.org/wiki/Robbed-bit_signaling
9600bps as the defacto terminal speed was older than 9600bps modems (those were the cat's pajamas in the mid-late 80s or so). It was simply "fast enough" for a terminal user and "slow enough" for the terminals of the day. I had one (a Volker-Craig 4404 if I remember correctly) that could not keep up at 9600bps and had to be used at something less, maybe 4800bps.
56K really was the maximum for telephone modems. This is because the telephone network, internally, transmitted 8 bit samples at 8K samples/sec - 64Kbps - except the occasional low bit got stolen for other signaling, so the net data rate was a bit less (and the result was inaudible to a telephone customer). Instead of trying to synchronize the modem to the stolen bits, it was simpler to just use the 7 that were reliable - thus 56Kbps. https://en.wikipedia.org/wiki/Robbed-bit_signaling
9600bps as the defacto terminal speed was older than 9600bps modems (those were the cat's pajamas in the mid-late 80s or so). It was simply "fast enough" for a terminal user and "slow enough" for the terminals of the day. I had one (a Volker-Craig 4404 if I remember correctly) that could not keep up at 9600bps and had to be used at something less, maybe 4800bps.