My understanding from discussions about modems in the mid 1990s was that the bandwidth limit for POTS-to-POTS modems was about 36kbit/s, because the signal has

My understanding from discussions about modems in the mid 1990s was that the bandwidth limit for POTS-to-POTS modems was about 36kbit/s, because the signal has to go DAC (modem) -> ADC (telco) -> DAC (telco) -> ADC (modem), and the modems at either end are not synchronized with the telco’s digital path. The thing that allowed speeds to increase to 56kbit/s was digital connections at the ISP end: typically the ISP would get a T1 or E1 circuit fed into a box that would act as 24 modems, but purely digital and synchronized with the telco. So the signal now went Ascend MAX (isp) -> T1 (telco) -> DAC (telco) -> ADC (modem), providing much less quantization noise.