K56flex (originally called the K56Plus) was a modem chipset from Rockwell that gave users the possibility of receiving data on ordinary phone lines at 56 kbit/s as opposed to the previous maximum of 33.6 kbit/s. K56flex was similar to the US Robotics X2 chipset.
After a brief period of competition K56flex and X2 were replaced by the ITU V.90 standard.
56 kbit/s transmission exploits the fact that most telephone exchanges are interconnected with digital lines and so can use a transmission technique on a twisted pair line that avoids the usual digital-to-analog conversion.
The actual improvement in transmission rates was often marginal.