K56flex
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K56flex (originally called the K56Plus) was a modem chipset from Rockwell and Lucent 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.
K56flex is a combination of two competitors efforts at 56K technology. Lucent developed the K56 protocol, while Rockwell developed the 56flex protocol. This occasionally led to incompatibilities between Lucent and Rockwell chipsets as their implementation of K56flex differed.
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.