US Robotics developed its x2 standard, so named for being double the speed of 28k modems. Rival manufacturer Rockwell soon dropped the K56Flex standard, which levied the same trick to up speeds.

US Robotics' (USR) x2 modem technology, which provides a 56Kbps transmission rate, may well be the last gasp of the traditional analog modem. The fact that 56Kbps is only possible in one direction ...