A modem standard developed by Rockwell for 56 kbps communications. K56flex Became more popular than the rival X2 but will be superseded by the official V.90 standard.

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An ITU standard (1998) for an analog modem that communicates at 56 Kbps downstream and 33.6 Kbps upstream. It was designed for ISPs and online services that are digitally attached to the telephone system via T1 and T3 circuits.

In practice, the downstream link is not faster than 45 Kbps in these PCM modems, so called because they use pulse code modulation downstream and standard V.34 upstream. Initially, two incompatible technologies competed in this arena: x2 from U.S. Robotics and K56Flex from Rockwell and Lucent. Such modems can be upgraded to V.90 if they contain software-upgradable memory chips. See V.92, V.34 and channel bonding.
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This was also true for dial-up modems when two 56 KBPS standards were introduced: Rockwell's (now Conexant) K56Flex and U.S.
It connects 48 dial-in-modem V.90, K56Flex, V.34+ or ISDN users to the Internet, IP LANs or corporate intranets.
State-of-the-art 56K V.90/K56flex performance with simultaneous support of both the ITU V.90 standard and K56flex, maximize the user's ability to be connected at the maximum possible speed.
There are two proprietary protocols (X2, K56Flex) and one standard (V.90) currently in use for 56Kbps modems.
For a long time, there were two competing standards--X2 (Rockwell Technology) and K56flex (Lucent Technology).
The Infonet Services Corporation has completed the upgrade of its dial-IP infrastructure enabling it to support both V.90 and K56flex technologies.
The system can be configured for ISDN, V.90, K56Flex, Fax and VoIP, with each port able to run a different protocol at the same time.
The Global Freedom 5660 PC Card has recently been upgraded to include 4MB of Flash memory, enabling all supported GSM phone drivers to be held in memory, as well as both V.90 and K56Flex protocols with auto-sensing capability.
The PCG-735 is a slim, light 200MHz Pentium/MMX notebook with 32 MB SDRAM, a 2.1GB hard drive, 12.1" HPA SVGA screen, removable 24X (maximum) CD-ROM and built-in K56flex modem (33.6 kps, upgradeable to V.90).
The VAIO 505 comes in two different configurations: PCG 505G ($1,999) comes with a 200 MHz Pentium processor, 32 MB of RAM (expandable to 64 MB), a 2.1 gigabyte hard drive, a software upgradable K56flex data/fax modem, a 10.4-in.
The new standard is a compromise that combines technology from 3Coms x2 standard and K56flex, a rival modem standard developed by Rockwell International and Lucent.
The reason: There's a competing technology out there, called K56Flex, that's about to be introduced, and the two technologies are not compatible.