An ITU standard (1998) for an analog modem that communicated at 56 Kbps downstream and 33.6 Kbps upstream. V.90 was designed for ISPs and online services that were digitally attached to the telephone system via T1 and T3 circuits.
In practice, the downstream link was 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 could be upgraded to V.90 if they contained software-upgradable memory chips. See
V.92,
V.34 and
channel bonding.