V.90 (redirected from PCM modem)

V.90

An ITU standard (1998) for a 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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(protocol)

V.90 - An ITU-T standard modem serial line protocol allowing download speeds of up to 56 kbps with upload speeds of 33.6 kbps. V.90 modems are designed for connections that are digital at one end and have only one digital-to-analogue conversion. As of 1998-02-06 the V.90 standard, formerly called V.pcm, has been given final approval by ITU-T. On 1998-10-27 the ITU-T announced that approval of the V.90 standard was completed. Interoperability testing is complete or in progress for several modem manufacturers. The V.90 standard reconciles two competing standards, X2 and K56flex. The ITU-T has initiated the approval process for a new all-digital version of the protocol, to be known as V.91. ITU Press Release 98-04. and NP-3.