From The Free On-line Dictionary of Computing: (http://www.foldoc.org/, Editor Denis Howe)
An ITU-Tstandardmodem 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.