V.90 is a A Tour of the Internet, Who Runs It, Standards Org, approved by the International Telecommunications Union (ITU-TS), for transmitting data downstream to modems at 56 Kbps (thousand bits per second). The V.90 standard was arrived at by combining the x2 technology from US Robotics (now part of 3Com) and the K56flex technology from Rockwell. Transmission upstream from a computer modem is slower than downstream (about 33 Kbps) since it requires digital-to-analog conversion.
56 Kbps transmission technologies exploit the fact that most telephone company offices are interconnected with digital lines. Assuming your Internet connection provider has a digital connection to its telephone company office, the downstream traffic from your local Internet access provider can use a new transmission technique on your regular twisted pair phone line that bypasses the usual digital-to-analog conversion. A V.90 modem doesn't need to demodulate the downstream data. Instead, it decodes a stream of multi-bit voltage pulses generated as though the line was equipped for digital information. (Upstream data still requires digital-to-analog modulation.)
Unlike Integrated Services Digital Network, the 56 Kbps technologies do not require any additional installation or extra charges from your local phone company. On the other hand, the maximum transmission speed of ISDN is twice that of V.90 at 128 Kbps. You also have the flexibility of combining digital and voice transmission on the same line.