x2 is a technology from US Robotics (now 3Com) for the downstream transmission of data over ordinary phone lines at 56 Kbps (thousands of bits per second). The 56 Kbps speed is achieved in the downstream direction only (to your home or business). Upstream speed is at the regular maximum speed of 33.6 Kbps. (The actual achieved downstream speed is reported by users to be about 53 Kbps.) x2 provided input to and has been replaced by the V.90 ITU-TS standard.
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56 Kbps 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-equipped 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 V.90 technology does 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.