X2

Definition - What does X2 mean?

X2 was a modem protocol developed by U.S. Robotics (now 3Com) to download data at 56 Kbps under pulse-code modulation without the need for modulation/demodulation. It used V.34+ to upload data at 33.6 Kbps using plain old telephone service lines.

X2 was replaced by the V.90 standard, which combines both X2 by U.S. Robotics and K56flex from Rockwell Semiconductor.

Techopedia explains X2

The X2 modem protocol overcame the long-held belief that 33.6 Kbps was the fastest data transmission rate possible over copper wires. Knowing that most phone switching stations were connected by high-speed digital lines, X2 eliminated the need to modulate/demodulate digital data using an analog carrier signal. As a result, the normal modulate/demodulate process was eliminated, allowing for a faster data transfer rate. If the Internet service provider (ISP) had a digital connection to its telephone office, the X2 modem now only had to decode the multibit voltage pulses, just as the telephone lines were designed to do originally.

However, X2 had some stipulations along with the higher downstream transfer rate:

The upstream data transfer remained at 33.6 Kbps with a maximum possible rate of 40 Kbps
The ISP had to provide a modem that supported V.90 at the originating end of the transfer
Noisy lines resulting from interference from other phone lines could reduce the maximum possible transmission rate