In the real world 56K is rarely seen due to FCC regulations. Actually, the speed given to you as the connection speed is probably not the actual speed you are running. Modems negotiate their connections constantly, adjusting speed as necessary. Modems increase the speed for good connections lowering the speed for poorer ones. Line static can even cause your modem to hang up the connection.
Distance can also be a factor.
The V.90 technology seems to do a better job of error correction. The overwhelming factor of any connection speed has always been phone line quality. At this time Essex uses digital V.90 modems at all its sites. X2 is a protocol used by US Robotics modems only, and in order to to serve all types of modems we have chosen not to use X2 protocol. Check with your modem manufacture s web site for information on upgrades.