Why does my modem connect at a speed lower than it's advertised speed (21.6, 24k, etc.)

The name of the modem (28.8, 33.6, 56k, etc . . .) can be a bit misleading: what would be a more appropriate description for a high-speed, v.34 modem would be "up to 28.8, 33.6kbps, or 56k". A v.34 modem does over 2000 line tests when it establishes a connection to determine the maximum speed of the connection that you're using. Not only that, but with a 56k modem, the maximum speed allowed 53kbps over standard phone lines.

Meaning even though the modem is capable of the full 56k connection, the fastest it could ever go is 53kbps. The maximum speed of your phone line is a combination of many factors: the quality of the wiring of the phone connection, the distance between you and your phone company's central office, the kind of switching equipment your phone company has you connected to, and the way the phone company is routing your call. When all is said and done, there are few areas that will get consistent 33.6 or 53kbps connections.