With a modem, you are dialing another modem. The modem speeds are matched at both ends and are negotiated to the fastest rate that both sides support through a process called handshaking. With xDSL, the devices that talk are limited by the constraints of modem technology in the traditional sense. They use a little different type of clocking mechanism. The standards allow for a maximum of 56k for modems, the DSL technologies are also constrained by distance to the CO. One thing the two share is the "sliding scale" speeds which get lower the farther away you are because of attenuation. (The signal gets weaker the farther you get, which translates to a slower speed). If you can picture a rock in a slingshot. When it first leaves the slingshot it is fast, but slows the farther it travels. Same principle, but for packets. So the devices would negotiate at the rate of speed when the rock hits the target. There are also some differences in the actual transmission, how much do you want to know? If this didn't answer your question sufficiently or you want more, please let me know and I will be happy to go into the remaining differences.
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