A 56Kbps modem gains the extra speed by using a digital connection from your ISP to the phone company, before the signal is converted to an analog one at the phone company's switch. Test your phone line using any modem by calling a test number. Keep in mind that the modem speed ratings are in bits per second. Some transmission overhead is required in both directions, so as a rule of thumb, divide the bit speed by 10 to get the speed in bytes. ISDN lines are symmetrical, meaning that the send and receive rates are the same.
Cable modems are even faster, using available bandwidth on cable-TV wiring.
Instead of dialing up to make a connection every time you want to access the Internet, as you do with telephone and ISDN lines, a cable connection actually adds your computer to a local area network operated by the cable company. As a result, your computer is connected to the Internet 24 hours a day, seven days a week. The cable connection is not symmetrical, but it is fast. You typically get 4Mbps download and 128Kbps upload transmission rates. The downside of the cable connection is that all the other cable-modem users on your system share the same bandwidth, so if many users are active at the same time, performance can degrade.