ISDN can provide data transfer rates of up to 128 kilobits per second. These speeds are slower than those of local area networks (LANs) supported by high-speed data communications technology. However, they are faster than those of analog telephone lines, which provide transfer rates of up to 28.8 kilobits per second.
In addition to the difference in data transfer rates, ISDN calls can be set up much faster than analog phone calls. While an analog modem can take up to a minute to set up a connection, you usually can start transmitting data in about two seconds with ISDN. Because ISDN is fully digital, the lengthy handshaking process of analog modems (those piercing, screeching noises) is not required.