An ISDN line requires a digital device called a terminal adapter. ISDN is available in most areas. 56k modems promise the highest speeds over standard telephone lines. ISDN, on the other hand, was designed from the start to provide digitally pure connections. ISDN consistently delivers true 128k speeds.
Cable offers a constant, always-on connection to the Internet.
The speeds vary from area to area. It can be faster or slower depending on available bandwidth. The number of people in your area that share bandwidth deterimines the available bandwidth. Cable is a bit like a party line or like a pipe. If only a few people are using the pipe, everybody gets great service.