Early modems used frequency shift keying and simple [frequency division duplexing]?. Later modems used first phase shift keying and then quadrature amplitude modulation with [echo cancellation]? to give progressively higher performance.
Modern audio modems (V.90 and V.92 standards) closely approach the [Shannon capacity]? of the PSTN telephone channel.
ADSL modems are also a kind of modem, the main difference being that they are not limited to audio frequencies over the telephone line. Recent ADSL modems use coded orthogonal frequency division modulation.
See modulation for a fuller list of modulation techniques.
Modems are the most popular means of Internet access, UCLA? 2001 study of American Internet users shows that 81.3% of them use telephone modem, and 11.5% cable modem, an order of magnitude more than any other method.