PSTN also has to handle the interaction with the networks of multiple service providers, from Cable to Cellular (mobile phone) systems. For a long time, the PSTN has been able to route calls which contain only musical tones, and this has enabled the use of the system for faxes, remote control, and data transfer via modems.
Although modems (modulator-demodulators) were slow ways to communicate digitally, they led the way for more recent improvements such as the ability to run broadband digital data on the same PSTN lines, at least as far as the local exchange. For example here is how cellular networks link into the PSTN.
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