Basic Operation of Modems over PSTN

Basic Operation of Modems over PSTN

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Modems agree to a particular encoding scheme to transmit and receive bits. In Figure 16-4, PC2 uses a higher frequency, shown as an analog waveform that moves up and down more quickly, to mean binary 0. PC2 uses a lower frequency (fewer movements up and down) to mean a binary 1. The receiverin this case a router with a modemuses those same rules to interpret the meaning of the signal. The telco passes the signal because the signal is just an analog electrical signal, and that's what the telco expects to happen on the local loop.

In the figure, the PC modulates (changes) between 30-Hz and 10-Hz signals to encode a binary 0 or 1, respectively. When the receiver interprets the analog signal, converting it back to binary digits using those same rules, the process is called demodulation. The term modem is a shortened version of the combination of the two words modulation and demodulation.