13.4.1 Modems
A modem (modulator–demodulator) converts digital baseband symbols into waveforms suitable for transmission over a particular physical channel and performs the inverse operation at the receiver. While earlier systems distinguished among baseband, VF, and RF modems, the same functional division remains useful: the distinction is primarily the transmission medium and the spectral constraints imposed by it.
Historically, three broad approaches were used:
- Baseband transmission. Baseband systems transmit line-coded digital pulses (see Appendix D) directly over a physical medium such as twisted pair or coaxial cable. Performance is limited by channel attenuation, dispersion, and bandwidth constraints. Repeaters or regenerators are required for long distances. Modern Ethernet and optical fiber systems are examples of high-speed baseband transmission employing line coding and equalization.
- VF modem. VF modems were developed to allow digital data to traverse analog telephone networks designed for speech (approximately 300–3400 Hz bandwidth). Digital data were modulated onto audio-frequency carriers using FSK, PSK, or QAM. These systems enabled data communication over the public switched telephone network (PSTN) without modifying the infrastructure. Standards such as V.32 and V.34 represent the culmination of this technology. With the widespread deployment of digital switching and broadband access technologies, VF dial-up modems are now largely obsolete.
- RF modem. RF modems translate baseband digital signals to radio-frequency carriers for wireless transmission. Modern RF modems incorporate digital modulation (PSK, QAM, APSK), pulse shaping, coding, and carrier synchronization within integrated transceiver architectures. Satellite modems, microwave radios, cellular base stations, and Wi-Fi systems are contemporary examples.