Modems (modulator-demodulator) are devices that convert digital data from computers into analog signals for transmission over phone lines, and vice versa. They enabled early online connectivity for timesharing, BBS's, and the internet. This write-up explains how they work, the progression of speeds from 110 baud to 56k with many stops in between, a table of key milestones, top makers, acoustical couplers for slow speeds, and common issues like extension phone interference.
Modems modulate digital bits (0s/1s) into analog tones or waves that travel over POTS (Plain Old Telephone Service) lines, designed for voice (300–3400 Hz bandwidth). The sending modem modulates data (e.g., phase/frequency/amplitude shifts); the receiving modem demodulates it back to digital. Handshaking negotiates speed/modulation. Early modems used simple FSK (frequency-shift keying); later ones added QAM (quadrature amplitude modulation) for higher speeds. The theoretical max on POTS is ~56 kbps due to line noise and Shannon's limit.
Modems started at low speeds for reliability, advancing as technology improved. The journey began at 110 baud for early teletypes and moved through 300, 600, 1200, 2400, 4800, 9600, 14.4k, 28.8k, and up to 56k. Costs fell rapidly in the 1990s with internet demand, from $500+ for 2400 baud to $50 for 56k.
| Speed (Baud/kbps) | Time Frame | Technology/Standard | Typical Cost (Adjusted to 2026 USD) |
|---|---|---|---|
| 110 baud | 1950s–1970s | Bell 103; FSK for teletypes | $500–$1,000 |
| 300 baud (0.3 kbps) | 1970s–early 1980s | Bell 103/V.21; FSK | $300–$600 |
| 600 baud (0.6 kbps) | Mid-1970s–1980s | V.22; PSK modulation | $200–$400 |
| 1200 baud (1.2 kbps) | Late 1970s–1980s | Bell 212A/V.22; QPSK | $150–$300 |
| 2400 baud (2.4 kbps) | 1980s–early 1990s | V.22bis; QAM | $100–$200 |
| 4800 baud (4.8 kbps) | Mid-1980s–1990s | V.32; TCM/QAM | $80–$150 |
| 9600 baud (9.6 kbps) | Late 1980s–1990s | V.32; advanced TCM | $50–$100 |
| 14.4 kbps | Early 1990s | V.32bis; optimized QAM | $40–$80 |
| 28.8 kbps | Mid-1990s | V.34; adaptive modulation | $30–$60 |
| 56 kbps | Late 1990s–2000s | V.90/V.92; PCM downstream | $20–$40 |
For 300 baud and slower (e.g., 110 baud), acoustical couplers were common in the 1960s–1970s. Users placed the phone handset into rubber cups; the modem sent/received tones through the speaker/microphone. Portable but noisy and error-prone; replaced by direct-connect modems in the 1980s.
When online, if someone picked up an extension the connection could be lost—adding noise, dropping carrier, or disconnecting. This was due to shared lines in homes; dedicated lines or call waiting guards mitigated it.
Modems bridged analog phones to digital computing, enabling BBS's, email, and the early internet. From 110 baud acoustic couplers to 56k, they democratized access before broadband. Costs plummeted with proliferation, fueling the online world.