This page covers dial-up from the phone network's side: how data was squeezed through a voice channel. The services that ran over those links (bulletin board systems, online services, ISPs, PPP) belong to the history of the internet.
What a modem does¶
The phone network was designed to carry voice, roughly 300–3400 Hz. It had no way to carry raw digital data. A modem (modulator–demodulator) solved this by turning bits into audio tones on one end and back into bits on the other. Over time, encoding schemes changed the frequency, phase, and amplitude of the signal in more and more elaborate ways to pack more bits into each tone.
Early modems used acoustic couplers. The telephone handset sat in rubber cups so the modem could whistle into the mouthpiece directly, because connecting equipment to the line electrically was not allowed until regulators opened up the network (see The digital network).
Speed timeline¶
| Year | Standard | Speed | Notes |
|---|---|---|---|
| 1962 | Bell 103 | 300 bps | Frequency-shift keying, full duplex |
| 1970s | Bell 212A | 1200 bps | Phase-shift keying |
| 1981 | Hayes Smartmodem | 300 bps | Introduced the AT command set |
| 1984–91 | V.32 / V.32bis | 9600 / 14,400 bps | Echo cancellation, trellis coding |
| 1994–96 | V.34 | 28,800 / 33,600 bps | Close to the theoretical limit of an analog line |
| 1998 | V.90 | 56k down / 33.6k up | Unified the rival K56flex and X2 standards |
| 2000 | V.92 | 56k down / 48k up | Quick connect, modem-on-hold |
The Hayes AT command set (ATDT for "dial using tones," ATH for "hang up," ATZ for "reset") outlasted the modems it was written for. Cellular modems and many embedded devices still use AT commands today.
Why 33.6k seemed to be the ceiling¶
By the 1990s the phone network was digital inside, running PCM at 64 kbps per call (see The digital network). A call between two analog modems went through two analog-to-digital conversions, one at each end's central office. Each conversion rounds the signal to the nearest of 256 levels and adds quantization noise. The Shannon limit for a channel with that much noise works out to roughly 35 kbps. V.34's 33.6k was essentially at that limit.
How 56k got around it¶
The breakthrough was noticing that ISPs could connect to the network digitally, using T1 or ISDN lines straight into the switch. For data travelling from the ISP to a home user:
- The ISP sends digital values that line up exactly with the 256 PCM levels the network uses.
- The data stays digital until the central office line card closest to the subscriber.
- Only one digital-to-analog conversion happens, and it introduces no quantization error because the values were already exact PCM levels.
- The home modem only has to tell apart which of those known levels it is receiving.
In theory that gives 8,000 samples × 8 bits = 64 kbps. In practice:
- Robbed-bit signalling on some T1 links took the low bit of some samples, leaving about 7 usable bits: 8,000 × 7 = 56 kbps.
- Regulators limited the power on phone lines, and the strongest PCM levels exceeded that limit. That cut practical speeds to about 53.3 kbps.
- The upload direction still went through an analog-to-digital conversion, so it stayed at V.34 speeds (33.6k) until V.92 improved it somewhat.
- Line quality, distance, and any extra conversions along the way (such as some remote cabinets) often pushed real speeds well below 50k.
The handshake¶
The well-known screeching sound at the start of every connection was the two modems negotiating out loud:
- An answer tone at 2100 Hz, which also disabled the network's echo suppressors.
- Announcing which standards each side supported (V.8).
- Line probing: sending test tones across the frequency range to measure the line's response and noise.
- Training the echo canceller and equalizer.
- Settling on a final standard and speed.
Many modems let users hear the handshake through a small speaker so they could tell a good connection from a bad one by ear. ATM0 turned the speaker off.
The end of dial-up¶
DSL (which reused the same copper pair at higher frequencies), cable, and later fibre replaced dial-up through the 2000s. Small numbers of people still use dial-up in places with no other option. As copper lines are retired and voice moves to VoIP, the compressed codecs and jitter on those links make modems unreliable or unusable (see VoIP and the retirement of copper).