modem

Research, standards and history

Everything the overture stands on

modem performs a piece of telecoms history. Below: the tools that already move data over sound for real, if speed rather than ceremony is what you are after; the standards this project implements once CONNECT appears; and the history that made the ceremony worth performing at all.

Read the phase-by-phase explainer

Standards and sources

What each part of the handshake was. The README and docs/ already carry the accurate detail on how this project uses them; these are the standards and references themselves.

  • Bell 103 modem (Wikipedia)

    The standard this project's connection implements once CONNECT appears - AT&T's 1963 300 baud modem, with the same originate and answer tone pairs.

  • Frequency-shift keying (Wikipedia)

    The modulation underneath everything past the overture: a bit is a shift between two frequencies, nothing more exotic than that.

  • ITU-T V.21

    The international 300 baud duplex standard that gave Bell 103's originate/answer split its ITU form outside North America.

  • ITU-T V.22

    1200 bit/s duplex, one step up the same lineage - a rung this project doesn't attempt to reach.

  • ITU-T V.34

    Up to 33,600 bit/s - the standard behind the screech everyone actually remembers, and the one the README is explicit about not implementing for real.

  • Dual-tone multi-frequency signalling (Wikipedia)

    What a dialled digit actually is: two simultaneous tones, chosen so no harmonic of one gets mistaken for the other.

  • Echo suppressor (Wikipedia)

    Why a call had to announce itself before data could flow - left switched on, these devices would have muted the very signal a modem needed to hear.

  • ITU-T V.25

    The recommendation that specifies the answer tone itself and the procedure for disabling echo control devices on the line - the reason the 2100 Hz ANSam tone exists at all.

The history

Why any of this was worth building.

Try the demo