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V.8 and V.34

V.8 is the short exchange at the start of a modern modem call where the two modems say which standards they support and agree on one. V.34 is the fastest standard for an ordinary analogue phone line, up to 33600 bit/s. It is also the base that V.90 and V.92 build on.

The code is dsp/dp_v34.cpp and the dsp/v34_*.cpp files. For every step of the start-up, the signals and the fixes made for other modems, see the technical details.

V.8: agreeing on a standard

When the answering modem picks up, it sends its answer tone. The calling modem replies with a list of what it can do; the answering modem replies with the ones they share; and the call goes on with the best of those. If the other modem doesn't support V.8 at all (older modems), D-Modem falls back step by step through V.32bis, V.22bis, V.21, V.23 and Bell 103 until one connects.

V.34: how it gets its speed

Before sending data, V.34 modems spend a few seconds measuring the line:

  1. They send test tones to find out how much of the frequency range the line carries and how noisy it is.
  2. They pick a symbol rate (how many signal changes a second) and a carrier frequency to suit the line.
  3. They train their equalisers (filters that undo the line's distortion) and echo cancellers (which remove the modem's own signal echoing back).
  4. They agree a data rate, from 2400 to 33600 bit/s.

If the line gets worse during the call, the modems retrain (measure again) or renegotiate to a different speed.

How well it works

Between two D-Modems on a simulated line, V.34 reaches its full 33600 bit/s on a clean line, and steps down as noise increases (for example 28800 bit/s at a 35 dB signal-to-noise ratio, 19200 at 25 dB). It also connects with every other company's modem in the test suite (Interop).

Half-duplex V.34 for fax

Fax machines can use V.34 too (T.30 Annex F), in a half-duplex form: one side sends the pages at up to 33600 bit/s, and a slow 1200 bit/s control channel carries the fax messages both ways between pages. D-Modem has this modem, tested with two of them back to back; see the technical details.