V.26 ter and V.26 bis: 2400 bit/s on a dial-up line¶

V.26 ter is the ITU's 2400 bit/s modem for ordinary phone lines, from 1984. Both ends send at the same time on the same pair of wires, each on the same 1800 Hz carrier, and each modem removes its own echo from what it hears (echo cancelling), as V.32 does later at higher speeds. It falls back to 1200 bit/s on a poor line. It was used mostly in France and in Europe; elsewhere V.22bis did the same job.

D-Modem has V.26 ter from the Recommendation's text. Two D-Modems connect with it at 2400 or 1200 bit/s. There are no V.26 ter modems in D-Modem's interop tests, so it has not been tried against another maker's.

Its older relatives:

• V.26 (and Bell 201, its "alternative B") are 2400 bit/s modems for 4-wire leased lines. D-Modem has them (AT&L2+MS=V26, see leased lines).
• V.26 bis is the dial-up version from 1972, before echo cancelling. D-Modem has it too: see V.26 bis below.

For the full technical description, see the technical details.

Using it¶

AT+MS=V26T       (or AT+MS=126, or AT+MS=V26TER)

Both ends need it: V.26 ter starts with an answer tone and its own handshake, not V.8, so a modem that is not set to it will not find it. AT+MS=V26T,0,1200,1200 keeps the call at 1200 bit/s.

The start-up takes about 7 seconds after the answer tone:

1. The answering modem says which speeds it can do.
2. The calling modem picks one and says so.
3. Each modem in turn sends for a second so that its echo canceller can learn its own echo.
4. Both send at once, and the data starts.

V.26 ter has no retrain: a line that gets too bad ends the call.

In simulation¶

Two D-Modems back to back on a simulated phone line (pump_test and dmodem_test) connect at 2400 bit/s:

V.26 bis¶

V.26 bis is the older dial-up 2400 bit/s modem. It has no echo canceller, so only one end can send at 2400 bit/s at a time. The other end can talk back slowly, at 75 bit/s, on a separate low tone (the "backward channel"), as V.23 does at 1200/75. It falls back to 1200 bit/s on a poor line.

V.26 bis lets the fast direction turn around (half duplex). D-Modem does not: like its V.23, the answering modem always sends at 2400 (or 1200) bit/s and the calling modem always sends at 75 bit/s. That suits a host or BBS sending screens to a terminal that types.

AT+MS=V26B       (or AT+MS=226, or AT+MS=V26BIS)

Both ends need it. The answering modem's +MS sets the speed: AT+MS=V26B,0,1200,1200 there makes it 1200 bit/s, and the calling modem follows, whatever its own +MS says (V.26 bis has no way to agree a speed; the caller tells it from the answerer's first signal). V.42 error correction is off, as on V.23.

The start-up takes about 1.2 seconds after the answer tone: the answering modem sends its synchronizing signal for a second, the calling modem starts its 75 bit/s tone, and both say CONNECT.

In simulation (pump_test, dmodem_test), two D-Modems connect at 2400/75 bit/s with no errors on a clean line, at 15 dB of signal to noise, at 8000 Hz, with a 4x level change and a 300 ppm clock difference, and with each modem's own echo 6 dB below its signal; AT+MS=V26B,0,1200,1200 connects at 1200/75. There are no V.26 bis modems in D-Modem's interop tests.