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V.8bis: agreeing on a mode before the modems start

When two modems connect, they normally go straight into V.8: the answering modem plays its answer tone and the two agree on a modulation. V.8bis (ITU-T V.8 bis) is an extra step that can come before that. In a few short tones and messages, the two ends tell each other which kinds of call they can make (plain data, voice and data together, video telephony, fax, text telephone) and pick one, before any modem sound starts. It was designed to work even in the middle of a telephone conversation, quietly enough not to disturb the people talking.

D-Modem needs it for V.70, voice and data at the same time on one call, where V.8bis is how the two ends agree to switch to that mode. D-Modem runs a V.8bis exchange at the start of a data call, then carries on with V.8 and the usual modulations; with AT+DSVD=1 at both ends it chooses V.70 (voice and data together), and with AT+ASVD=1 at both V.61 (ASVD), whose own start-up then takes the place of V.8.

For the full technical description, see the technical details.

Turning it on

V.8bis is off unless asked for, with V.251's command:

AT+A8E=1,1,,1

The last number turns V.8bis on (1) or off (0, the default). It must be on at both ends for the exchange to happen.

  • Answering, the modem sends V.8bis's opening signal before its answer tone and waits up to 3 seconds for a reply. If the caller does not reply, it goes on with the normal answer tone, so a caller without V.8bis still connects (about 3.5 seconds later than usual).
  • Calling, the modem listens for that opening signal as well as the answer tone, and replies to it. A modem that answers without V.8bis is called as usual.

AT+A8E? shows the settings and AT+A8E=? what is accepted.

What the user sees

Nothing changes on the AT command side: ATD and ATA work as usual and CONNECT reports the speed. With both ends on, the call takes about two seconds longer to start, the time of the exchange.

Limits

  • Only the data mode and voice and data (V.70 with AT+DSVD=1, V.61 with AT+ASVD=1) are chosen; the other modes V.8bis knows (video telephony, fax, text telephones) are not.
  • D-Modem uses the exchanges where the answering modem stays the answering modem of the start-up that follows. A caller that asks the other way round (V.8bis transaction 7, or an answerer that sends its choice first) gets the normal start-up without V.8bis instead.
  • The commands V.251 has for a terminal to run V.8bis itself (+A8M, the +A8x reports) are not there.
  • It has been tested between two D-Modems, not yet against another maker's V.8bis modem.