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Modem relay (V.150.1)

A modem call over the internet normally carries the modem's sound, digitised, all the way: the far end's modem has to hear D-Modem's tones through every gateway and codec in between. Modem relay (ITU-T V.150.1) does it another way. A gateway near the far modem answers that modem's tones itself and sends only the data, as plain bytes, over the internet. Lost packets are sent again, so the data arrives intact even on a poor network, and the codec in the middle no longer matters.

D-Modem can be the other end of such a call. dmodemd on SIP talks V.150.1 directly: it is its own gateway, so the far gateway sends the data straight to it and no modem sound is involved on D-Modem's side at all. Secure telephones (STE) and some carriers' modem gateways use this.

For the full technical description, see the technical details.

Turning it on

Modem relay is off unless asked for:

SIP_V150=yes dmodemd ...

dmodemd then offers modem relay on the data calls it makes, alongside the normal audio, and accepts it on calls that offer it. If the far end does not take it up, the call goes on as an ordinary audio call, so nothing breaks when the far end has no V.150.1. Fax calls and voice calls are never relayed.

What the user sees

Nothing changes on the AT command side. ATD and ATA work as usual.

  • Calling out: the modem dials and waits for the answer tone as usual. The far gateway sends that tone as an event, which dmodemd plays to the modem. The modem then switches to the relay, the gateways agree a modulation, and CONNECT reports the speed the far modem connected at (for example CONNECT 33600).
  • Answering: the modem answers, dmodemd sends the answer tone as an event, and the calling gateway switches the call to the relay. Then CONNECT as above.
  • Hanging up: ATH (or the far end hanging up) ends the relay and the call. The far gateway ending the relay gives NO CARRIER.

The speed in CONNECT is the far modem's speed with its own gateway; the data between the gateways goes as fast as the network allows, and dmodemd keeps it to that speed for the modem.

Limits

  • The modulations offered are V.34, V.32bis, V.22bis and V.21. The far modem's link to its gateway decides the speed.
  • dmodemd sends data as plain bytes and does not compress it. A far gateway that leaves its modem's V.42bis or V.44 compression for dmodemd to undo (one that is not "double trans-compression", mg=2) is cleared down if its modem compresses.
  • The far gateway must use the audio answer tone procedure (CDSCselect=1, the default V.150.1 behaviour). A gateway that switches the call to voice band data instead ends the relay, and the modem reports NO CARRIER.
  • It has been tested against the test suite's own gateway, not yet against a commercial one.