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Summary

One of the most often things asked here is the support for full-duplex operation in combination with voice modems drviven by the Unimodem TSP. This articles contains new and definite information about full-duplex support with Unimodem.

Introduction

Some months ago when I did some research on that topic and finally found a modem where full-duplex mode was indeed working (of course this was not just some kind of speakerphone mode!!). Meanwhile I have managed to look through the source code and was able to prove my findings.

Here is a summary of the facts:

Unimodem supports modems in full-duplex mode on W2k and later OSs under these conditions:

1. The modem supports full-duplex line operation

Full-duplex speakerphone support is not sufficient!!! Many modems advertise FD, but this only refers to FD speakerphone. Please see SpeakerPhoneMode for more info.

2. The modem comes with an inf file that is authored for full-duplex

The required entries are documented nowhere. So don't expect any modem to have these entries. Currently there is one and only one modem I know of that comes with these entries in mdmx5560.inf which is included in a default Windows installation: XJ-CC5560 56K Data Fax Full-Duplex Voice from USRobotics - it's a PCMCIA modem

3. The client application uses TAPI3 and TAPI3 media streaming

It does not work through the wav devices, so just don't try it

How it works

The Unimodem TSP checks for the presence of the modem registry entry "StartDuplex". If it finds this entry it advertises to have an MSP. This is the CSA MSP.

If the client application is TAPI3 and uses ITTerminalSupport, TAPI3 then uses the CSA MSP (instead of the generic WAVEMSP which is normally used for all TSPs without specific MSP). The TAPI3 application selects the terminals onto the media streams and when the call is connected and streaming starts, the

CSA MSP communicates with the serial wave driver to send and receive audio: Full-Duplex streaming is now in progress.

At this point in time I can't say how big the potential of this finding might be. It is very sure that you won't be able to turn any modem into a full-duplex modem. On the other hand during research I have read through various AT command references where in at least one case I could see a command that was for starting full-duplex operation (and this command was different from speakerphone duplex operation). I think this was a conexant chipset.

I had tried to get a sample of the USR modem mentioned above without success, but I was able to acquire a 3COM PCMCIA modem with the same chipset. I patched the inf file for the USR modem and was finally able to make it operate full-duplex.

Test yourself

If you are interested in doing some research and you believe that your modem can operate in full-duplex mode, you can try the following procedure:

Make a copy of the modem's inf file and rename it. Edit this copy:

• first rename the modem in the Strings section. This is important because this allows you to install it in parallel with the current driver (as long as it is NOT a PnP modem, then you can have only one copy at a time)

• copy the lines with "StartPlay" and "StopPlay" and rename these to "StartDuplex" and "StopDuplex"

• find out the StartDuplex command from your modem's AT command reference and enter it

• find the line with "WaveHardwareID". If it reads "HALFDUPLEX", I'd say chances are better than if it reads "RockwellVoiceModemWave" or "3COMModemWaveDevice", but actually I don't really know yet. Change the value to "FULLDUPLEX"!

• I don't believe that there are other changes to make, but I don't know!

• Save your inf file and install your modem with the new file.

To verify and test the installation:

• open device manager and have a look at the audio devices. If you cannot see "Unimodem Full-Duplex Audio Device", then the installation has failed!

• Test your modem with a TAPI2 application (like Julmar's Phone) and the Windows sound recorder. (Make sure that this works with the original inf file) If this does not work, then your modem is probably not compatible with the Full-Duplex wave driver and I would suggest to give up.

• Even if the previous test works, this does not mean that FD will really work.

• Open the Windows Phone Dialer and make a call trough your modem. If you can talk and hear the other side at the same time, you were successful...

During testing you should always have a look at the modem log. In case of success, you will see something like "Starting Duplex" (don't remember the exact message).

Don't be disappointed if it doesn't work. I guess it will work with just very few modems.

Best regards,

Matthias Moetje