Is V.32bis Modem Transmission Reliable Over VOIP With G.711 and Grandstream HT802?

Anyone know if V.32bis modulation is generally supported over VOIP analog telephone adapters  (such as Vonage/Grandstream HT802)?    G.711 PCM is supported for voice but I'm unclear if V.32bis can pass seamlessly over the PCM encoding or if V.32bis support is explicitly needed.   I do see on the Vonage support page that analog fax is not supported.      https://businesssupport.vonage.com/articles/answer/Grandstream-HT802-ATA-Setup

Todd...  VOIP adapters use a CODEC (Coder/Decoder) to convert Analog to VOIP and V/V.  We have a VOIP system at work that I work on and we have what we call Analog Gateways which do the same function as your HT802.  There are various codecs that are available, G711 being one of them.  There's a bit more here.  G723 stuck in my mind and they do say in that link that it can be used for fax/modem links. (V32bis is a modulation scheme for faxes and modems)  In our system we have to specifically set a port as a fax port else a fax will not work, though an analog phone will.  On a previous phone system which had a bit more access to the settings I remember changing a port to G723 (I think) which got a security system modem working.  So Yes you do have to have V32bis support specifically mentioned, or at least G723. In you case, it seems your adapter only supports G711 which is basic voice and won't work with Faxes or modems.  It's possible that Vonage may be able to supply another adapter with another codec or a choice of codecs, that will work with Faxes.  I'd contact them directly and see if they can help you.  A quick search for "Fax Adapter" on Vonage got me this page which has this article on the CISCO Analog Telephone Adapter (ATA) which supports Faxes.  I have used them before on a CISCO VOIP system but don't remember much about them, and only used them for phones, but they do say they will support faxes and I know we did use them for that in other locations.  You'd be able to pick up one on Ebay fairly easily and cheaply I would think, they are pretty old now.  Good luck with it.

Thanks David, good info, I appreciate the quick response.  This particular scenario is a bit difficult as it is a customer who owns and manages the Grandstream HT802 ATA (analog telephone adapter), connecting to our V.3600 modem.  So I could not contact the Vonage Business group without an account, but the consumer side of their business assured me the HT802 would not support the transport of modem data.  Taking that consumer level support with a grain of salt, I searched further and I did find vocoder implementation solution spec sheets which show fax/modem data support over G.711 and G.726 vocoders.  (That chart shows G.723 having extremely low bit rates, 5.3 - 6.3 kbps, and no modem support, so you were probably thinking of G.726)  Both use pulse code modulation (PCM) and I believe G.711 is what telcos traditionally used for voice calls.  Also with searching device user forums, it appears as VOIP ATAs may support modem data using V32.bis by disabling any echo cancellation techniques on the line and network, and setting the ATA to specifically use the PCM vocoder to 'pass-through' the modem data.  It can still be hit or miss from what I have gathered.  The Grandstream HT802 has an additional issue in that being a low lower device, they evidently did not provide adequate power to generate the RING voltage and current required to complete the call.  Gotta love consumer grade electronics!

  This is chart I found, I thought the source was an ST application note but cannot find it now.

Hi Todd...ref G726 you might be right, that rings a bell.  Could you post the link to the table as it's too small to read...  One trick that might work is to set your customer's fax machine (the one on the ATA) to 4800 BPS or even 2400 BPS.  Make sure you set maximum Tx and Rx speeds to that.  This will force other machines dialling into or out of that machine to use those speeds.  They should work over a bit of wet string, so might get thru the G711.  I've used this a lot on our network when remote centres complain about fax problems and it usually works well.

Thanks