[Asterisk-Users] Dial in modem speeds over VoIP?

[Mike Ciholas]

Hi all,

I like to have a dial in modem on my toll free number so that
when I or my employees travel, they can always get in for net
access to read email if no better method is available.

Right now, my Panasonic KX-TD1232D PBX receives the call on a
POTS line and routes it to an analog modem.  The speeds achieved
are around 24 kbps due to the digitization of the KX-TD1232D
introducing quantization errors (it is a "digital" phone system, 
which means it digitizes on the way in and the way out).

Now I am considering moving my toll free number to a VoIP service
like nufone.  But how does that affect analog modem calls?

Consider an analog modem somewhere in the PSTN calls through a
toll free number handled by a VoIP service that is then routed to
an * box at my place of business.  The * box routes the call out
an FXS card to an analog modem.  What sort of modem performance
should I expect?

The signal gets digitized by the local CO at the calling site
then sent to the VoIP provider (presumably as 64 kbps ulaw
encoding).  They potentially compress the signal using some codec
(which may be quite a bit less than 64 kbps in data rate),
potentially introducing delay jitter, and certainly introducing
latency.  Then I get the voice in packets and send them out an
FXS port on the * box.  This introduces additional delay, jitter,
and noise.  Sounds like the modem might not even be able to train
much less transfer bits.

Does anyone have experience trying this or educated insights into
how well this would work?

I imagine that many residential VoIP providers (say Vonage on
ATA-186 boxes) must have had modems connected to them.  This
would be similar effects but in reverse.  Does Vonage have 
guidelines/comments about using modems on their service?  Has 
anyone tried this?

Related question: is there any way to avoid going out the * box 
on FXS to an analog modem?  That is, do we have software that can 
DSP the audio stream and recover the modem data bits directly?  A 
kind of virtual modem module?  That might make it much better as 
it avoids one more digital to analog to digital conversion.  It 
would essentially be the code found is so called "soft modems" 
but taking it's input from packets rather than sampling a phone 
line.

-- 
Mike Ciholas                            (812) 476-2721 voice
CIHOLAS Enterprises                     (812) 476-2881 fax
2626 Kotter Ave, Unit D                 mikec at ciholas.com
Evansville, IN 47715                    http://www.ciholas.com