[asterisk-dev] Virtual Modem Pool

John Lange john at johnlange.ca
Tue Sep 30 22:58:00 CDT 2008


I have to agree. That is the wrong approach.

I'm not a modem pool expert but Cisco makes routers that take T1 PRI
cards designed for _exactly_ this purpose. Given the decline in dial-up
perhaps you can find one on ebay for a reasonable price?

It would be nice if you could use a T1/PRI card from Digium or Sangoma
for this but the sticking point is that in order to do modem signalling
you need a DSP (digital signal processor) or software that simulates a
DSP (that's what SpanDSP is all about for Asterisk & faxing).

Unfortunately software DSPs do not scale very well so there is little
chance you could use software DSPs for more than a few channels at once.

Hardware DSPs are reasonably expensive and that explains why a WinModem
(which is a software DSP) is more expensive than a "true" modem which
uses a hardware DSP.

In any case you would never do this:

Device <--> POTs <--> VOIP Gateway <--> IAX2 <--> ??? <--> Clear Text

Modems simple _do not_ work on VOIP. It would look more like:

Device <--> POTs <--> Modem Gateway <--> Serial or clear text

What about high density modem cards? You can get at least 8 modems on
one card. Put a few of those in a linux box and configure it for dialup.

I have no idea of the cost but I'd be willing to bet this is by far your
most cost effective way to go.

Regards,
-- 
John Lange
www.johnlange.ca


On Tue, 2008-09-30 at 16:15 -0500, Steven S. Critchfield wrote:
> Seems like a very stupid way of doing this. Modems do not do well over
> VoIP connections. Even if you could get a connection to be made and
> stay up for long, any jitter in the VoIP connection will kill your
> throughput. You will still be sending the same amount of data per
> VoIP call and getting much less end to end bandwidth.
> 
> You would be SOOO much better off if you just looked at contracting 
> with some service like TiVo did to provide POP access and do modem to
> PSTN to POP, then you have internet access for your download. 
> 
> Just for the quick math.
> 
> uncompressed VoIP ~ 80kbps bandwidth from you to your VoIP gateway
> 2400bps signal that might be usable over voip.
> 
> hmm, you loose 76kbps of data trying to support that idea per line in
> use.
> 
> ----- "Brad Silen" <brads at qualityprocess.com> wrote:
> 
> > We are looking to deploy thousands of hardware devices connected to
> > the PSTN
> > which will upload data and download firmware updates using v.90
> > modems. It
> > will be deployed to a demographic which does not have Internet
> > access.
> > 
> > We are hoping to avoid setting up an old fashion modem pool, POTs or
> > T1-PRI,
> > and hope to access the PSTN through a SIP Trunk or IAX2.  This
> > solution
> > would be both cost effective and scale to handle peak loads; For
> > example,
> > when a firmware download is required.
> > 
> > Ideally we would like our application servers to send/receive using
> > TCP/IP
> > sockets with the virtual modems which are being driven by the VOIP
> > infrastructure.
> > 
> > The network might look like:
> > 
> > Device <--> POTs <--> VOIP Gateway <--> IAX2 <--> ??? <--> Clear Text
> > on
> > TCP/IP Socket
> > 
> > Solve for ???
> > 
> > Has anyone used Asterisk in this way?
> > 
> > Is there any reason why the VOIP Gateway (SIP Trunk or IAX2) data path
> > would
> > prevent modem communication?
> > 
> > Is there any similar solution terminating a v.34 connection (aka Fax)?
> >  A
> > Fax solution would verify the ability to send data via the VOIP
> > pathway and
> > offer sample code as a starting point.
> > 
> > Would we extend the Asterisk concept of an "extension"?  For example,
> > instead of forwarding the traffic to a SIP Phone the virtual modem
> > would be
> > an "extension" which converts the data stream to ASCII clear text. 
> > Or, what
> > would be the suggested architecture choice in Asterisk?
> > 
> > Note, I am very open to better, easier, or more clever solutions.
> > 
> > If there are service providers offering this type of virtual modem
> > pool
> > please have your shameless commerce division email me directly.  I
> > suggest
> > they not respond to the list since I am concerned it would violate the
> > rules
> > of this list.  I have not been able to find a solution and expect to
> > contribute.
> > 
> > 
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