[asterisk-dev] Virtual Modem Pool
kelvin at zft.co.uk
Wed Oct 1 07:11:18 CDT 2008
I ran a dial-in service for 12 years, just shut it down this month. About
4,500 cash registers dialled in every night using URS modems over POTS.
I use Cyclades T1/E1 PCI cards which terminate all ISDN channels on Linux
tty devices, just as though the caller was dialling in to a V.90 (?) modem
pool. I bought the first PCI card for $4,500. The last one I installed was
purchased on eBay for $50 (no kidding). They work for years with zero
> 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:
> ï»¿Device <--> POTs <--> VOIP Gateway <--> IAX2 <--> ??? <--> Clear Text
> Modems simple _do not_ work on VOIP. It would look more like:
> ï»¿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.
> John Lange
> 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
>> ----- "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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