[TriLUG] VoIP SIP MODEM?

[sholton at mindspring.com]

Jon Carnes <jonc at nc.rr.com> fills in the missing details:

> What you are talking about is simply a codec. It would take your UDP
> packets (not SIP - SIP is only used for control and setup, data/voice
> uses RTP which is a udp based protocol), and convert the UDP packets
> into signaling that could understood by a modem.

That's for the "far end".  I was thinking more of a near-end solution.

Here's the hypothetical scenario.  There's a BBS system I want to
talk to. It isn't "on the net", the only access is through dial-up to a
56K modem. But I (hypothetically) don't have a POTS line, so when 
it leaves my computer it's already encoded for VoIP. Somewhere
betwix hither and yon an ATA decodes my bitstream into the 
"squak and buzz" the Hayes (TM) at the far end is looking for.
I want to generate that bitstream in software, rather than having 
to use a real Zoom FaxModem looped back into a local ATA 
to get it.

> I don't know of any VoIP codecs that do this. It would be great to have
> one for faxing. Right now we have to loop the faxing signal thorough a
> Fax card/modem to let the hardware inside it decode/encode the fax
> signaling.

Yes. A similar problem.

Clearly the solution would involve a codec. but I'd need to use one
of the "standard" encoding/decoding schemes which the far-end 
VoIP-to-analog phone adapter can understand.

An my interface to this scheme (at my end) should look like a 
combination of /dev/modem (so that minicom recognizes it) and 
the 56K modem hardware (My Zoom) and the (local) ATA 
creating the encoded RTP session, but without all that hardware.

A "new" codec for this wouldn't be the complete solution, since I don't
get to choose what codec gets used at the far end, although I imagine 
the performance characteristics for a codec targeted at modem-type
"sounds" would be different that one optimized for human voice.

In theory, such a solution should be able to adapt to whatever codec
the far-end negotiations were willing to accept. If the only acceptable codec
were a low-quality G.711 then the channel might to be able to support 
the full 56Kbps, but negotiate to a lower rate depending on what the
modem-behind-the-VoIP-to-analog-phone-adapter was able to handle.

Oh well. An interesting problem to muse about....
 
-----Original Message-----
From: Jon Carnes <jonc at nc.rr.com>
Sent: Jun 24, 2004 7:32 PM
To: Triangle Linux Users Group discussion list <trilug at trilug.org>
Subject: Re: [TriLUG] VoIP SIP MODEM?

On Thu, 2004-06-24 at 10:08, sholton at mindspring.com wrote:
> Perhaps someone on the list can assist.
> 
> In a VoIP discussion recently, a question came up: 
> If I'm using pure VoIP (from IP phones on a LAN, and/or VoIP
> software on a PC) but need to connect to a modem, such as on 
> a BBS system, but don't want to use a kludge like 
>   PC -> modem -> SIP box -> Internet -> PSTN gateway -> modem -> BBS
> what kind of software device would I need for the client end?
> 
> I'd google on this, but I don't know what it would be called.
> 
> I'm thinking it would be some sort of software driver which
> looks like a modem to the kernel and apps but produces 
> and consumes SIP packets from the ethernet interface which 
> contain the CODEC-encapsulated analog squalking of a modem.
> 
> Is there a name for a beast like this?
>  

What you are talking about is simply a codec. It would take your UDP
packets (not SIP - SIP is only used for control and setup, data/voice
uses RTP which is a udp based protocol), and convert the UDP packets
into signaling that could understood by a modem.

I don't know of any VoIP codecs that do this. It would be great to have
one for faxing. Right now we have to loop the faxing signal thorough a
Fax card/modem to let the hardware inside it decode/encode the fax
signaling.

Most times we can live without faxing in a VoIP world. After all, we're
pushing around data, so we can just send the document directly - or scan
in the document and send it as a nicer picture than you would get with a
Fax.

Good luck - Jon Carnes

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-- 
Steve Holton
sholton at mindspring.com