[asterisk-users] Sip Node w/ 4 wire audio & AT command set call supervision

[John Todd]

On Dec 6, 2008, at 7:55 AM, George Bean wrote:

> I have several discontinued Sierra Wireless MP775 mobile GSM/EDGE  
> radio modems. These devices were originally installed in emergency  
> vehicles to provide data and voice access along with GPS reporting.  
> They have external RF connections for GSM/EDGE and GPS signals. The  
> baseband side includes four wire audio, RS-232 serial, USB and some  
> parallel digital I/O for panic switches etc. GSM voice connections  
> are established by sending AT commands via the RS-232 serial or USB  
> ports and utilizing a handset connected to the four-wire audio jack.  
> Here is a link to the Sierra Wireless web pages for the MP775.
>
> http://www.sierrawireless.com/support/mp775.aspx
>
> My interest comes in finding an inexpensive way to connect an  
> Asterisk PBX or similar system to the PSTN via GSM when POTS and  
> Internet service isn’t available or is too costly to connect. In my  
> case, I’m considering building a house at the end of a long  
> unpopulated stretch of dead end road and the cost of trenching and  
> cabling from the nearest telco POP is prohibitive.
>
> I would like to find a way to connect these modems to my network so  
> they appear as a SIP FXS device. This would require a device  
> generating/reading AT commands and passing baseband audio on the  
> front end and SIP emulation on the backend. I’m sure there must be a  
> way to do this with a pc but to minimize power consumption; I would  
> prefer to use something like a small single board computer with a  
> Geode processor. The latter, having multiple RS-232 and audio ports,  
> ought to be capable of handling at least two of the MP775’s.
>
> Has anyone seen any hardware, software or combination that would  
> allow me to accomplish such an interface?
>
> Regards,
> George

I've not heard of any such hardware/software combination to do this,  
so I apologize for the semi-useless reply.  However, I'd be very  
interested in any solutions you come up with, since I've got a few  
similar older WaveCom modem devices which probably do mostly the same  
thing - I think I picked them up for about $6 each on eBay, and I see  
a few there now for $30-$50 range.  I bought them just for SMS, but  
they are fully-functioning GSM phones if you want to wire up the right  
pins on the connector to speaker/microphone parts.

You also should take a look at eBay which has a number of SIP-to- 
mobile (SIP to GSM, SIP to CDMA) conversion units.  Type "sip gsm" in  
the search term box.  There's even one for $150 that seems to use  
bluetooth to connect SIP to Bluetooth phones (which can be done  
directly with Asterisk, though it may take some significant fist- 
slamming to get the drivers all set up correctly.)  Maybe you have an  
older Bluetooth phone to use the lower-cost option, or maybe the  
integrated SIP/GSM system makes sense.  If you're going to spend the  
money for a purpose-built solution, you might want to make sure that  
whatever gateway you get also supports SMS in some fashion.  Some of  
us are waiting for the drivers to get Asterisk to talk directly to the  
PORTech devices for send/receive SMS capability.  (hint, hint)

If you take the modem/soundcard route, perhaps there are some useful  
cards or things here -  http://www.ramseyelectronics.com/ - I haven't  
dug through, but they seem to have lots of small-end hacker-friendly  
bits and pieces which might do some of the line-level conversions you  
would require. You might connect directly to an Asterisk-driven  
soundcard, or you might use an FXO/FXS type conversion as well and  
connect to a single-line analog PC card that works with DAHDI/Zaptel.   
Get out the soldering iron!

JT

---
John Todd
jtodd at digium.com        +1-256-428-6083
Asterisk Open Source Community Director