[asterisk-users] Using voice modem as poor man's FXO in Asterisk 1.8

Stuart Longland redhatter at gentoo.org
Tue Mar 1 18:05:35 CST 2011


On 03/02/11 01:37, Daniel Tryba wrote:
> On Mon, Feb 28, 2011 at 09:41:38PM +1000, Stuart Longland wrote:
>> Indeed, most motherboards do come with Ethernet on board.  This one came
>> with one gigabit Ethernet interface.  However, we needed another for a
>> connection to an ADSL router (acting in bridged mode so we do the PPPoE
>> directly).  Thus, a 10/100Mbps Ethernet card was installed to provide
>> the second port needed.
> 
> You can free the PCI slot if you use VLANs on the internal interface
> to seperate the internal and external traffic. This requires a switch
> with vlan support, shouldn't could much more than $80.
> 

I'm aware of this, it still requires the expense of:

- a VLAN aware switch	($80 apparently; currency not stated)
- a FXO card		(estimates at AU$200 upwards)

and the need to take down the server to replace a PCI card, then set up
kernel drivers for the new card.

Even a USB FXO device, I still need kernel drivers, but I should be able
to just run `make menuconfig`, enable the modules, `make modules
modules_install`, plug it in and I'm done.  Ethernet based devices
shouldn't need kernel modules.

There's also regulatory requirements: here in Australia since I'm
plugging into the PSTN, it needs to carry the ACMA's regulatory
compliance mark.  So buying something from overseas isn't an option.
It's less of an option for me as I do not possess a credit card, and so
many companies out there seem to think we're born with them.  Thus
ideally, I'm looking for where I can source one in my local area.

I can picture the exchange with the salesman behind the counter now.

	"Hi, I'm looking for a FXO card…"
	"FX-/What/??"

These are barriers to people adopting systems such as Asterisk in my
opinion.  I figure there's some sort of technical reason for why a modem
can't work in theory, but I'm yet to hear one.  Yes, I'm new to IP
telephony.

The closest I've seen is that they don't have a DSP, as I say, big deal
in this day and age … DSPs are just specialised CPUs in many cases, and
while there'll be a penalty in terms of CPU load, I'm sure it can be
done in software.  We're dealing with what, 8kHz sample rate, 16-bit
mono PCM?  So, 128kbps?  I've seen my old computer do realtime DSP on
faster streams than that.

Full duplex may be another matter.  As I mentioned, I know the modem I
have here can do full duplex on the headset port, and I'm certain the
sound card can do full duplex.  At worst, I just make a cable that plugs
PC line out -> modem microphone in, PC line in -> modem speaker out.
And I might have to use ALSA's "plug" plug-in to rate convert from 48kHz
to 16kHz or 8kHz, not difficult.  I then just write 'ATA' to /dev/ttySX
when I see "RING", or 'ATDTxxxxxxxx' when I want to dial.

If someone can enlighten me on some technical details I might have
missed, I'm all ears.

Still, at least I have the source code there, and so it is possible for
me to code up a solution for myself. :-)
-- 
Stuart Longland (aka Redhatter, VK4MSL)      .'''.
Gentoo Linux/MIPS Cobalt and Docs Developer  '.'` :
. . . . . . . . . . . . . . . . . . . . . .   .'.'
http://dev.gentoo.org/~redhatter             :.'

I haven't lost my mind...
  ...it's backed up on a tape somewhere.



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