[asterisk-dev] Data Modem calls passthrough

[Brian Candler]

On Mon, Sep 04, 2006 at 07:18:33PM +0500, Anton wrote:
> Does anyone know is possible to pass DIALUP modem calls 
> over, through asterisk? Nothing extra needed I think, say 
> asterisk is in-between of 2 telcos, connected over E1 from 
> both sides. Modem calls is already DATA, so will it work, 
> or asterisk will make some sort of codec resampling, so 
> original E1 data will be lost?

There's some useful background here:
http://www.soft-switch.org/foip.html
(it talks about fax rather than data modem, but the same things apply)

Basically: if you want data modem calls to work, you have to either:

(1) use a G711 codec, have *very* low latency and jitter in your network,
turn off all echo cancellation, silence suppression, dynamic jitter
buffering etc, and cross your fingers; or

(2) use V.150 "modem over IP", which is the data modem equivalent of T.38
"fax over IP". Here, the modulated audio is demodulated at the first
endpoint, passed as data over the network, and remodulated at the far end
gateway. However there are very few implementations of this that I'm aware
of, and Asterisk doesn't have it.

Regards,

Brian.