Newb help: understanding my needs


hi all,

I’m a bit new to all the asterisk stuff. Sort of a hobby’ish desire now, maybe I’ll try to benfit from it practically later.

Anyway, I want to basically use my computers with soft-phones on them (recommendation for win machines?) to be able to connect to my pots line. Asterisk seems to be the way to go. What I don’t understand is why the asterisk server can’t just use a regular modem to connect to my pots line.

The way I understand this is that my computers will use voip (iax or sip) to my asterisk server. I’m thinking it gets decoded there and can be carried over the modem to my pots. So I guess I’m saying that my modem in this case is behaving as an FXO device (doesn’t it always?)

I understand there are some cheap options to get an fxo card for my asterisk server, but this is more of a technical question.

I’d also like to hear of your topologies to see how you’re using asterisk at home. thanks!


The simple answer is “because it can’t!”

The simplified version of the complicated answer is…

A modem can’t act as an FXO adapter because the modem has no mechanism for passing audio through from one side to the other.

Modems work with binary, not analog. The binary data stream from the computer is translated in the modem to a series of tones usually using (i think) trellis code multiplexing. The modem has circuitry that generates the correct tones and modulates and phase shifts them to encode the data.

Modems also demultiplex, demodulate and decode the incoming signal, from the modem at the other end of the phone line, and send it to the computer as a binary data stream.

There’s only a limited number of sounds a modem can make - and they don’t bear very much relationship to the sound of your voice!

Modems have most (all?) of the signalling capabilities of an FXO adapter, but none of the audio capabilities.

Of course, that doesn’t mean you couldn’t use a suitable one as an FXO adapter - if you were lucky enough to find one that was capable of it. You’d just have to reprogram its firmware and write a driver for the computer side!

Site Admin, is there any chance of doing a sticky message about this topic? It comes up so often.