Running a modem via Sipura thru Asterisk


We’re running Asterisk (1.02 I think) as our PBX, connected to an ISDN30 with around 20 snom 190’s dotted around the office. I have two users that have requirements for a standard analogue line connected to the modem in their PCs which they use for connecting to banking services.

If I connect a sipura adaptor to the LAN, I can make voice calls thru it no issue, but if I connect the modem, I can get it to dial out but the handshaking process doesn’t seem to completed correctly. Any ideas on settings I could tweak to resolve this issue. Anyone else got modems working through their Asterisk box, using the LAN to connect to /from the box, and standard voice circuits outbound.


That’s a limitation of VOIP. It really doesn’t work with FAX or modem calls well at all.

It assumes that if you have a network connection, the easiest thing to do would be to connect directly to the service (in your case a bank) by using the network directly. Both you and the bank would have to be connected to the same network or your two networks to the internet.

If it’s going to work at all, use only G711a or G711u (alaw or ulaw) as your codec with the adapter.

Slow down the modem as much as you can tolerate. (28.8 vs 56k) It’ll give you the best chance to connect.

Alternatively, get one of the Digium cards with two (or more) FXS ports. I’ve successfully gotten fax calls to work across those. I imagine that modem calls will also succeed.

Cheers Dufus

The weird thing is I’ve got another sipura that I can use to make / receive faxes without any issues, and additionally a credit card machine dials-out thru the other line one the same box to authenticate payments which is obviously a modem function.

I guess both the fax and the ccard terminaly are possible running at 9.6Kbps (cc as there’s so little data transfer) and maybe that works. Connecting to the bank might run at upto 56 (not sure of the modems our end) which might cause the issue. I think all our services are running alaw currently.

I might try swapping the sipura’s around and seeing what happens, which should show whether its just a config issue or whether its as you say something that really doesn’t work well over IP.

One day all things like banks, telcos will allow information to be collected via IP, rather than requiring boring bloody dial-up which is so last century dahling.