[Asterisk-Users] FXS with v.90 modem support?

[Steve Underwood]

Dustin Wenz wrote:

> I have a couple of devices that need an analog modem to communicate  
> outside of our Asterisk system. Most FXS gateways don't seem to  
> support this... I have a stack of Sipura 2002's that are, AFAIK,  
> worthless for this purpose.
>
> I've heard that Digium's IAXy FXS will work with modems, but I can't  
> find any reference to that in their documentation. There is also the  
> VegaStream Vega 50 that claims to support v.90 on 8 FXS ports per  
> unit. Does anyone have experience with these devices, or can  
> recommend anything else?

Then important things here are that you must only digitise once, and 
that the path must be clean and free of timing slips. Things like

modem <-> FXS <-> asterisk <-> FXO <-> PSTN

will not work. The signal is digitised, returned to analogue, then the 
PSTN will digitise it again. In general VoIP boxes of any kind will not 
work, as they do not guarantee a clean path. What should work is

modem <-> FXS <-> asterisk <-> E1 or t1 <-> PSTN

However, a lot of people have problems getting even this configuration 
to give clean enough results for V.90 to work.

Steve