[Asterisk-Users] Analog modems behind an Asterisk server?

[Rich Adamson]

> Hello, we would like to hook up analog modems behind an Asterisk server, 
> and we're very interested in the experiences that others have made when 
> attempting that. We assume that there are no inherent problems with 
> modems in respect to the Asterisk software, but it appears that the 
> FXO/FXS hardware restricts this kind of a setup to lower data 
> transmission rates, is this correct?
> 
> Currently, we only transmit at 1200bps, is this rate problematic with 
> Digium cards? Up to what data transmission rate are Digium cards known 
> to work reliable? We do not think we'll ever go beyond 9600bps, can we 
> do this with a let's say TDM400P?
> 
> Will future Digium hardware improve the situation or will this stay the 
> same in the future?
> 
> How is hardware from other vendors performing when using analog modems?

The success rate of moving modem data through a digium analog TDM card 
(fxo & fxs ports) varies and appears to be somewhat related to the
exact motherboard in use. The card very very frequently has an issue
with missed data across the pci bus (card to motherboard). The missed
data negatively impacts any modem call regardless of whether its a
fax machine or pc modem. You are likely to have less then a 50% chance
of making work correctly.

Before a pile of people jump in to say "it works for me", keep in mind
that various types of modems use different modulation schemes and some
are more sensitive than others to distorted audio (missed data). In
very general terms, the higher the modem speed the more likely it will
be negatively impacted by the distortion (missed data).

If you're not familiar with modem technology, I might add there are two
primary items that are directly related to the modem's audio across
analog lines. The "baud" rate of analog signal on the wire and the
bit rate of encoded digital data. You might have a current modem that
allows you to change the bit rate (digital side), but on most modems
you have no control over the analog baud rate (or modulation scheme).
So, changing the modem's bit rate won't impact how well the modem
actually works through the TDM card.

Some people have reported that point of sale and credit card authorization
boxes have worked via the TDM card. However, the modem's used in that
equipment typically are very slow speed modems that were intended to
function in any business environment including those with noisy 
telephone lines. Those have a higher possibility of success, but
should not be interpreted as being the same as a modem used with PC's,
etc.

Bottom line... you will have far less then a 50% chance of making any
PC modem work at acceptable speed through a TDM card.

The latest code for the Sipura boxes (spa3k) appear to have addressed
modem signals (fxs to fxo). I just upgraded two spa3k's to that latest
firmware, but have not attempted to use any modem through it. Might
check to see if anyone else on the list have tried it. The firmware
was just released in the last day or two, so it might take a little
while for folks to try it.