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

[Christian Schnell]

Rich Adamson wrote:
>>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.

Since we will assemble the Asterisk hosts ourselves we'll have full 
control over the hardware components, including the motherboard. So, if 
we choose carefully, can we expect stable results?

> 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.

Our modems are intended for use on noisy lines, that's also the reason 
why we stick to such low data transmission rates (1200 baud, 1200bps), 
we're really not planning to use anything that transmits more than 
9600bps. We've successfully bridged calls across various ISDN a/b 
adapters and also across Voip/FXS-Gatway adapters (tiptel innovaphone 21).

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

Hmm, I think I'll give it a try once I find stable hardware components.

> 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.

Thank you very much for your insightful reply,
Christian.