[Asterisk-Users] FXO as modem (was: * fax reliability between ISDN PRI and FXS ports)

[Rich Adamson]

> >> I have been investigating on this list and found that faxing is not
> >> reliable between Zaptel
> >> cards and that Digium does not support nor
> >> recommend fax over the TDMXXB interfaces.
> >>
> >> Is this true ?  Will fax not be reliable enough for my customer ?
> >
> > True... the TDM card (or its drivers) has an issue with missed frames
> > that seriously impacts its ability to handle _any_ modem-type calls.
> 
> Question: There are problems using FXS ports to pass data and get high
> speed access, but can an FXO port be used as a modem? Nothing fancy
> required, 12 or 2400 baud should be ok (9600+ would be nice, but practical
> over nice). I know there is something like this with the "FAX" extension
> already. (Not sure if there is a "DATA" equivelent). Is this practical?
> Has anyone done this?

There is some sort of issue passing data from the chipsets on the card
via the pci bus to the OS drivers. The issue applies to all data
regardless of whether it originates on a fxo or fxs module. The missing
data (or missed frames) negatively impact any modem use, regardless
of whether you call it a fax machine, pc modem, or otherwise.

It's my understanding (which I've not attempted to verify) that some
folks have made point-of-sale modems (eg, credit card validation)
function, but one has to wonder how reliable that really happens to be.

The _sympton_ suggests that something unusal is happening every ten to
twelve seconds (as observed with 100% cpu utilization noted with
'vmstat 1'), and that recurring symptom negatively impacts the pci
transfer of data to/from the card. It is entirely possible the point-
of-sale modem use works if the actual data transfer falls within that
ten second interval (given that point-of-sale transactions are typically
very small short-duration bursts of data).

Although analog voice is also impacted by the issue, the human ear
does not recognize the resulting distorted audio.

So, the long/short of it is that a modem signal passing through this
card will not result in anything that anyone would consider usable.