[Asterisk-Users] Qs about FXO/FXS cards

[Rich Adamson]

> I have been wondering if the spa 3000 would make a good PSTN interface
> for an * box where POTS is the only available (or practical) service.
> Have you implemented this? Are there any limitations or known issues?
> The SPA2000 sure seems to work well as an ATA, even had good luck with
> fax over IP using g.711 and the fax detection in zaptel and the SPA
> (turns off echo cancel dynamically when the CNG tone is heard I
> believe).
> 
> Can you use the FXS and FXO ports at the same time, for two separate
> calls via * ?

Yes, it works fine.
 
> The SPA 3000 is small enough that a half dozen of them would be
> manageable, any more than that and your are usually in the T1 price
> range for service anyways.

The down-side to the spa3k is that its rather difficult to configure 
since they've provided so many different config options and their user
manual does not address much beyond a basic config.

For my home use, I inserted the spa3k into the pstn line in such a way
as to avoid remedial spousal training. :) All house phones are attached
to the fxs (line 1) port.

This specific config supports:
- all outgoing fxs -> pstn calls are passed through to the fxo port
  without asterisk being involved. (eg, avoids 911 and training issues)
- all outgoing fxs calls prefixed with a "8" are routed to * for
  completion (regardless of what follows the 8).
- all outgoing fxs calls matching "3xxx" (* extensions) are routed to
  * for completion.
- the fxs port was configured to register with *, and therefore the fxs
  port is also an exten from asterisk's perspective (eg, * exten's can
  dial the fxs port)
- distinctive ringing is implemented as:
   - incoming pstn calls (via fxo port) ring normal
   - incoming voip & * calls use a distinctive ring
- the fxo port was configured to register with * (different creditials
  from the fxs port), and therefore asterisk can place calls through
  the spa3k fxo port.
- I specifically did _not_ want * involved with incoming pstn (fxo)
  calls (in this case), but rather wanted those to ring through to the
  fxs port directly. (Avoids complaints when care and feeding *)
- the spa3k is running v2.0.11(GWg)

The only difference between the above and using the spa3k as an inbound
pstn gw is the definitions for the pstn (fxo) port. If you dig through 
the voxilla.com forum postings, you'll see where that also has been
implemented. However, that fxo -> * connection is even less clear. I had
that working several months ago, but that wasn't my specific objective
so I didn't attempt to document it.

The problem with most of these external gateways (regardless of vendor)
is that it almost requires a knowledgable person with a packet sniffer
and a lot of trial & error mucking around to find the appropriate
combination of parameters to accomplish a specific task. In my specific
implementation above, the key turned out to be extensive use of dialplan
strings that were not very well documented.