[asterisk-users] "Legacy" analog data modems and Asterisk

[Erik Jacobs]

I did some poking around on the Googleweb and was unable to find a concise
answer to my situation.  I have some guesses and some theories about what
will work and what might not work, but I'm sure that others have followed
this path before.

Currently we have a large number of customers that we support via analog
modem.  To make a long story short, it's very difficult for these customers
to be able to provide VPN access into their networks of our hardware due to
various security and large-company bureaucracy issues.  Therefore, the modem
connections remain.

We are considering an Asterisk-based PBX for an upgrade to our existing
Panasonic DBS72, which is a fine system but simply doesn't cut it for the
things we need to do.  However, this poses the problem of what to do with
the modems.

Preface the following with this:  We have *0* desire to terminate calls via
IP.  We're using Asterisk for the ease of adding phones locally and
remotely, not because we want to save money via IP calling (which would be
improbable, as our 6 PSTNs have unlimited local and long distance + DSL).

Options (in no particular order):

1) Connect Asterisk to existing 6 PSTN lines using FXO.  Connect existing
modems to Asterisk using FXS.  Data speeds will probably be sub 14.4k, which
is not acceptable.

2) Upgrade PSTN to PRI.  Connect Asterisk to PRI and connect modems to FXS.
Anyone have an idea about the potential data speeds here?

3) Connect Asterisk *AND* modems to PSTN using splitters.  Does anyone know
what happens if someone is using a PSTN with the modem and Asterisk tries to
use an FXO?  Is Asterisk "smart" enough to detect that the PSTN is currently
in use?  Or is it like your little sister and it will pick up the phone
while you're dialed into a BBS and knock you offline.... (ahh, those were
the days).

4) We make PPP connections to our customers with the existing modems (for
the most part), so I'm not sure that there would be any way to somehow hook
the modems up to the Asterisk box and have the Asterisk make the connection.
This would very likely involve some extraordinarily complex routing tasks
and, as we're looking to a 3rd party Asterisk PBX provider, I don't think
we'll have the access to the guts of the hardware to do this.

5) The most simple and least elegant -- unplug the phone line you want to
use for modem from the FXO and plug it into your modem.  Que sera, sera.

Sorry that my first post is a huge plop, but it's an interesting situation
that I've been going back and forth about for a while.

Plus, Asterisk sure beats a $20k Altigen setup.

Erik Jacobs
Project Engineer
erik.jacobs at crystalcc.com