Any experience with Comcast digital voice for OOB (offlist is fine)


Eric Tykwinski
TrueNet, Inc.
P: 610-429-8300
F: 610-429-3222

You're asking if a VoIP link could be used with traditional modems to do OOB management?

I'm pretty sure the answer is a flat no: any modems faster than 1200 bps
depend on phase change modulations that VoIP codecs can't begin to deal
with; FoIP is implemented by spoofing: the TA detects CNG tone, and spoofs
a datalink.

I don't know of any TAs that will do that with regular data modems.

-- jra

I've had problems with DTMF originating from comcast voice in the past (going into t1/pri from xo terminated on Cisco-ISR with voice modules).

Was a pain to troubleshoot. I would be interested to hear your results, much depends on how they implement the service.

- Jared

As others have said modems require POTS or at least a PBX line. Also isn’t the hand-off fog VoIP ethernet? You wouldn’t be able to stick that into the RJ-11 port in the modem. It would be easier to use the comcast internet connection with some sort of IPsec tunnel for OOB. It’s cheap and mostly reliable.

If you’re looking for a better solution see the thread on OOB gear RE: opengear. They are multi-port and support, POTS, wifi and 3G for access.

Thanks all,

Jared, sorry I forgot about out-of-band touch tones and should of specified better, the client was looking to use a modem like most guessed.
I suggested using a cellular option since POTS wasn't available, as most gear usually has that as an option, and it looks like US Robotics makes a serial connection modem at that.

I do remember though something about a modem over VoIP protocol being developed, something like Jay was saying about Faxing over VoIP, but I guess it never took off. My guess being relying on the same line as an internet connection would be about that smart anyways.


Eric Tykwinski
TrueNet, Inc.
P: 610-429-8300
F: 610-429-3222

FoIP (Fax over IP) is "dodgy" for a couple of key reasons:

  a) Different call capabilities, during setup, of SIP
     and T.30, where a portion of the segment is TDM.

  b) Different FoIP modes, during setup, where one end
     uses SIP and the other uses T.38, with an
     inability to propagate these capabilities across
     the circuit.

  c) Signaling delay between initial INVITE and 200
     OK, introduced due to SIP-SS7-SIP conversion.

  d) TDM-IP-TDM conversions causing incorrect training
     across the circuit.

  e) Low-rate codecs in the transit path that are not
     signaled to the end points, e.g., end points
     running at g.711 while an intermediate device
     runs at g.729.

My guess is modems would have the same issues.

In general, FoIP solutions have a better chance of working
if a circuit provisioned to transmit a fax is all-IP (or, at
the very least, limits the TDM-IP conversions).

If all else fails, scan and e-mail.