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

Mark Tinka mark.tinka at
Sat Mar 1 21:39:27 UTC 2014

On Saturday, March 01, 2014 10:20:29 PM Eric Tykwinski 

> 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.

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.

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