[cisco-voip] Fax over Voip (was:Color Cisco Phones)

[Robert Singleton]

On Fri, 2007-02-16 at 12:26 -0500, Lelio Fulgenzi wrote:
> A couple of questions....
>  
> - can you not reduce the speed of the G3 faxes to a lower speed?

We are currently down to 9600 on all the machines that allow the
setting, but according to my research, simply lowering the bit rate does
not necessarily change the audio encoding requirements. Without getting
into the deep detail, the bits in a modem audio stream are encoded into
carefully orchestrated phase and level changes in the carrier tones.
Reproducing the analog waveform from end to end is critical to the
transmission of the data. The bit rate presented to the modem and the
audio requirements of the analog path are not theoretically related to
one another. Even if the bit stream was reduced to 2400 bps, the audio
path must still be able to conduct a V.34bis modem carrier with it's
complex bit encoding phase and level changes.

In practice, it seems that reducing the bit rate on the fax machines to
at most 14.4kbps can increase reliability somewhat. I think this may be
due to the error correction facilities inherent to the V.34bis standard
which, hidden from the user, retransmits data as needed. Reducing the
data feed rate to 14.4kbps probably increases the reliability by
allowing a huge bit error rate to be concealed by error correction
without overrunning any buffers.

In our case, 9.6kbps doesn't seem much more reliable than 14.4, probably
because the calls tend to be longer and thus more susceptible to other
long haul problems.

> - how about the vg224 ports? we are looking at expanding our solution
> to a remote site and it's either vg224s or fxs ports in the router. i
> just want to buy what i know will work.

Sorry to go so long about modems.

We have a borrowed IAD-2400 equipped with 12 FXS ports. This old box
does not appear to support VLANs effectively (this from someone who
should know these things; sadly that is not me) so it's hard to ensure
that QoS is in effect. Part of me says, don't worry about QoS because
the IAD-2400 and our router are one the same Catalyst 6509 switch and
may even be on the same module within that switch, we have a clear
channel DS3 from here to the CoLo where the gateways are and the router
and gateways are on the same 3560 over there. I think that if we have
any congestion problems, we would be worrying about the fax machines
later.

The appeal of the FXS solution is that with no more than a couple of
exceptions, everywhere I have VoIP phones, I have a 2811 and its rare
that I have more than one or two faxes. In those places where I do have
more than a couple, I tend to have andwhere from 8 to 17 of them, so I
will probably need another solution there. The ATAs work wonderfully for
cordless phones and paging systems and they are so easy to deploy, so
the removed units probably wont go to waste.

Robert