Lee Howard wrote:
On Sun, Sep 24, 2006 at 01:58:21PM -0700, Christopher Corn wrote:
A couple of faxing methods im confused about.
The pass through method, sending fax data over G711 codec
versus
Relay method, t30 to t38 conversion
Can someone explain to me why the pass through method doesn't
require t30 to t38 conversion ( or does it do it?)? i believe
the conversion to t38 is so that it can be routed through a
packet network and then back to t30 so that the fax machine can
understand. why is it that if you use a pass through method, and
your still passing through a packet network, you dont need to
convert to t38 and t30?
Be careful about your wording. People here generally refer to "pass
through" as T.38 pass-through and not G.711 pass-through.
I think that if you understood how faxing works you would see that
your questions here don't really make sense.
In traditional PSTN faxing you have a total of two endpoints
performing T.30 protocol. In a simplified form, the sender takes
scanner image data and modulates it (into an audio waveform) and then
passes that audio over the PSTN to the receiver which demodulates it
(takes the audio and turns it into data again). As long as the
demodulated data is identical to the original data, then everything
should be okay... for the most part. However, if you start to
consider audio corruption on the PSTN, then that's where difficulties
start to ensue. If you have some audio, modulated data, and then you
compress it or fracture it or otherwise corrupt it, then there's no
possible way that the demodulator is going to be able to come up with
the original data.
Now introduce VoIP telephony... where a small amount of audio
corruption (jitter) is anticipated on the UDP channel... and mix it
with faxing and hopefully you can see how it just doesn't work well.
VoIP is packetized audio passed over an IP network. Packetized audio
is nothing new. ISDN circuits have had it for a long time now. Those
circuits are digital - meaning the audio waveform is digitized at 8000
Hz... so the audio is represented with bytes and are packetized into
frames. Those traditional digital circuits are designed to prevent
any loss of that data. VoIP works similarly, except that the medium
is lossy UDP/IP networking.
ISDN doesn't packetize voice. ISDN is a strict circuit switched TDM system.
Since VoIP works on *IP* networks, and since IP networks already
handle data communication very well, there really is no reason to
perform the modulation or the demodulation - just send the raw data
through. So that's basically the punchline of T.38... it's fax
protocol without the traditional modems involved. Then you have FoIP.
However, these days the world is a hybrid of VoIP and PSTN
environments (mostly PSTN still), and thus anyone using T.38 will need
to have a "gateway" involved somewhere along the call path that can do
that traditional modulation/demodulation. That is what the T.38
gateway is. If a T.38 relay does not act as a gateway (i.e. no
modulators) then it performs only T.38 pass-through - meaning it only
is useful for situations where calls are end-to-end T.38 or where an
external FoIP service provider is used.
Because of the way things work T.38 gateways will not only need to
have traditional modems (hard or soft) but will also need to perform
T.30. So when faxing with T.38 and the call is not end-to-end T.38
then you have at least three points along the call path performing
T.30 (versus the traditional scenario of just two).
So, to answer your questions...
Why does using G.711 not require T.38? Because from the viewpoint
that the question was given, G.711 and T.38 are competing approaches.
T.38 was designed to replace G.711. You can packetize G.711 audio
just fine without converting it to anything else. So when faxing with
G.711 T.38 is not involved because its basically mimicking the
old-style traditional PSTN faxing, except that the audio is passing
over a different (less-reliable) medium.
So the reason that T.38 exists is because UDP/IP is lossy and is not
therefore reliable for the purposes of faxing with G.711 unless the
communication can be guaranteed to be nearly lossless. For those that
work on lossy channels, G.711 will just not work reliably.
Lossless channels are only a part of it. If you look at
http://www.soft-switch.org/foip-with-real-atas.html you will see
examples of other problems that happen with a wide range of ATAs. Once
that have FAX support modes, yet cann't possibly ever work with FAX.
Steve
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