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.
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.
Lee.
_______________________________________________
--Bandwidth and Colocation provided by Easynews.com --
asterisk-users mailing list
To UNSUBSCRIBE or update options visit:
http://lists.digium.com/mailman/listinfo/asterisk-users
