[asterisk-users] spandsp (foip)

[Lee Howard]

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.