[Asterisk-Users] T.38 fax summary

[Lee Howard]

On 2005.02.25 05:53 Rich Adamson wrote:
> Steve Underwood,
> 
> Would you mind summarizing where/how T.38 functions, and maybe how it
> compares to the analog fax environment for the asterisk-users arhives?

I don't mean to speak for Steve, so I hope that Steve will still reply 
if he chooses to, but I like the question, and since I know enough 
about T.38 and fax to answer at least in a general sense, I will.

In a traditional analog fax you have modulated audio data, that is, the 
data stream is converted into an audio representation by the 
transmitter, and the receiver demodulates the audio stream to produce 
the data stream.  A lot of data gets packed into very small portions of 
audio, which is why fax over VoIP (T.38 is not VoIP, it is FoIP) is 
unreliable - any jitter will likely cause data loss.

There are no modulators in T.38.  So take the fax procedure, but 
instead remove the data modulation/demodulation part.  T.38 devices 
communicate raw data through the IP network, and the IP network is as 
good at communicating data as the PSTN is as good at communicating 
audio.  So if you could have a full T.38 delivery route from fax sender 
to fax receiver, the data never once gets converted into an audio 
signal - it doesn't need to be.

That's the gist of things.

Lee.