[Asterisk-Users] T.38 fax summary

[Mark Eissler]

On Feb 25, 2005, at 10:20 AM, Lee Howard wrote:

> 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.
>

Sort of...but no. Fax requires a codec that supports the frequency 
spectrum of a POTS audio channel. Currently, that means that anything 
other than g.711 won't work since the other popular codecs achieve 
their efficiency by dumping frequencies humans can't hear (just like 
mp3). The problem isn't typically g.711 because that's the codec that 
is generally used by the digital telco world. A common problem when 
discussing g.711 often is packet size vs bandwidth limitations. T.38 
can alleviate this problem because it doesn't rely on a codec.

The bigger problem with faxing over VOIP is related to lost packets and 
timing issues (jitter). Lost packets are the death knell for fax 
because it isn't very tolerant of missing data. How do you complete an 
image with missing data??? AFAIK T.38 can't do anything to recover from 
packet loss...the fax machine needs to be tolerant of it. Ironically, 
ECM was introduced to recover from information loss when transmitting 
faxes over analog lines but ECM can actually cause problems when used 
with T.38. If you can turn ECM off that's the best thing to do when 
using T.38. Besides lost packets though if you have to consider packets 
arriving at weird timing intervals (jitter).

The fax machine needs to get its data in a steady stream. This is 
supposed to be a realtime transmission after all. While T.38 can absorb 
some of the problems triggered by latency and jitter, when the problem 
becomes too excessive it tanks just as quickly as faxing without T.38.

So with those barriers out of the way what is it that T.38 tries to 
accomplish? Instead of sending a fax over VOIP as a stream of sampled 
audio, the protocol intercepts the audio at the endpoints and 
packetizes it as blocks of data instead. The receiving gateway must 
know how to handle the data stream so it can convert the fax back into 
a T.30 fax data stream for POTS. During the session, progress is faked 
so that the two fax machines don't think the transmission has 
stopped...that's a crucial step because it takes time to convert and 
send/receive the fax reliably.

I think the best arsenal for faxing over VOIP today is to have a good 
broadband connection, g.711, and a fax machine where YOU can set the 
max transmission speed. Sadly, the last part seems to be missing quite 
often. I've noticed that HP actually mentions faxing over VOIP in the 
documentation for their 7410 all in one machine and, more importantly, 
they include support for changing transmission speeds. Way to go HP!

-mark

--
Mark Eissler, mark at mixtur.com
Mixtur Interactive, Inc. - at - http://www.mixtur.com