[asterisk-users] fax / t38 gateway

[Kristian Kielhofner]

On Mon, Oct 27, 2008 at 2:49 PM, Wilton Helm <whelm at compuserve.com> wrote:
> Thanks Brendan for the explanation.  There is one other idea that struck me,
> but again, I don't know if it has any merit.  My thinking is to keep FAX as
> FAX and electronic as electronic, rather than introducing a new hybrid
> approach.  Obviously Entering FAX from an electronic source is as old as the
> FAX modem, and Exiting it electronically is as old as E-FAX, not to mention
> other alternatives.
>
> Is it feasible to simply specify the codec as ulaw or alaw (depending on
> jurisdiction, I forgot the g numbers) for calls originating from the FXS or
> whatever the FAX is coming from?  Obviously, the bandwidth would be higher
> in that case, but you can't get around the laws of physics.  Yes it is lossy
> compression, still, but it is the simple, predictable form of lossy
> compression that the modem in every FAX machine already is programmed to
> cope with.  The only problems I can see would be if the provider who handles
> the call refuses to accept that codec, or transcodes it to something else.
> I don't know the likelihood of either of these.
>
> Wilton
>

Wilton,

  Many providers will "allow" you to do faxing via g711u/g711a (G711u
mu-law is used in "T" countries, G711a a-law is used in "E"
countries).  Of course they will "allow" it - fax modems talk to each
other just like we do.  They're just doing it with much less tolerance
to error and variations in the audio.  The provider's gateways,
however, should detect the fax tone and disable echo cancellation,
etc.

  What this discussion is forgetting are the issues inherent with
packet networks:

- latency
- jitter
- packet loss

  Standard fax machines communicating via some ATA with a G711u RTP
stream cannot correct for these situations.  In some severe cases. the
modems might not even be able to train.  V.x modem standards were not
designed for packet networks.  For this reason many faxes (especially
at higher speeds) will fail (depending on the state of the network)
when using a G711*, "pass-through", or "clear channel" codec.

  You will have a much higher rate of success faxing with G711u over
your LAN than a congested cable modem, for instance.

  That's what T.38 is for.  It doesn't even use RTP, it uses UDPTL
(UDP Transport Layer) or TCP (rare) to manage the transport of data
and correct for transmission errors in various parts of the OSI stack.
 As we've said before the "support" for this standard varies and often
times just doesn't work.

- G711u will fail depending on the condition of the network.
- T.38 will fail depending on the type(s) of equipment used.

  Faxing via VoIP is largely a crap shoot.  However, it is important
to focus on T.38 because I feel these interop issues can *eventually*
be resolved.  No one is ever going to "fix" the issues with packet
networks*.  That's why they are packet networks.  We will have much
better luck working towards T.38 interop.


* Obviously they are some "fixes" like MPLS, etc, but that doesn't
really help those of us trying to make do with the internet, for
example.
-- 
Kristian Kielhofner
http://blog.krisk.org
http://www.submityoursip.com
http://www.astlinux.org
http://www.star2star.com