[asterisk-users] fax / t38 gateway

[Jonn R Taylor]

This is caused by your DSL/cable modem buffering the data internally. To resolve this you need to feed the modem at a data rate that prevents it from buffering. I solved this by using a shaping bridge server that does nothing but control the data in and out of the internet connection. I have steady 21ms ping times to the server that my RTP traffic comes from.

I do faxing over VOIP every day and it works! But it only works if you have a stable internet connection with an SLA, which I have. Also having an ISP that only uses a pure IP network makes a big difference. Some DSL providers use ATM to transport IP and this causes high packet loss when their network is busy.

T.38 is a great idea but we need large scale implementation of it, with standards for it to work.

Here is my hylafax log from last week. I removed the numbers for privacy reasons.

Facsimile received since last week:

       Pages     Time Pg/min Errs TypRate TypData
          1     3:17    0.3    0   14400 2-D MMR
         26    16:55    1.5    0    9600  1-D MH
          2     0:46    2.6    0   14400 2-D MMR
         13     5:13    2.5    0   14400 2-D MMR
         14     5:48    2.4    0   14400 2-D MMR
          5     2:33    2.0    0   14400 2-D MMR
          1     0:28    2.1    0   14400 2-D MMR
          1     0:31    1.9    0   14400 2-D MMR
------------------------------------------------------------------------------------------
Total                                              63    35:31    1.8    0

-----Original Message-----
From: asterisk-users-bounces at lists.digium.com [mailto:asterisk-users-bounces at lists.digium.com] On Behalf Of JD
Sent: Tuesday, October 28, 2008 2:20 PM
To: Asterisk Users Mailing List - Non-Commercial Discussion
Subject: Re: [asterisk-users] fax / t38 gateway

Gordon Henderson wrote:
> but it's very do-able, given good Internet connections.
[...]
> I think your statements were just a bit too strong - I agree 
> wholeheartedly about the V. protocols and copper, but I've found in 
> practice that faxing over IP is not just theoretically possible, but quite 
> do-able, and is someone wanted to send me a fax (I have no fax machine, 
> they're so 80's :), then I'd not hesitate to setup a VoIP DID and give 
> them the number...
>
> Gordon
>   

Fair enough. However, my experience has been that defining "good 
Internet" for faxing over G.711 is not entirely self-evident.

I've seen some really lagged environment work with FAX over G.711 like a 
dream. And some fast ones break badly:

While I don't have full explanations for this, it _seems_ like the 
following holds true:

good ping time sequence for faxes--no jitter (all times in ms):

   40, 40, 40, 40, 40, 40, 40, 40...

A bad one -- high jitter low latency:

  5, 10, 5, 20, 18, 7, 12, ...

A _really bad_ one -- "shifting" jitter:

  20, 20, ... , 20, 20, 30, 30, 30,...

No provable explanation as to why.

John

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