[Freeswitch-users] Successful faxing with spandsp / T38 and stats

Mitch Capper mitch.capper at gmail.com
Wed Jun 27 05:01:48 MSD 2012

Hello everyone,
I have been working on faxing for awhile now and figure I would post
back with some results and method.  Much of my method I have
documented on the wiki about faxing but some more details on my method
are also below.  Faxing overall is a horrific thing and when I first
looked into VOIP faxing I did not think I would be able to achieve
nearly the success rate I have been able to with FreeSWITCH.

These stats were collected over 1000 individual faxes and over 4000
pages sent to over 500 unique fax numbers.   I would guess a lot of
these numbers are geographically clustered around the northwest of the
US but I doubt that should matter a whole lot.
Up until last week I had but 1 fax that didn't complete (it was an 80+
page fax that failed twice mid-fax before we just emailed it so it
probably would have completed with some extra time).

In the last week I have had two I have yet to get through (not sure
why or if I have finally found fax machines I just can't send to or
what have you).   I was blown away with the current success rate.  I
did not collect as detailed stats the entire time I was logging data
(probably only 3/4's+ of the total faxes have full detailed stats).
With all that said the below numbers should be decently reflective of
real world use.

First on the method:

I try a progressive retry system,  I try one method if it fails I wait
60 seconds then try the next.  I use two providers, flowroute and
t38faxing.com.   Flowroute I highly recommend they are a great
provider and have worked very well for faxing.  T38Faxing.com / EZCALL
are quite bad and I highly recommend avoiding (see bottom of email for

My 8 methods are: T38,ECM / ECM / T38_DISABLED,ECM / T38,SLOW /

Slashes delimit the method and commas delimit the flags for that
method.   Slow=fax_disable_v17,  T38=fax_enable_t38_request,
T38_DISABLED=fax_enable_t38=false  BACKUP=T38Faxing.com (T38faxing.com
fails if t38_request is enabled so thats why T38 is not used with

If the line is busy I don't move to the next method nor if there is a
channel error.

The methods are tried in order they are not tried randomly.  Randomly
would make the below stats better but I care about fax through a fast
as possible so I try in order of what I thought had the best chance of

average secs per page: 28.055
average tries until success: 1.33 (not including busy).

Method Usage Rates:
        75% T38,ECM success rate: 82% avg secs/page: 27.4
        13% ECM success rate: 46% avg secs/page: 29.5
         7% ECM,T38_DISABLED success rate: 60% avg secs/page: 28.3
         2% T38,SLOW success rate: 56% avg secs/page: 45.7
         1% SLOW,T38_DISABLED success rate: 57% avg secs/page: 40.6
         1% ECM,BACKUP success rate: 57% avg secs/page: 35.8

Failure Reasons:
        40% The call dropped prematurely
        25% No response after sending a page
        11% Invalid response after sending a page
         6% Invalid ECM response received from receiver
         5% Received no response to DCS or TCF
         2% Unexpected message received
         2% Timed out waiting for initial communication
         2% The HDLC carrier did not stop in a timely manner
         1% Received bad response to DCS or training
         1% Disconnected after permitted retries
         0% Timed out waiting for the first message

Hope you enjoy the method and stats, honestly with this rate of
success and either a few better backup providers or even a single hard
line for faxing over you could run a very respectable faxing service
using FS.


As I mention on the wiki certainly one very important thing other than
trying various methods is quality providers.  Flowroute handled 99% of
the faxes above obviously and did a great job.     T38Faxing however I
cannot recommend avoiding more.  I used them as my backup provider
after surveying #freeswitch a bit and while I normally try to avoid a
brand bash it has just be a horrible experience.   They have a very
poor backend which would be livable if their support was not equally
poor.  They frequently don't answer the phone and their support email
system has been down for awhile before (emails bounce) along with
their control panel for their website.   Their entire faxing service
wasn't working for over a week when contacted they said they were
adjusting their north american routing and it took over a day before
it was back up.   So overall I would suggest finding a different
provider from t38faxing.

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