[Freeswitch-users] Fax Transmit Rate - force to 9600?

Andrew asims1979 at hotmail.com
Wed Feb 28 23:39:27 UTC 2018 

Thanks Vallimamod, that gives me some additional info we weren't aware of (re: preamble and t38 max bitrate).


I noticed you mentioned disabling v17; which would allow the bitrate to increase to 14400, which is where we're seeing the problem. We want to force/limit the max bitrate under all circumstances (t38 or t30 over g711) to 9600, which we know works.

Some additional info;

Doing some further digging; the problem doesn't appear to specifically lie with t38; from what we can see some fax calls come in via t30 over g711 which also fail with a bit rate of 14400.

Given the preamble info you mentioned, could it be we're sending the sending fax our capabilities (i.e. max bitrate = 9600) and it's ignoring this and sends it across at 14400? I would assume that most, if not all faxes have the capability to send at 9600 if requested?

Given our issue relates to incoming (i.e we're the receiving end), is there anything else we can do within FS to force the lower bit rate?

cheers,
A



________________________________
From: FreeSWITCH-users <freeswitch-users-bounces at lists.freeswitch.org> on behalf of Vallimamod Abdullah <vma at vallimamod.org>
Sent: Thursday, 1 March 2018 4:34 AM
To: FreeSWITCH Users Help
Subject: Re: [Freeswitch-users] Fax Transmit Rate - force to 9600?

Hi,

>From the error message, it looks like the remote fax drops the call when it sees no match between capabilities (it's the receiving fax who sends the preamble with its capabilities first.)
According to the source code, with v17 disabled, the max bitrate over t38 is 9600, else it's 14400. So I guess you need to enable v17 to communicate with faxes with min bitrates greater than 9600.

Best Regards,
--
Vallimamod Abdullah
SIP Solutions
vma at sipsolutions.fr<mailto:vma at sipsolutions.fr>
.

On 28 Feb 2018, at 02:32, Andrew <asims1979 at hotmail.com<mailto:asims1979 at hotmail.com>> wrote:

Hi all,

We've been trying to get our fax implementation as reliable as possible using FS; we've had some great results with inbound faxes with the following items set in the incoming dial-plan:


action application ="answer"/>
action application="playback" data="silence_stream://2000"/>
action    set    ignore_early_media=true
action    set    absolute_codec_string='PCMU,PCMA'
action    set    fax_enable_t38=true
action    set    fax_verbose=true
action    set    fax_use_ecm=true
action    set    disable-v17=true
action    set    fax_v17_disabled=true
action    set    fax_disable_v17=true
action    set    fax_enable_t38_request=true
application    rxfax    /tmp/FAX-IN-${uuid}.tif

application    hangup

The one thing that we have identified with this configuration is that 99% of inbound faxes come through fine when the fax transfer rate is negotiated at 9600 bps (as a result of thefax_v17_disabled=true setting).

However, we still see quite a few incoming faxes reported as coming in with a fax transfer rate of 14400 bps, which have a 100% fail rate.

I understand that the two ends need to negotiate a rate; but how do we force (or only offer) the rate of 9600 or lower? My understanding is that the fax_v17_disabled=true setting should cover that? My first thought was that the remote end was forcing the rate at 14400, regardless of what we were sending back in terms of fax capabilities.

Not sure if it helps, but the two error messages we receive when faxes fail due to the 14400 rate are:

- Unexpected DCN while waiting for DCS or DIS, and
- The call dropped prematurely

My thought was, If we could find a way to get all faxes coming in at 9600, we'd be able to improve the reliability.

Looking through the wiki and previous list messages, the fax_disable_v17=true setting seems to be the documented approach, but I'm not sure why we're still seeing faxes come in at a higher rate.

Any advice on this or are we missing something with regards to locking in the lower speed?

cheers,
A

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