Fax transmission level too high?

I’m having ongoing difficulties sending and receiving faxes reliably. I have medical practices regularly faxing us and having calls drop mid-fax for no obvious reason I can discern. “fax show stats” gives me the following (presumably this is just stats since the last restart):

FAX Statistics:
---------------

Current Sessions     : 0
Reserved Sessions    : 0
Transmit Attempts    : 1
Receive Attempts     : 11
Completed FAXes      : 12
Failed FAXes         : 9

Spandsp G.711       
Success              : 0
Switched to T.38     : 1
Call Dropped         : 0
No FAX               : 0
Negotiation Failed   : 0
Train Failure        : 0
Retries Exceeded     : 0
Protocol Error       : 0
TX Protocol Error    : 0
RX Protocol Error    : 0
File Error           : 0
Memory Error         : 0
Unknown Error        : 0

Spandsp T.38        
Success              : 3
Call Dropped         : 3
No FAX               : 0
Negotiation Failed   : 0
Train Failure        : 0
Retries Exceeded     : 6
Protocol Error       : 0
TX Protocol Error    : 0
RX Protocol Error    : 0
File Error           : 0
Memory Error         : 0
Unknown Error        : 0

I’ve discovered one of our phone companies provides a fax-back test service. You send it a fax and a few seconds later it sends back information about your fax.

Sending and receiving to this service has never dropped out. The results I get back tell me:

Overall performance:          Poor
Transmission level:           Poor
Training signal quality:      Excellent
Transmission speed:           Excellent
Facsimile image transmission: Poor

The recommendation says:
Your facsimile transmit level is 5.7 dB TOO HIGH. Please contact your supplier

Other information it gives is that there was 1 “unknown G3 code line”, 4 “empty image lines, excl RTC” and 699 “unfilled scan lines”. All of these it says should be 0.

Is any of this relevant when faxing through Asterisk? Does it tell me anything useful to help me understand what’s failing? Any guidance greatly appreciated.

Where is the SIP to ISDN conversion taking place?.

I didn’t think there’s any ISDN involved here but honestly there’s a lot I don’t understand, especially when it comes to T.38 faxing. I mean, doesn’t T.38 bypass the whole modem scenario? In which case dB don’t seem very meaningful?

That’s the point. The problem is occuring where the modem is being simulated, which will generally be at the ISDN to SIP boundary. I was trying to confirm that that was at your ITSP.

Typically a fax machine, on the public network, will connect analogue to the local exchange, ISDN to the ITSP and SIP to the VoIP user. If that describes your topology, any excess signal problem lies with the ITSP.

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Okay… you might need to dumb this down for me a bit. I have several SIP accounts with a provider, one of these I’m using exclusively for sending and receiving faxes. If I’m understanding you correctly I’m sending a digital stream from my asterisk box to my SIP provider; that is then getting converted to an audio stream and relayed into the public phone network - and at that step the signal is too loud? So the problem might not be in my setup, but an issue with my provider?

If you’re using a connected fax machine, I recommend you check the baud rate is set to 9600.

"Why you need to lower the baud rate to 9600.

Faxing over VoIP can present certain challenges which require a slower transmission speed, so that most of the data get to it’s destination. Packet loss, jitter, and latency all add up to corrupt a fax transmission over VoIP. When a fax machine determines that data is lost, it will shut down the transmission, even though with some bits of data lost you would still have a workable document. By slowing down the transmission speed to 9600, you increase the probability that most of the data will make the trip."
https://www.voipmechanic.com/voip-fax-settings-9600-baud.htm

I’m sending and receiving directly into asterisk with spandsp. I’ve tried dropping the transmission rate to 9600 but it does not improve my success rate. The test fax service mentioned above shows I’m getting an error rate of 0 bits in 100,000 at 14400bps.

@garyg: T.38 is a mechanism to get round the limitations of VoIP, by transmitting the underlying data in digital form.

I did a whole bunch more testing that was increasingly confusing … and then I found a document that suggests this whole thing has been a wild goose chase and it’s just that the fax test service I was using is too “sensitive”. http://www.bnsgroup.com.au/bns-group/helpdesk/knowledgebase.php?article=169

I guess the cause of my failed faxes is something else. Back to the drawing board!