T.38 Fax Failures? Let’s Troubleshoot with Tim!

Today we’re handing over the blog to Tim Klein, a Technical Service Representative II here at VoIP Innovations. He works on the front lines assisting our customers and is a wealth of information. Take it away, Tim!Tim Klein, a Technical Service Representative II here at VoIP Innovations

T.38 faxes can fail for several reasons, but this post will touch upon the more common ones.

Since the faxes are sent across the public internet, if they were to traverse a major backbone that was having issues, then this could cause the faxes to fail. Anywhere from latency to packet loss can add to these failures. Familiarizing yourself with the errors can give you an idea as to what the issue is and where to act.

The HDLC Carrier Did Not Stop in a Timely Manner

This means that our fax server initiated a fax transmission with the far end, but there was a timing error that could not be resolved. This happens when the far end sends their Digital Identification Signal indicating their fax capabilities, such as data rate. Our fax server would then respond with our Digital Command Signal indicating our transfer rate. Usually faxes will start at 14,400 which is the Maximum bits per second speed. If that is too fast for the far end, then the appropriate response would be a Failure to Train. Our fax server would then negotiate a slower speed, but if the Term End does not respond then the end user would receive an error that the HDLC carrier did not stop in a timely manner.

  1. Timed Out Waiting for Initial CommunicationThis error indicates that our fax server is waiting on the far end to send its initial Digital Identification Signal to continue the T.38 negotiation. The fax cannot proceed any further until the far end sends its signal first. If our fax server does not receive the signal, we will make attempts to talk to it by sending a CNG tone. Or, we would simply put a fax calling tone wherein our system would be playing fax tones trying to communicate with the far end. If we do not receive anything, our fax server would then tear the call down, and the end user would receive the error in their inbox.
  2. Invalid Response after Sending a PageThis error indicates the far end did not respond with the proper message to continue the fax flow and the fax was terminated. An example would be our fax server sending an End of Procedure message indicating there are no more pages to be sent, and the Term End also responding with an End Of Procedure in lieu of a Message Confirmation, indicating the last page was received. Below is a screen shot from a Wireshark packet capture illustrating this behavior. As you can see the arrow pointing to the right is our fax server sending the EOP, and the arrow pointing to the left is the Term End responding with the same thing; thus, causing the fax error.

Email to Fax

  1. Disconnected after Permitted RetriesThis error occurs when our fax server is trying to negotiate speeds with the Term End. When we start out at 14,400 bits per second, if that is too fast the Term End should respond with a Failure to Train. If they do not, and they keep sending us their receiving rate, the fax will fail with that error.
  2. Far End Cannot Receive at the Size of ImageThis would indicate the attachment is greater than the standard 8.5 x 11 document size. You can check this in Adobe Reader by clicking File->Properties->Page Size. Oftentimes scanned documents will exceed these dimensions which can cause failures with the Email-to-Fax service. To overcome this potential problem, you can use a third-party program such as Cute PDF to re-save the file before attaching it. Cute PDF has a default size of 8.5 x 11 so the only thing you are doing when using it is shrinking the document you have to fit the standard paper sized used in most fax machines.


T.38 fax failures are not limited to these errors. Troubleshooting fax failures can be difficult due to how faxes are set up. All faxes start as voice calls until fax tones are detected. Once these are detected the negotiation process begins for T.38 and if any packets were to break down during this process can cause the faxes to fail. If this were to happen, you should check the number you are faxing to ensure the far end is indeed playing fax tones. You can confirm this by placing a test call to the destination. If the far end is playing fax tones make note of how long into the call the term end starts to play these tones. If the term end is taking quite some time, then that can be the cause of the failures. This is because the sending side is waiting to hear fax tones, but if it is taking the far side too long, then the sending side may disconnect the call. If that is the case then the Term party would need to investigate their equipment.

Customers have seen an increase up to 98% success rate when using our Efax service as we recently turned up a new vendor. Using our Efax service allows our support team to pull the necessary Wireshark packet captures in the event of a failure to quickly identify where the issue is originating from. We can then advise the customer the necessary steps to resolve the issue.

Hopefully this post gave you more insight to the Email to Fax service. If you ever need assistance with anything at all, please give me or one of my fellow Technical Support Representatives a call!

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