Fax T.38 troubleshooting guide

Faxing is an application that relies on a variety of parameters and can sometimes become tricky to troubleshoot.  This article covers common cases to help you troubleshoot why your faxes are failing.

Common Failures

• No T.38 re-Invite initiated
• Negotiation/training issues
• Failures in image transmission
  • Corrupt pages
  • Incomplete/partial images
  • Missing pages

Identifying the problem

The most important piece of information in resolving a faxing issue is to understand what the problem is. Here are some pointers that will guide you in the right path:

Payload / Transport Method

Faxes are transmitted over IP using T.38 or G.711-pass-through.  T.38 is an ITU-T recommendation to transmit fax data over IP networks. In general T.38 is considered a more robust transport method for faxing thanks to its redundancy controls to mitigate network performance issues such as packet loss.

The other method to transmit fax is G.711. In this case fax data is transported in-audio. G.711 although less robust is a simpler approach and a good option to overcome T.38 interoperability issues.

One of the most common faxing issues that the sending and receiving machine support different transport methods, one G.711-passthrough and the other T.38. To solve this issue you can opt to transcode the call using a VSXi MST3 or MBT. 

Direction

Is faxing failing in one (inbound, outbound) or both directions? A lot of times only one direction fails due to how faxes are negotiated. The receiving machine often times sets these parameters and this explains why a fax may only fail in one direction.

Frequency

Is the problem reproducible always or just some times? Around 8% failure rate or below is considered normal for faxing over SIP trunks.

Always reproducible issues are likely related to a signaling and SDP negotiation problem. If your faxes are always failing it is mandatory to first look at the signaling and the existence and success of re-INVITEs. If re-INVITEs are being responded to and acknowledged it is worthwhile to look into the SDP of the INVITE and 200 OK.

The T.38 protocol is sensitive to network performance factors such as latency,  jitter and packet loss. If you get inconsistent results your fax problem could be related to a network link issue.

Phases of a faxing

Understanding the different phases during the transmission of fax is important to understand and resolve the problem. There are five phases:

• Phase A- Call Setup(Call Establishment)
• Phase B- Negotiation and Training
• Phase C- Image Transfer
• Phase D- Post Message Procedures
• Phase E- Releasing the Call/Fax

 

Phase A: Call Setup

During phase A the fax endpoints will attempt to initiate the process by first detecting the CNG and CED fax tones. The detection of CNG and CED happens before re-INVITEs take place and the main purpose of it is making sure that the receiving end is indeed a fax capable device. If the fax machines fail to detect the CNG the re-INVITE will not occur.  This would indicate that the CNG tone is distorted, the signal is not strong enough or the device is out of spec.

If the T.38 re-INVITE took place problem area will likely be on phase B.

 

Some devices that are typically fax software solutions instead of hardware will re-Invite themselves to T.38 instead of waiting on the receiving side to do it.  They should wait until the receiving side sends the CED tone in response to the CNG tone before doing so.  This is typically fine unless there is a re-Invite collision, although this will typically work itself out.  

 

Phase B: Negotiation and training

During this phase the fax devices exchange their capabilities. 

The receiving device sends Digital Identification Signal(DIS) describing its capabilities. The sending device sends Digital Command Signal(DCS) confirming what it wants to do based on the receiving devices capabilities and its capabilities. These capabilities/settings include:

• Data rate: Speeds of the fax transmission include 14400, 9600, 4800 and 2400.  Lowering the speed can sometimes increase reliability or allow success (e.g. from 14.4k to 9.6k).
• Image resolution
• Error Correction Mode/ECM: If faxes are failing, disabling ECM may fix the issue.
• Encoding
• Page size

The calling fax machine will send training frame of zeros to check that the negotiated speed is ok(Training). The called fax machine will send a CFR(Confirmation to Receive) if the training passed.  If the training does not pass they will negotiate a slower speed.

 

Phase C: Image transfer

In this phase the sending device sends image based on agreed capabilities in the previous phase. Below is an example of data transmission using ECM at a speed of 9600.  You can tell its ECM based on the hdlc-data.

 

An example of non-ECM data transmission at a speed of 9600 is below.  This is easy to tell it says non-ecm-data.

 

Phase D: Post message procedures

If you have gotten this far things are looking good. On this phase and beyond only a fax failure is most likely the result of a network performance issue especially if it's a multi-page document. Some of the messages to look for include:

• Message Confirmation (MCF) - Page received.
• Multi-page signal (MPS) - Return to phase C.  More pages to transmit.
• End of Procedure (EOP) -  Done transmitting pages.  Go to Phase E.  There will be a MCF after this as well to confirm they received the last page.

Some of the error messages you may see in this phase include: Retrain Negative (RTN) if the last page was too damaged to be used.  RTN indicates to re-train at lower speed and resend.

Another error message is Retrain Positive(RTP). RTP indicates that there were some errors with the page, but its acceptable.  The sending side can either re-train/re-transmit or move on to the next page. 

 

Phase E:  Release the call

Are you still with us? This phase is straightforward and will be led by the Disconnect (DCN) message indicating that there's no more fax data to be transmitted.