Our RightFax support exists to not only help keep your RightFax server healthy and operational, they also seek to teach you as much about faxing as possible. As our latest support tip, we are focussing on of the basics: how fax transmissions work.
Why are we going back to the basics? Because there are many errors that can occur during a transmission, and understanding the process can often help you troubleshoot and fix an issue.
How Do Fax Transmissions Work?
There are five phases of a standard fax message, and different errors can occur at each phases.
Phase A: Call Establishment
This is the stage where the fax machine will identify itself as a fax machine. The calling fax machine will use a tone called a CNG (Comfort Noise Generation) to let the device that answers know it is a fax machine. The receiving fax machine will then answer with a tone that alerts the sending fax machine whether or not it is able to receive faxes at V.17 or V.34 speeds.
Phase B: Pre-message Procedure
Once the fax speed is determined, the two machines move on to the next phase where the receiving fax responds with two pieces of information: CSI (Call Subscriber Information) and DIS (Digital Identification Signal). This information identifies who owns the fax machine and what its capabilities are.
The calling fax machine then responds with it’s own information in the form of a TSI (Transmitting Subscriber Identification) and DCS (Digital Command Signal). Finally, the calling machine will send a test in the form of a TCF (Training Check). If the test passes, the receiving machine will send a CFR (Command to Receive). If it fails, it will send an error message in the form of an FTT (Failure to Train).
Phase C: In-message Procedure & Message Transmission
If the test passes, the fax machines move into the phase where the intended fax message is actually sent. The calling fax machine will first start with another test, known as a Short Training, followed by a resync. The fax is then broken down into frames of code and transmitted. You want to see that ECM (Error Correction Mode) has been enabled. This tracks the frames that are sent.
When the information is sent, the calling fax machine will then send an RCP (Return to Control for Partial Page) message letting the receiving machine know it is finished transmitting the fax.
Phase D: Post-message Procedure
In this phase, the calling fax machine will finish its transmission by sending a PPS (Partial Page Signal), which is essentially a list of what was involved in the fax (page, block and frame counts). Once the transmission is complete, the receiving fax machine will check to see that the message has the right amount of frames and, if is correct, will respond with an MCF (Message Confirmation). If an error has occurred, it will respond with a PPR (Partial Page Request), letting the calling machine know which frames are missing and that the message needs to be resent.
If Error Correction Mode is not enabled, a different sending and confirmation process is enabled.
Phase E: Call Release
The final phase is just a disconnect from the call. It is still possible to get a transmission error message here if the disconnect takes too long to establish. The sending fax machine will send a DCN (Disconnect) or a DCN error if the disconnect was unsuccessful. Keep in mind that this does not mean that the fax transmission was unsuccessful.
You can learn more about this process (and see a visual demonstration) by watching our video on this topic at RightFax University.