T.38 and ECM (Error Correction Mode)
I've noticed when receiving faxes, even though I have ECM enabled on my endpoint, it's never actually used.
Our switch has ECM enabled globally, but there are different types of it. The ECM is negotiated only for high speed faxes and if both actual fax machines support it
What is ECM and how does ECM work?
Question: What is ECM and how does ECM work?
Answer: ECM is an optional transmission mode built into fax machines or fax modems. ECM automatically detects and corrects errors in the fax transmission process that are sometimes caused by telephone line noise. This generally means an ECM coded fax will be more likely to succeed on a noisy line.
The page is divided into what is known as Partials Pages which in turn are comprised of Frames. Once the receiver has received a Partial Page, it examines its frames (using check-sums) and then advises the transmitting fax of any missing or in error. The transmitter then need only resend the frames in error rather than the whole page. Note that using ECM on a link which exhibit packet loss will greatly increase the transmission time as a lot of frame retransmission will be required. ECM should not be seen as a solution to packet loss.