Managed by the FaxBack Support Team
This article applies to NET SatisFAXtion (all versions)


Fax pages are missing scan lines.
Example of a page that lacks ECM and has poor signal quality.


Faxes are sent in pieces called scan lines. Each scan line is one pixel tall and the page size long. When a scan line is missing or contains corrupt data, that scan line will be missing or incomplete on the page. If error correction mode (ECM) is available from both sides (sender and receiver) these scan lines will not pass a checksum test, and the receiver will request that the sender, resend those bad scanlines. If ECM is not available, due to one side not supporting it, page data can be missing.

Signal quality degradation is the problem, when two fax devices, where ECM is negotiated out of the call, or is not enabled. If you see a lot of this type of problem, the signal degradation issue is likely on your side. If only a select few sent faxes are missing scan lines, the degradation problem is on their side, or even someplace in-between. It could be an up stream provider has fallen back to a VoIP G.711 or G.729 voice codec.

Make sure both sides (sender and receiver) support ECM and have not disabled it. If the receiving side does not support ECM, it will not be used. Additionally, if a carrier or equipment in the route the call takes, does not support ECM, it will also be removed, even if both sender and receiver do support ECM.

Determine if ECM was enabled on a given fax.

Generate a received report from the fax server. Locate the fax entry and find the value of "Image Transfer Method" to see if ECM is listed. If ECM is not present in that field, ECM was not used. Note the two fields only listing 1D. ECM was not present on those two receive events.

To see how many missing or corrupt scan lines there were in that fax, look at the value in the "Number Of Errors" Field. If Signal Quality was an issue, and scan lines were missing, you would see that number reflected in Number Of Errors


If ECM is not available from the sending side, this issue can only be solved at the telephony side, and is not a software issue.
NOTE: Brooktrout TruFax cards, do not support ECM at their hardware level, even if it is enabled by the fax server software.

Copyright © FaxBack, Inc.