Yesterday we successfully transferred G4 Fax for the first time over OCTOI.
One endpoint was my recently-acquired Canon FAX-L260i machine - one of the few ISDN fax machines sold in Germany which could actually do G4.
The other endpoint was @manawyrm using a software G4 fax implementation (RVS-FAX on Windows).
We tested both inbound and outbound transmission and took raw B-channel bitstream captures.
In the service manual of the L260i I discovered that it’s possible to print D-channel and also B-channel protocol traces of the last fax; see below for a [scan of] the printed B-channel trace of an outbound fax transmission with two traces.
It’s a bit funny that the layers are completely mixed. SABM/UA are [obviously] the outer X.75 layer (L3), while TCR/TCA are part of the T.70 layer (L4), and CSS/RSSP/etc. are part of the T.62 protoco layer (L5).
I’m planning to walk through those traces this wednesday night at the upcoming RetroNetCall