G4 Telefax / Group 4 Facsimile Service

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

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The wireshark dissectors for T.70 and T.62 are now available from osmocom/wireshark-mate: wireshark MATE configuration for use with GSM/UMTS/LTE protocols - wireshark-mate - Osmocom gitea

Despite them being lua, you still need a wireshark version resembling current master as I first had to introduce support for calling “decode as…” to the X.75 dissector, and then fix a bif in the lua dissector support.

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