Retronetworking

h1. Telefax G4

G4 (Group 4) is a rarely-used version of the Telefax service which works only over ISDN.  Contrary to the much more commonlty used G3 (Group 3) for analog telephone lines, G4 does not include any Modem.  Instead, it natively uses digital transmission for the Telefax data over ISDN B-Channels.

h2. Protocol stack

The protocol stack looks like this:

* G4 Fax specific data as per T.6, T.563 etc.

* T.62 Document Layer

* T.62 Session Layer

* T.70

* X.75

* ISDN B-Channel

h3. B-channel captures

* raw B-channel capture of a G4 fax that @manawyrm sent using RVS-FAX 3.0 on a Windows PC to the [[Canon_FAX-L260i]] of @laforge on 2024-03-01

** attachment:isdntap-manawyrm-00493065022141-2152-20240301-193039-1-tx.raw

** attachment:isdntap-manawyrm-00493065022141-2152-20240301-193039-1-rx.raw

* soft-hdlc decode of the above capture (using "hdlcsync":https://gitea.osmocom.org/cellular-infrastructure/osmo-rbs/src/branch/master/hdlc/hdlcsync.c)

** attachment:isdntap-manawyrm-00493065022141-2152-20240301-193039-1-tx.hdlcsync

** attachment:isdntap-manawyrm-00493065022141-2152-20240301-193039-1-rx.hdlcsync

h3. wireshark dissectors for ITU-T T.70 and T.62

simple/basic LUA dissectors for use with current wireshark can be found in https://gitea.osmocom.org/osmocom/wireshark-mate - they make use of the [recently introduced] X.75 dissector in wireshark. Make sure you have a wireshark version that includes the following changes:

* https://gitlab.com/wireshark/wireshark/-/merge_requests/14715 _packet-x75: Add sub-dissector table for "Decode as..."_

* https://gitlab.com/wireshark/wireshark/-/merge_requests/14720 _wslua_dissector: Support 'decode_as' from DissectorTable:try()_

The are "decode as" dissectors, so you need to

* select the X.75 payload and "Decode as" T.70

* then select the T.70 payload and "Decode as" T.62