retronetcall - Group 4 Facsimile Service (Fax G4)

G4 Fax was specified in 1985 as a fully digital fax system designed for ISDN and packet-switched networks, unlike analog G3 Fax

Uses a layered protocol stack consisting of:

• X.75 as layer 2
• T.70 protocol for segmentation and transport
• T.62 protocol for session and document handling

No interoperability between G3 and G4 fax systems - they are completely separate technologies

Very rare/limited deployment despite being technically advanced:

• Few physical G4 fax machines were ever produced
• Known implementations include Canon L260i and RVS-COM software fax

Key technical capabilities:

• Mandatory black & white service, optional grayscale/color
• Multiple resolutions: 200, 240, 300, 400 PELs per 25.4mm
• Digital transmission at 64kbps over ISDN
• Document and file transfer capabilities

TIFF-G4 image format originated from G4 Fax specifications and became popular for document archiving

Sessions are unidirectional - new session needed for bidirectional document transfer

Protocol provides reliable transmission with:

• Acknowledgments and retransmissions
• CRC checksums for error detection
• Segmentation for large messages

Development effort underway to create open source G4 Fax implementation using Lua-based Wireshark dissectors

Protocol traces can be captured from physical G4 fax machines which print communication logs