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Appendix F

Group 3 Fax Technology

Fax Protocols
Group 3 Protocol
Phase A - Call Setup
Phase B - Pre-Message Procedure
Phase C - Image Transfer and Message Transmission
Phase D - Post-Message Procedures
Phase E - Call Release
Non-Standard Facilities Frame

Fax ProtocolsTop of Page

The International Telecommunications Union (ITU) is the international standards body for telecommunications. In 1968 it published the standard for Group 1 (G1) fax protocol; in 1976 it published Group 2 (G2) protocol; in 1980 it first published Group 3 (G3) protocol.

Group 3 protocol is specified in several standards. T.4 and T.6 specify the image transfer protocols. T.30 specifies the session management procedures which support the establishment of a fax transmission. It allows the two stations to agree on such things such as transmission speed and page size. Since Group 3 protocol is specified for the switched analog network, and it is an all-digital procedure, it must use modems. The modems are specified in additional ITU standards: V.21 (300 bps) for the T.30 procedures, and for image transfer V.27ter (4800/2400 bps) and, optionally, V.29 (9600/7200 bps), V.33 (14400/12000 bps), and V.17 (14400/12000/9600/7200 bps).

The following International Telecommunications Union (ITU) specifications provide detailed information about facsimile:

Group 3 ProtocolTop of Page

To understand Group 3 fax protocol, and the options it offers the system designer, it is important to understand the basics of T.30. T.30 sets the session-control procedures for Group 1, Group 2, and Group 3. It divides a call into five phases:

The session-control procedures used to control the call from Phase B to Phase E (Call Release) use High-level Data Link Control (HDLC) frames at 300 bps as defined in V.21.


appfa.gif

Figure 8. Call Phases





 Phase A - Call Setup Top of Page





Phase A verifies that there is a fax terminal at each end of the transmission. The calling fax terminals and called fax terminals send tones at the beginning of a fax call because Group 3 is intended for transmission over the voice network. Therefore, the calling terminal periodically transmits a CalliNG tone (CNG - 1100 Hz for 0.5 sec.) that identifies it as a fax terminal. The called fax terminal answers with CallED station identification (CED), a 2100 Hz tone that lasts for three seconds. 






 Phase B - Pre-Message Procedure Top of Page





Phase B identifies and selects the required facilities on the fax terminals involved in the transmission and negotiate which end will transmit, at what speed, for what page size, etc. Phase B begins with the called station transmitting the mandatory facsimile control field, Digital Identification Signal (DIS), a packet which characterizes the called station's capabilities, including:




DIS, and the other facsimile control fields (DTC and DCS), may each be associated with one or two optional packets. The first optional packet identifies the terminal sending the control field (station ID). The second optional packet is a non-standard facilities frame whose content is not specified by T.30. This non-standard field provides the opportunity for a fax terminal manufacturer to transmit customized data to remote fax terminals. You can use NaturalFax to set and read these fields in a standardized manner.



The calling fax terminal, which is still in control of the session, can request the called fax terminal to receive or to send. During normal fax operation, the calling fax terminal requests that the called fax terminal receive. During a fax polling operation, the calling fax terminal requests that the called fax terminal transmit.



If the calling fax terminal wishes to send, it determines the facilities of the called fax terminal from the received DIS, selects the parameters of the fax session, and sends the appropriate DCS frame to the called fax terminal. The called fax terminal acknowledges with a Confirmation to Receive (CFR). 



To initiate a poll, the calling fax terminal sends a Digital Transmit Command (DTC). As with DIS, the DTC can be sent with a station ID and the non-standard field. The non-standard field maybe used to identify the particular fax document to be sent as well as the polling fax terminal's password. If polled, the called fax terminal assumes control of the session. If it has nothing to send, it sends a disconnect to the calling fax terminal and hangs up. Therefore, the calling fax terminal should always send before polling. If the polled fax terminal has a document to send, it sends DCS, the command to receive. 






 Phase C - Image Transfer and Message TransmissionTop of Page





The Group 3 in-message procedures are specified in T.4 and T.6. The page is divided into horizontal picture elements (pels) of, nominally 1728 pels/line of 
215 mm, and vertical pels of either 3.85, 7.7, or 15.4 lines/mm. (normal, high, and super high resolution). The minimum transmission time per line is specified in Phase B.



T.4 specifies the G3 data-compression coding schemes, often referred to as Huffman encoding. One-dimensional (1D), run-length encoding involves fixed codes for white/black run lengths (for example, the number of contiguous white or black pels). Two-dimensional (2D) encoding provides additional compression by encoding two lines at a time; the second line specifies changes from the first. A special code is used for End Of Line (EOL). T.6 specifies the MMR coding scheme. In MMR, encoding lines are not separated by EOL codes. Six EOLs at the end of a page indicates the end of the page and marks the end of Phase C. 



Group 3 faxes can also be transmitted using the T.30, Annex A, error-correcting standard. It uses the HDLC transmission control procedures used in the session-management procedures in Phases B, D, and E to transfer the image data in Phase C. HDLC includes cyclic-redundancy error checking. This allows the receiver to detect errors in the received image data. If there is an error in a received data packet, the transmitting end retransmits at the request of the receiver (Automatic Repeat Request - ARQ). 






 Phase D - Post-Message ProceduresTop of Page





When document transmission is complete, the sending fax terminal can:




When document transmission is complete, the receiving fax terminal can:






 Phase E - Call ReleaseTop of Page





The sending fax terminal sends an EOP frame, followed by a DCN (disconnect) frame. The sending and receiving fax terminals will then release the call.






 Non-Standard Facilities FrameTop of Page





ITU T.30 allows a non-standard message, one not defined in T.30, to be included with most negotiations. These non-standard frames are called NSF when the called fax terminal sends DIS, NSS when the calling fax terminal responds to DIS with DCS, and NSC when commanding the called fax terminal to send (a poll). A non-standard frame begins with the country code of the manufacturer, the manufacturer's code, and any additional ASCII information (limited by allowable transmission time of three seconds per frame). The non-standard frame transmitted by the remote terminal will only be read if the first three bytes match the Country Code and manufacturer's ID parameters for this terminal.



The non-standard messages are used by some manufacturers to negotiate use of proprietary features when two machines with the same manufacturer's ID code fax to each other.











 
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