Group 3 protocol

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:

• 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

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 b/s as defined in V.21. The following illustration shows the call phases:

Phase A - Call setup

Phase A verifies that a fax terminal is at each end of the transmission. Because Group 3 is intended for transmission over the voice network, the calling and called fax terminals send tones at the beginning of a fax call . The calling terminal periodically transmits a CalliNG tone (CNG - 1100 Hz for 0.5 seconds) 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

Phase B identifies and selects the required facilities on the fax terminals involved in the transmission and negotiates which end will transmit, at what speed, for what page size, and so on. Phase B begins with the called station transmitting the mandatory facsimile control field, digital identification signal (DIS). This field is a packet that characterizes the called station's capabilities, including:

• Group
• Data rate
• Vertical resolution
• Image encoding
• Page width capabilities
• Maximum page length capability
• Handshake speed
• Error-correcting mode

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 can be used to identify a particular fax document to be sent as well as the password of the polling fax terminal. 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, which is the command to receive.

Phase C - Image transfer and message transmission

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. A sequence of 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 (CRC) error checking which allows the receiver to detect errors in the received image data. If there is an error in a received data packet, the transmitting end re-transmits at the request of the receiver (automatic repeat request - ARQ).

Phase D - Post-message procedures

When document transmission is complete, the sending fax terminal can

• Return to phase B to switch directions (turnaround polling).
• Return to phase B to change resolution, paper size, or transfer data rate.
• Send an end of message (EOM) and return to phase B.
• Send a multi-page signal (MPS) and return to the beginning of phase C for the next page.
• Send an end of procedure (EOP) and proceed to phase E.

When document transmission is complete, the receiving fax terminal can

• Send a message confirmation (MCF) and proceed as instructed by the sending fax terminal's post-page message (EOM, MPS, or EOP).
• Request retraining, which forces a return to phase B.

Phase E - Call release

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

Non-standard facilities frame

ITU T.30 allows a non-standard message (one not defined in T.300) 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 a maximum allowable transmission time of three seconds per frame. The non-standard frame transmitted by the remote terminal is read only if the first three bytes match the country code and manufacturer's ID parameters for this terminal.

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