Fax communications (whether IP is involved or not) are dependent on some fundamental protocols. These protocols are defined by the ITU-T Recommendations of T.30, T.4, and T.6. If you really want to understand how Group 3 (G3) fax machines communicate, then everything is detailed in these specifications.
However, most of us do not need the level of knowledge detailed in these specifications but instead a general overview of their functions is more than adequate. In the figure below you can see a graphical representation of how T.30, T.4 and T.6 are used in a basic fax transaction.
In the figure you can see that T.30 occurs end-to-end between the fax machines. T.30 defines the signaling between the fax machines and specifies the message format for this signaling. For those familiar with data protocols, the messages are based on High-Level Data Link Control (HDLC) frames. These HDLC frames are then modulated using the V.21 specification and sent at a slow 300 bps. Fortunately, the messages are small in size so the 300 bps transmission speed is not a major cause of delay.
Using T.30 messages the fax machines decide on the parameters that will be used for the sending of the fax page. This negotiation of the page sending parameters defines attributes such as the page resolution, page transmission speed (a speed higher than the 300 bps used by T.30 must be negotiated), page size, and whether Error Correction Mode (ECM) will be used or not. Additionally, T.30 manages the transmission and reception of the fax pages and it handles the call disconnect process.
As shown in the figure, the T.4 and T.6 protocols only pertain to the actual fax pages themselves. Specifically, T.4 and T6 define how the fax page information is encoded, which correlates directly with how efficiently the fax information is compressed. Within the T.4 protocol, the Modified Huffman (MH) and Modified READ (MR) encoding methods are defined. In T.6, the Modified Modified READ (MMR) encoding algorithm is detailed. From a text compression efficiency perspective, MMR is the best, followed by MR, followed by MH. During the T.30 negotiation of the page transmission parameters, one of these encoding methods is decided upon.
For those of you familiar with VoIP I have found that the following analogy may be helpful for understanding the roles of the T.30, T.4 and T.6 protocols. You can think of the fax signaling protocol, T.30, as acting like a voice call control protocol, such as H.323 or SIP. Similarly, the fax encoding protocols of T.4 and T.6 define the encoding algorithm for the fax information, much like a codec defines the encoding algorithm for the voice information. Hopefully, this analogy and the figure above clarifies the roles of T.30, T.4, and T.6. When dealing with fax communications it is good to have a basic understanding of these protocols and their roles.
David Hanes, CCIE 3491, is an engineer within Cisco's Customer Advanced Engineering (CAE) team, where he assists with fax over IP (FoIP) solution design, troubleshooting, and new product testing. David is the co-author of the book Fax, Modem, and Text for IP Telephony and regularly speaks and provides training on Cisco FoIP solutions. Since joining Cisco in 1997, David has held various positions within the Cisco TAC organization including Team Lead for the Multiservice Voice Team and TAC Escalation Engineer for VoIP technologies.