The original T.30 Recommendation was established in 1988 by the International Telecommunications Union (ITU) and defines the procedures which are required for document transmission between two fax devices in the public switched telephone network (PSTN). The T.30 standard has been amended many times.
The T.30 standard uses five phases:
– Phase A - Call establishment
– Phase B - Pre-message procedure
– Phase C - In-message procedure and message transmission
– Phase D - Post-message procedure
– Phase E - Call release
Phase A, call establishment, sets a connection between the sending and receiving devices to handshake (exchange fax tones) . T.30 call establishment may be handled manually or automatically. In manual operation at the sending terminal, the caller dials the number and hears the ringing tone. When the receiving device answers the call, the sending caller hears the fax tone or may have a verbal exchange depending on whether the receiving device is automatic or manual. The sending fax device then transmits the handshaking tone to the called terminal. In automatic mode at the sending device, the device dials the phone number and when the receiving device answers the call and hears the initial tones, the sending device automatically transmits the fax handshake.
In Phase B, T.30 pre-message mode identifies capabilities and then orders the common class of capabilities, then confirms the acceptable capabilities. The T.30 identification part consists of capabilities identification (e.g, bit rate, page length and width, data compression format, scanning per line, etc.) and the confirmation part consists of the receiving device sending a confirmation to the sending device.
In Phase C, the T.30 in-message mode occurs simultaneously with message transmission and controls the signaling which means that the control signals are transmitted with the message (e.g. in-message synchronization, error detection and correction, and line supervision). In the second part of Phase C, the T.30 message transmission proceeds which is handled under Recommendation T.4, which covers the modulation and demodulation of signals along with compression methods.
In Phase D, the T.30 post-message mode includes information such as end-of-message signaling (sent by the sending device to tell the receiving device that all the pages have been sent and to return to phase B for reception of another document), the multi-page signaling (sent by the sending device to tell the receiving device that a page of the document has been fully sent and to return to Phase C for reception of the next page in the same document), the end-of-facsimile signaling (sent by the sending device to the receiving device to tell that all the pages in the document have been fully transmitted and that no further pages are coming and to proceed to phase E), confirmation signaling (sent by the receiving device to confirm message receipt in response to any of the above information sent by the sending device).
T.30 is still in effect today, although the use of the PSTN is now giving way to use of the Internet. T.30 still works at the device-level, and then interacts with T.37 or T.38 to make faxing compliant with IP (Internet Protocol) networks.