• Modern fax communication is based on the Group 3 (G3) fax standard, which is composed of the ITU-T specifications of T.30, T.4, and T.6
• T.30 details the signaling, T.4 specifies the Modified Huffman (MH) and Modified Read (MR) encoding methods, and T.6 covers Modified Modified Read (MMR)

Basic G3 Fax Transaction

• The optional Called Subscriber Identification (CSI) and Non- Standard Facilities (NSF) messages may be transmitted along with the DIS message

• The optional Transmitting Subscriber Identification (TSI) message may be transmitted along with the DCS message

• Blue messages indicate low speed T.30 signaling and gold messages represent higher speed modulations used in training and page transmission

Calling Tone (CNG):-

CNG is the calling tone generated as a first indication of a fax call from the originating fax machine. The CNG tone is a pure tone with a frequency of 1100 ± 38 Hz (tone frequency tolerance is ±38 Hz) with 0.5 seconds on and 3 seconds off 

Called Tone CED:-

The CED is the called tone, which is also called the answering tone (ANS). The tone is a flat tone signal of 2100 ± 15 Hz for a duration of 2.6 to 4 seconds 

Digital Identification Signal (DIS):-

Dis contains all capabilities of the receiving fax machine.

Digital Command Signal (DCS):-

It gone sound similar to DIS. its the capabilities agreed by Originating fax Machine.

Training Checks:-

The training check frame is the phase of a fax call when the sending fax transmits a sequence of zeros to the receiving fax machine at the highest common data rate negotiated during prior phases. This check determines whether the line quality is adequate to pass information at the desired rate. If the line quality is good, the receiving fax machine receives this sequence of zeros without error. If line quality is not good, bit errors will occur during reception, and the total amount of zeros is not received.