These operating modes also known as TDM or ARQ-56, conforming to the CCIR recommendations 342-2 and 242. Four 50 Bd Baudot channels are interleaved to form a time multiplexed aggregate bit stream. Multiplex frames of 56 bits are used.

ARQ-M4-342 and ARQ-M4-242 systems operate at a Symbol rate of 172 or 192 Baud on the radio link.

The ITA-3 7-bit alphabet is used allowing error detection. The ITA-3 alphabet is a balanced code in which each character has a 3 to 4 ratio between mark and space bits (balanced code). ARQ-M4-342 and ARQ-M4-242 are full-duplex systems.

Full-duplex systems send a repeat request (RQ) character to the remote station if a character error has been detected or the distortion or fading becomes excessive. This results in the re-transmission of the last 3 or 7 characters preceded by the RQ request control character.

According to the CCITT recommendation, the repetition cycle may span 4 or 8 characters, as is the case with ARQ-E. The longer RQ-cycle of 8 characters has never been monitored.

In addition to the time multiplexing of several channels (division channels); each division channel may be further subdivided into sub-channels resulting in a multitude of possible modes of operation. At present however no transmissions with sub-channel division are known. Systems employing sub-channel division may be recognized by the rhythmic blinking of the Error indication (system state) in the decoder status bar.