What type of error control system is used in AMTOR ARQ (Mode A)?

AMTOR (Amateur Teleprinting Over Radio) features two primary modes for error control: Mode A and Mode B. Mode A is known as ARQ, which stands for Automatic Repeat reQuest. In ARQ mode, the transmitting station sends data in short blocks. The receiving station checks each block for validity. If the block is received correctly, it sends an acknowledgment. If an error is detected, the receiving station automatically requests a repeat of that specific block until it is received flawlessly. This makes Mode A highly reliable for point-to-point communications. In contrast, Mode B uses Forward Error Correction (FEC) where characters are sent twice, which is better suited for 'roundtable' broadcasts where the receiving stations cannot transmit a request for a repeat.