What is the role of ARQ in a transmission system?

ARQ stands for Automatic Repeat Request (or Automatic Retransmission Request). It is a standard error-control method used in digital communications, including many amateur radio digital modes. When a station receives a data packet, it checks for errors using a checksum. If the data is corrupted or missing, the receiving station automatically sends a signal back to the transmitting station requesting that the specific packet be sent again. This automated process ensures that the final message is received flawlessly. It has nothing to do with data compression or encryption.