Automatic Repeat Request is a communications mechanism typically used over serial communications or modem communications lines. The receiver will send back an ARQ message to the transmitter to indicate that the last block should be retransmitted. Note that there is usually a predetermined number of times a block of data may be retransmitted before the connection is reset or dropped.
Also, some type of error detection scheme must be employed to allow the receiver to verify the integrity of the data. Cyclic redundancy checks are often used for this purpose. ARQ can only be used in bi-directional links and will not work in simplex environments.
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