V.42

V. 42 is an error correction protocol promoted by the ITU-T, a technical standards group within the United Nations]. Its function is to allow the receiver to immediately request re-transmission of any lost data packets.

V.42 essentially turns an error-prone communications path into an error-free path so that whatever is sent over the path gets delivered without errors. However, it doesn't guarantee how quickly the error-free data will be delivered to the receiving end. V.42 is generally always included in dialup modems.