Pactor is the most popular of the TOR modes and has Automatic Correction Request (ARQ) .. the sending station sends a packet burst of less than one second and then listens for an automatic response from the receiving TNC which tells it OK or resend based on an error correction information included with the packet .. The theory is deliciously complex but once the TNC is set up right, near error free copy is received .. eventually .. I think of this as the "Burp" mode to help remember it .. The sound is a series of 0.8 second transmission every second .. in between, the other station replies but cannot be heard in this clip ... Because the sender was trying to connect, another story altogether.