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SenderChannelReceiver
DATA
Ready
Sender
Saved original ·
Channel
Receiver
Waiting
SenderReceiver
Frame encoding and ARQ
Row parity · P columnColumn parity · parity byte
ACK accepts the frame; NAK reports a detected error. A NAK or a timeout lets Sender resend its saved original. Undetected corruption produces ACK and cannot trigger ARQ.
Earlier send
Try a changed message that passes the row checks, then one that passes both rows and columns. Compare correct DATA with a lost ACK.
Seven data bits and one parity bit per character; blocks contain up to eight characters. This model combines ACK and NAK. Resend shows a clean repeat; real channels may corrupt another copy.