Data transmission: modes, methods and error checking

Serial and parallel

Serial sends one bit at a time down one wire; parallel sends several bits at once down several wires. Parallel looks faster and is, over very short distances. Over longer distances it suffers skew (bits arriving at slightly different times because the wires are not identical) and crosstalk (signals interfering between adjacent wires). That is why USB, SATA and network cables are all serial — the exam wants this reasoning, not just the definitions.

Simplex, half-duplex, full-duplex

Simplex: one direction only (keyboard to computer). Half-duplex: both directions, one at a time (walkie-talkie). Full-duplex: both directions at once (telephone call).

Why errors happen

Electrical interference, signal attenuation over distance, and timing problems all flip bits. Every method below detects errors; only one corrects them.

Four checking methods

Parity adds one bit so the number of 1s is even (or odd). Cheap, but fails when two bits flip. Checksum adds the data values, sends the total, and the receiver recalculates. Echo check sends the data back for comparison — doubles the traffic and cannot tell whether the error happened going or coming back. Check digit (ISBN, barcodes) catches typing and transposition errors at the point of entry.

Automatic Repeat reQuest

ARQ combines error detection with acknowledgements and timeouts: no ACK before the timeout, and the sender retransmits. This is detection plus retransmission, not correction. Genuine correction needs a parity block (row and column parity), where the intersection of the failing row and failing column identifies the exact bit to flip.