Why have high-speed connections between chips and boards moved from parallel transmission over many wires to serial transmission over a few fast wires?

Answer: A. In parallel transmission, arrival-time differences between wires (skew) make it hard to raise the speed

Parallel transmission requires the bits on many wires to be captured together with one clock. As speed rises, arrival-time differences caused by tiny differences in wire length (skew) become significant compared with the clock period, so the speed cannot be raised further. Serial transmission recovers the clock from the data on each pair of wires, avoiding skew, so the speed per wire can be much higher. It also reduces the number of pins and traces.

• A. Correct. Fewer wires also mean less routing and fewer pins.
• B. Serial links are electrical too and consume power.
• C. It was never banned.