Digital communications: serial and parallel transmission, the data rate (bit rate and baud), multiplexing (time-division and frequency-division), and error detection with parity and checksums.

Serial (up to 2 marks): serial transmission sends the bits of a word one after another down a single line. Advantage: it needs only one (or a pair of) wires, so it is cheap, avoids the skew between separate wires, and works well over long distances.

Parallel (up to 2 marks): parallel transmission sends all the bits of a word simultaneously on separate lines. Advantage: it is faster for a given clock rate because a whole word moves at once; it suits short, fast links such as inside a chip or between nearby boards.

Long distance (up to 1 mark): serial is used over long distances, because running many wires is expensive and parallel lines suffer timing skew and crosstalk over distance.

Markers reward serial (one line, cheap, long distance) and parallel (all bits at once, faster, short distance), and serial as the long-distance choice.

Parity (up to 3 marks): a parity bit is added to each data word so that the total number of $1$s is even (even parity) or odd (odd parity). The receiver counts the $1$s; if the parity is wrong, a single-bit error has occurred and is detected.

Limitation (up to 1 mark): parity detects an odd number of bit errors only; if two bits flip, the parity is unchanged and the error goes undetected. It also cannot correct the error, only flag it.

Bit rate (up to 1 mark): the bit rate is the number of bits transmitted per second (bits per second).

Markers reward making the total count of 1s even/odd and checking it, the two-error (even number) limitation, and the bit rate as bits per second.