Communication methods

Serial transmission

Serial transmission is a method for transmitting data from one device to another. Serial transmission sends one bit at a time, down the same wire, from the sending device to the receiving device. Very high speeds can be achieved where fibre optic cables are used to carry the signal.

Skew and crosstalk

Parallel transmission has two key problems:
Skew: which is where the slightly different properties of each individual wire causes the individual bits to arrive out of synch.
Crosstalk: when the signal transmitted on one wire interferes with the signal on another wire.
Crosstalk increases as the frequency of the signal increases and both skew and crosstalk increase as the length of the cable increases.

Asynchronous transmission

Asynchronous transmission is where data is transferred one byte at a time, preceded by a start bit and followed by a stop bit. The start bit alerts the receiving device so it can synchronise it's clock and receive the data. The data is transmitted with the 8th bit as a parity bit. In the example below odd parity is being used to ensure there is an odd number of 1s in each transmission. As there are already three 1s and that is odd the party bit is a 0.

Knowledge check