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Serial transmission is where one bit is sent at a time.

Serial transmission uses a single wire to send data one bit at a time.

Diagram 1 shows an example of serial transmission.

Diagram 1

Parallel transmission is where multiple bits are sent at once.

Parallel transmission uses multiple wires to send a lot of data simultaneously. This seems better than serial transmission, but in reality it's a lot worse:

• In a perfect world data from all channels would be perfectly synchronised so that they are all sent at exactly the same time. In reality however, there is a delay between the clock and each data channel, meaning that it is difficult to synchronise parallel transmission at higher bit rates.
• Also, imperfections in the wires used to transmit the data could mean that the data isn't received at the same time (because data in some wires travels faster than data in other wires), which causes a delay at the receiving end. This is called data skew.
• Crosstalk (interference) between wires can also be an issue over longer distances, resulting in corrupted data, making parallel transmission less reliable than serial transmission.
• Also, the extra wiring required for parallel transmission makes it more expensive than serial transmission.

Therefore, serial transmission is cheaper, faster, and more reliable than parallel transmission.

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Where might parallel data transmission be used instead of serial transmission?

Over short distances, such as the control buses inside a computer, because the effects of data skew are negligible.

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