Parallel and Serial transmission

In serial transmission, data is sent along a single data connection, one bit at a time. A popular example is the USB protocol.

In parallel transmission, multiple bits of data are sent simultaneously, through the use of multiple data connections. For instance, with 8 cables, 8 bits can be sent at a time.

Parallel connections are typically reserved for communications within devices – for example, connecting the system memory with the CPU.

Comparison

From a cost perspective, serial transmission is preferred: the cables are cheaper (fewer data lines required), and the hardware at the ends of the cable is simpler.

A serial connection can be far longer than a parallel one – in parallel connections, a phenomenon called ‘skew’ can affect the reliability of data transfer, and is particularly problematic as lengths increase. Skew is where the data from different wires arrives at slightly different time.

As electrical signals pass through wire, the resistance of the cable causes attenuation of the signal. In order to relay messages across long runs of cable, the signal must be regenerated at regular intervals; this is achieved by using amplifiers at various points along the cable. The hardware required to do this is simpler and cheaper for serial communication than it is for parallel transmission.

Parallel data transmission obviously has the potential to be able to handle a higher volume of data than serial transmission, due to the additional connections.