Data Transmission WJEC Revision

Serial and Parallel Transmission

Data can be sent in one of two ways – serial or parallel. In serial data transmission, bits are sent via an interface, one bit at a time, over the same data line. Very high data transfer rates can be achieved using fibre optic cables.

In parallel data transmission, the bits in one byte are sent simultaneously along separate lines.

Parallel data transmission is used almost exclusively inside the computer because of the bus systems, but serial transmission is used where the distance is greater than a few metres.

The advantages of using serial transmission are: the cost saving, as only one data channel (two wires) is needed; serial transmission can travel longer distances than parallel; and the interface involved is simpler.

The advantage of using parallel transmission is that it is faster as more bits are sent at one time. e.g. To transmit 8 bits in parallel only requires one time unit whereas in serial it would require 8 time units, therefore this is faster than serial.

The disadvantage of using parallel transmission is that because each line is unique, the signals do not always arrive at the same time. This problem is known as “skewing”, and the problem increases with the distance covered.

Simplex, Half Duplex and Full Duplex transmission

Different data transmission methods are available: • Simplex – transmission can only take place in one direction. This type of transmission could be used for example when the sending device such as temperature sensor never requires a response from the computer.

• Half duplex – transmission can take place in both directions but not at the same time, as for example in a citizen’s band radio. This type of transmission is often used between a central computer and terminals.

• Full duplex – data can be sent in both directions at the same time. Most interactive computer applications use full duplex transmission.

Digital Transmission of Data

Digital transmission of data is preferable to analogue as it is less likely to become corrupted or be affected by interference or degradation.


A multiplexor is a device which receives data from several independent sources and combines it into one input signal of data that can be transmitted over one communication channel. This increases the efficiency of communication and saves on cost of individual channels. At the receiving end, the multiplexor (sometimes called a de-mulitplexor) separates the single stream of data into its separate components.


The process that prevents all data being sent to all parts of a network, and therefore makes more efficient use of the data lines.

computer science

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