Parallel Transmission

When several bits, comprising the data item, are transmitted simultaneously, each along its own separate channel, it is called parallel data transmission. For example, if eight bits represent a data item then obviously there must be at least eight distinct channels between the sender and receiver, plus as many additional channels as are required for control information.

Although parallel transmission is universal within the computer, for high speed data transfers on various buses, it is rare in environments outside that of the internal computer structure and connections between the computer ands close peripherals e.g. the CENTRONICS printer interface. Perfectly viable reasons exists why parallel transmission is not employed if data transfers are required to be carried out over anything other than short distances.

• In parallel transmission all the bits in a data item are transmitted along separate channels simultaneously. However, due to natural aberrations in the structure of each line (e.g. resistance), the signals do not arrive at the receiver at the same time. This problem is known as a skew and obviously this fault will increase in severity as distance between sender and receiver grows larger.
• Since each bit requires its own line transferring several bits in parallel means providing several channels, several transmitters and several receivers - thus the cost is substantial
• As distance increases it may be necessary to correct for signal degradation, due to voltage loss or interference, by means of expensive low-loss cables and special signal amplifiers - clearly this would again substantially increase the cost.

An alternative method of achieving parallel transmission is the use of multiple tones or frequencies to encode a character. This technique is sometimes used to transmit the numbers dialled by a push-button telephone. Pushing a button on the telephone keyboard generates two tones - a low tone indicating the row in which the button is located and a high tone indicating the column. These two tones are transmitted simultaneously over the line connecting the telephone handset to the local exchange - obviously this technique is very much like frequency division multiplexing.

Serial Transmission

The difficulties encountered when implementing parallel transmission over long distances can be eliminated by using serial data transmission. The source of the problems in parallel transfers is the use of multiple lines to transmit data bits simultaneously. However, by employing the serial approach this does not occur as just a single line is used and the bits, comprising a data word, are sent one bit at a time.

Advantages of serial transmission

• Savings in cost - only one data channel is needed instead of several
• The problem of skewing does not arise

Disadvantages of serial transmission

• To transmit 8 bits in parallel only requires one time unit whereas in serial it would require 8 time units
• Problems associated with synchronisation