Basically any data transmission system can be considered as consisting of three components:

• a SENDER which transmits the messages
• a CHANNEL (or circuit) which carries them
• a RECEIVER which accepts them

The diagram below shows this pictorially:

Although in most cases the channel is bi-directional (duplex), allowing messages to be sent from either end, there are some exceptions to this rule.

When information is exchanged between two communication entities there are many areas where agreements have to be established, before transmission can be commenced - in other words there have to be protocols.

The definition of a protocol, commonly used is:

• a set of rules governing the format, timing and error control of messages exchanged between communication entities in a network.

However, it is imperative to realise that these comprise of only some of the areas where agreements have to be established.

As the use of data communication systems becomes more widespread it becomes increasingly more essential that appropriate standards are set up, to enable different systems and equipment from different manufactures to be connected together. Standards within each country were originally developed by the body responsible for the public telephone network.

On an international platform the ISO OSI Model has become the recognised standard to which most modern networks are built. Thus it is important to realise where exactly the protocols, encoding data, flow control and error control would be implemented on this scenario.

Lastly, as technology has been enhanced new techniques have been developed in the approach to data transmission:

• Multiplexing
• Switching