2.2 Signal theory
This section looks how data is sent across a network from one computer system to another. All computers communicate using a variety of media (light, radio, electrical and microwave). The principles are based on electronics and physics.
Digital signalling methods
The following shows the properties of a data transmission being sent from one computer system to another.
The sine wave has two properties of interest: amplitude and frequency.
A in the following diagram represents amplitude or strength of the signal, and can be explained simply by the volume or loudness. The higher the amplitude of the signal, the louder and stronger it is – the lower the amplitude, the quieter and weaker it becomes. With any transmission, higher amplitude signals will travel further.
For different systems, amplitude has a different meanings, as follows.
• Radio and microwave both use the same method of transmission: radio waves. For all radio waves, the amplitude of the waveform is measured in metric terms (meters or millimetres).
• All cables reply on electrical current, the strength of which is measured in volts. The current in a normal data cable is +/-5 – any higher a voltage may damage the sensitive computer equipment. A telephone cable can carry up to +/- 50 V. The range of the signal switches from positive voltage to negative voltage and is referred to as AC (alternating current).
• With light, the brighter the light source, the stronger the signal. Most fibre optic cable use infrared or laser-generated light; the difference between these two light sources affects the distance the signal can travel and the speed of the line.
F represents the frequency of the signal. The frequency is the rise and fall of the waveform from zero to bottom, then to the top and back to zero, shaped like a rollercoaster ride. This is called a cycle and is measured in hertz (Hz). A low frequency signal has a smaller number of cycles per second; a higher frequency signal can have billions of cycles per second (such as GHz or MHz)
Representing data electronically
Based on the technology used on a sine wave, data is transmitted as a square (or digital) wave. All computers use binary in which each bit of information is represented as a 0 (zero) for the off state and 1 (one) for the on state. The binary is organised in chains of bits, called bytes or words according to the system that is going to use it. For example, 01001100 is a single byte that represents the decimal value of 76 or the ASCII or AMERICAN STANDARD CODE FOR INFORMATION INTERCHANGE, value of ‘v’.
Sending data from one computer to another is called encoding and various formats exist according to the system being used (wireless, fibre or electrical cable). Common formats are Manchester encoding or Huffman coding.
Encoding is not a new concept – it is more than 160 years since Samuel Morse developed Morse code to be used on the new electrical telegraph.
He created a system of combinations of two signals (a short pulse called a dot and a long pulse called a dash), similar to the binary zero and one. The system was invented so that the telegraph operator could key in a message in any language at a relative fast speed.
Encoding that is used to send data across a computer network is based on a signal ‘square’ wave, which is an adaption of the sine wave.
In order to avoid data being lost or the computer system being confused (which would cause an error), there are two simple rules when sending data.
1. All binary zero (off) are sent at high amplitude so there is no confusion with the ‘power off’ of no signal being sent. This is like the Morse code dash.
2. All binary ones (on) are sent at a mid-range amplitude to contrast with the rules of the binary zero. This is like the Morse code dot.
The following diagram that the frequency of the signal is fixed (although this value will vary according to the speed of the transmission medium), but the amplitude is varied and based on a zero or one coming through the line.
To ensure that data is successfully transmitted, an agreed common method is used for sending the data, one that can be managed by all computer systems.

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