The following information is based on my UNI studies on wireless communication systems.
Wireless Signals: All forms of electromagnetic energy- gamma rays, radio waves, even light- travel through space in the forms of waves. These waves are know as electromagnetic waves. They travel at the speed of light :186,000 miles per second (300,000 kilometres).
802.11 use wireless transmission use electromagnetic (EM) waves as the medium, not air or empty space
There are two basic types of waves by which wireless data are sent and received: infrared light and radio waves. Infrared light some of which is invisible, has many characteristics that visible light has , because it is adjacent to visible light on the light spectrum. Yet it is a better medium for data transmissions because it is less susceptible to interference from other sources of visible light.
Each wave length within the spectrum of visible light represents a particular colour. This because the differing wavelengths of light waves bend at a different angle when passed through a prism. Which in turn produces different colours. The colours that visible light produces are red, orange, yellow, green, blue, indigo, and violet. Visible light is sometime referred to as ROYGBIV.
Infrared wireless systems require that each device have two components: an emitter, which transmits a signal, and a detector, which receives the signal. An emitter is usually a laser diode or a light-emitting diode(LED). Infrared wireless systems send data by the intensity of the light wave. The emitter sends a narrowly focused beam of infrared light.
(TV remote is an example of this type of device)
Infrared wireless transmission can be either directed or diffused. A directed transmission requires that the emitter and detector be directly aimed at one another( line of sight).
A diffused transmission relies on reflected light. With diffused transmission the emitters have a wide focus beam instead of a narrow beam.
Infrared wireless systems have several advantages. Infrared light neither interferes with other types of communications signals(such’s as radio waves) nor is it affected by other signals. Expect light.
Infrared wireless light does not penetrate wall, the signals are kept inside a room, this makes it impossible for someone else to listen in on the transmission signal.
Infrared wireless systems have several serious limitations
Lack of mobility: directed infrared wireless systems use a line of sight principle, which makes it difficult for users because the alignment between the emitter and the detector would have to be continually adjusted.
Range: limited range of coverage. Directed infrared systems required line of sight and cannot have anything place in-between the infrared beam, which means they need to place close together to ensure that nothing obstructs there path, due to the angle of deflection, diffused infrared can cover a ranged of 50feet(15m), and because diffused infrared requires a reflection point, it can only be used indoors.
Speed: limitation of speed, diffused infrared can send data a maximum speeds of only 4Mbps. This is because of the wide angles of the beam lose energy as it reflects. The loss of energy results in a weakening of the signal. The weak signal cannot be transmitted over long distance, nor does it have sufficient energy to maintain a high transmission speed, resulting in a lower data rate.
Because of the limitation, infrared wireless systems are generally used in specialised applications, such as data transfers between computers, digital cameras, handheld data collection devices, PDAs, electronic books and other similar mobile devices.
Radio waves provide the most common and effective means of wireless communications today. Radio waves travel the space or air similar to that of electromagnetic wave, electromagnetic waves that travel in this fashion is called radio wave(radiotelephony). When an electric current passes through wire, it creates a magnetic field in the space around the wire. As this magnetic field radiates, it creates radio waves. Radio waves like light and heat waves, are electromagnetic waves, they move outward , usually in all directions from the source.
Unlike infrared light and heat radio waves are free from some of there limitations, radio waves can travel great distances and penetrate most solid objects.
Analogue and digital data are transmitted over radio waves. An analogy signal is one in which the intensity of the waves( voltage or amplitude) varies and is broadcast continuously.
Digital signal consists of discrete or separate pulses, as opposed to an analogy signal, which continuous. A digital signal has numerous starts and stops throughout the signal.
To transmit a digital signal over an analog medium, it requires a device know as a modem(MOdulator/DEmodulator) is to be used. A modem takes the distinct digital signals from a computer and encodes them into continuous analog signal for transmission over analog phone lines. The process of encoding the digital signals(bits) onto an analog wave is called modulation. The modem at the receiving end of the connection then reversers the process by decoding the analog signal into its original digital signal
Wavelength: is the distance between any point in one wave cycle and the same point in the next cycle
Frequency: is the number of time a cycle( which composed of one top (positive) and one bottom(negative peak) occurs within one second. Frequencies are measured by the amount of cycles per second. The term hertz(Hz) is used instead of cycles per second.
Radio transmitter send what is know as a carrier signal. This is a continuous wave(CW) of constant amplitude(measured in volts) and frequency. The up and down movement of the wave is call an oscillation signal or sine wave.
A CW by itself carries no useful information. Only after it is modulated does it contain some kind of information which is then called a carrier signal or carrier wave.
J. L. Olenewa (2014). Guide to Wireless Communications, ( Third Edition). Boston:CENGAGE Learning