1. Radios generate sine waves to transmit signals that
carry audio, video and data messages
A radio has a transmitter Tx to generate the signal and
a receiver Rx to receive the signal
◦ The transmitter encodes the message (using a modulation
standard) onto the sine wave and transmits the wave using an
antenna
◦ The receiver receives the sine waves from its antenna and
decodes the signals back to 0s and 1s
A sine wave signal looks like
this on a oscilloscope
http://openclipart.org/detail/173971/sco
pe-sine-wave-by-sangear-173971
2. Radio wave signals are separated by using different
sine wave frequencies
Wavelength is the time between the start of a wave
and the end of the wave
Frequency is the number of wavelength cycles per
second
Here are sine waves at different frequencies
3. The electromagnetic spectrum is the range of all
possible frequencies of electromagnetic radiation
◦ The use of frequencies in the electromagnetic spectrum is
regulated in most countries and managed by the ITU
International Telecommunication Union and other bodies
◦ Radio bands (portions of the electromagnetic spectrum) have
been allocated for the Wireless LAN signals
Bluetooth 2450 MHz band
IEEE802.11 / WiFi 2.450 GHz and 5.8 GHz bands
◦ These radio bands are part of the ISM open spectrum.
◦ ISM stands for Industrial, Scientific and Medical
◦ ISM equipment such as microwave ovens can generate signals
using the same frequency as Wlan devices causing interference
4. Radio Frequency (RF) signals are available to any device with
an antenna within the signal range
◦ RF signals can pass through walls and be received outside the building
◦ RF signals diminish in strength as they move further from the source
of the signal
◦ Attenuation is the rate at which a signal looses its strength
RF signals can be distorted or diminished by interference:
◦ Other equipment and devices using the same frequency on the ISM
bands
Microwaves, cordless phones, Bluetooth devices, wireless
video cameras, outdoor microwave links, wireless game
controllers, fluorescent lights all cause signal interference
5. Radio signals can pass through obstacles OR be absorbed (water
and concrete) OR be reflected (metals and glass)
Signals are diminished
◦ when obstructed by obstacles inside buildings
Metal cabinets and lifts
Building materials
◦ when obstructed by obstacles outside buildings
Weather (rain) and other buildings
People (bodies will absorb some of the signal !)
6. Bands are divided into channels. A channel is the frequency
used to transmit a signal. The channels are defined in the
IEEE standards
◦ The 2.4 GHz band is divided into 13 channels that are 5 MHz apart
For 802.11 b there are 3 non overlapping channels with a channel width of
22MHz
Channel 1, 6 and 11 (and 14 in Japan)
For 802.11g/n there are 4 non overlapping channels with a channel width
of 20 MHz
Channel 1, 5, 9 and 13
For 802.11n with a channel width of 40 MHz
Channel 3 and 11
◦ The 5 GHz band is divided into 40 channels that are 5 MHz apart
Using the channels is regulated by individual countries
For Australia channels 36, 40, 44, and 48 for indoor use and 149,153,157,
161 and 165 for outdoor use
