Chapter 4 Wireless Technology
157
radio waves
electromagnetic
waves with a
frequency range of
10 kHz to 3,000,000
MHz.
spread spectrum
a transmission
technique that uses
multiple channels to
transmit data either
simultaneously or
sequentially.
frequency hopping
a spread spectrum
technique that
transmits data on
multiple channels
simultaneously.
direct sequencing
a spread spectrum
technique that
transmits data on
multiple channels
sequentially.
Radio Wave Transmission Techniques and
Networking
Radio waves are electromagnetic waves with a frequency range of 10 kHz to
3,000,000 MHz. They have the longest wavelength when compared with microwave
and infrared. Radio waves are used in LANs. Radio wave-based networks adhere
to the IEEE 802.11 and Bluetooth standards and operate at 2.4 GHz.
Radio Wave-Based Transmission Techniques
To communicate between radio-based wireless network devices, several
transmission techniques are used: single-frequency, spread-spectrum, and orthogonal
frequency-division multiplexing. You must become familiar with these techniques
because most wireless network technologies are described using these terms. The
techniques are based on the technology, the frequency band of operation, and the
manufacturer’s idea of the best way to achieve a high data rate. A high data rate
not only relies on how fast the data can move between two points, but also on how
much data has to be retransmitted because of interference.
Transmission techniques divide an allocated frequency band into many
separate frequency ranges, or channels. After the frequency band is divided, a
carrier wave is generated for each of the channels.
Spread Spectrum
Spread spectrum is a transmission technique that uses multiple channels to
transmit data either simultaneously or sequentially. The term spread spectrum refers
to transmission channels spread across the spectrum of the available bandwidth.
Spread spectrum transmission works in a similar manner that highways
work. A highway system consists of several separate lanes to carry vehicles. You
can think of spread spectrum as several radio wave paths designed to carry
radio waves. In the highway system, trucks carry supplies to a store. Each truck
uses a separate lane. If one of the lanes is blocked (interference), the other lanes
can still carry the supplies to the store. This is the same method employed by
spread spectrum. If one of the channels is blocked by radio interference, the other
channels can still carry the radio wave data.
In the spread spectrum technique, data can be transmitted on multiple
channels simultaneously or sequentially (one at a time). The spread spectrum
technique that transmits data on multiple channels simultaneously is called
frequency hopping. The spread spectrum technique that transmits data on
multiple channels sequentially is called direct sequencing.
Spread spectrum is the chosen transmission method of most wireless
technologies. Transmitting data on multiple channels decreases the likelihood of
interference. Interference is typically limited to only one or two of the channels.
The other channels in the frequency band are free to carry data undisturbed.
Data that is lost can be easily retransmitted on a channel that is not affected by
the interference.
Frequency hopping
Frequency hopping is also referred to as frequency hopping spread spectrum
(FHSS). Frequency hopping is used with wireless devices that use the 2.4-GHz
radio band. The 2.4-GHz frequency has a bandwidth of 83.5 MHz. Rather than
use the entire range as a single channel to carry radio data, the frequency band is
divided into 79 1-MHz channels. See Figure 4-9.