Wireless LANs - LAN Wiring - page 315

“800” MHz. Broadcast television operating frequencies range from 54 to 800 MHz,
microwave ovens are typically around 1250 or 2400 MHz, and microwave radio
and satellite links operate in various bands to over 10 GHz. Many radar systems
operate in a couple of bands around 1.3 GHz (C band) and around 9 GHz (X band).
Similar operating frequencies are used in other countries, with worldwide coordina-
tion done through the International Telecommunication Union (ITU).
The most significant bands for wireless LANs (WLANs) are 900 MHz (actu-
ally 902-928 MHz), 2.4 GHz (2400-2483 MHz), and 5 GHz (5150-5250 MHz,
5250-5350 MHz, and 5725-5825 MHz). These are existing bands designated for
industrial, scientific, and medical applications.
Most of the current WLAN equipment uses the 2.4 GHz band (such as 802.11b
and g), while some of the newer high-speed equipment is targeted for the 5 GHz band
(802.11a). This band's low-power, unlicensed operation minimizes potential interfer-
ence by limiting the distance that the signal can travel. Other popular WLAN tech-
nologies include Bluetooth and WiMAX. An older cellular wireless networking
standard, CDPD, and a newer GSM wireless standard, GPRS, exists in the cell phone
bands, but they are wireless WAN access standards, not really WLAN standards.
Modulation
The information we transmit over a radio frequency is (RF) placed on the carrier
wave by a process called modulation . A wide variety of modulation schemes exist,
but they basically are variations of either amplitude or frequency/phase modulation,
sometimes with multiple carriers. The AM broadcast band is the simplest example
of amplitude modulation. The carrier is simply increased or decreased in intensity
by the amplitude of an audio signal. In contrast, FM broadcast transmission makes
the frequency of the output vary in proportion to the audio signal.
Wireless networking uses several variations of these modulation methods to
allow high-speed data to be modulated into an RF signal. Popular modulation tech-
niques include one that causes the base or carrier frequency of the transmission to
hop around among a select number of discrete frequencies, while encoding the data
varying both the amplitude and phase of the signal. This encoding is similar to the
encoding and modulation methods we use for high-speed modems, with the addi-
tion of this frequency hopping behavior. Another modulation technique digitally
spreads the transmitted signal within a channel bandwidth.
The formal name for either of these jumping-bean or spread-out transmissions
is spread-spectrum operation . We will cover more on the reasons for using this tech-
nique a little later. The two spread spectrum techniques described above are “fre-
quency hopping” and “direct sequence,” although there are others.