Hardware Reference
In-Depth Information
wireless speakers, security devices, microwave ovens, and the Bluetooth short-range networking
products use. Although the increasing use of these products is a potential source of interference, the
short range of wireless networks (indoor ranges up to approximately 150 feet and outdoor ranges up
to about 300 feet, varying by product) minimizes the practical risks. Many devices use a spread-
spectrum method of connecting with other products to minimize potential interference.
Although 802.11b supports a maximum speed of 11Mbps, that top speed is seldom reached in
practice, and speed varies by distance. Most 802.11b hardware is designed to run at four speeds,
using one of four data-encoding methods, depending on the speed range:
•
11Mbps
—Uses quaternary phase-shift keying/complementary code keying (QPSK/CCK)
•
5.5Mbps
—Also uses quaternary phase-shift keying/complementary code keying (QPSK/CCK)
•
2Mbps
—Uses differential quaternary phase-shift keying (DQPSK)
•
1Mbps
—Uses differential binary phase-shift keying (DBPSK)
As distances change and signal strength increases or decreases, 802.11b hardware switches to the
most suitable data-encoding method. The overhead required to track and change signaling methods,
along with the additional overhead required when security features are enabled, helps explain why
wireless hardware throughput is consistently lower than the rated speed.
Figure 17.3
is a simplified
diagram showing how speed is reduced with distance. Figures given are for best-case situations;
building design and antenna positioning can also reduce speed and signal strength, even at relatively
short distances.
Figure 17.3. At short distances, 802.11b devices can connect at top speed (up to 11Mbps).
However, as distance increases, speed decreases because the signal strength is reduced.
Because of its slow speed and support for the weak and easily cracked WEP encryption standard,
IEEE 802.11b is obsolete in new installations. However, 802.11b-compliant hardware can
interconnect with 802.11g and 802.11n hardware as long as mixed mode and WEP encryption (or no
encryption) are used in network configuration.
IEEE 802.11a
The second flavor of Wi-Fi is the wireless network known officially as IEEE 802.11a. 802.11a (also
referred to as Wireless-A) uses the 5GHz frequency band, which allows for much higher speeds (up
