Hardware Reference
In-Depth Information
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 ofconnecting with other products to minimize potential interfer-
ence.
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
changesignalingmethods,alongwiththeadditional overheadrequiredwhensecurityfea-
tures 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 pos-
itioning 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 dis-
tance increases, speed decreases because the signal strength is reduced.
