Wireless Home Networking - Index of
Wireless Home Networking - Index of
Wireless Home Networking - Index of
You also want an ePaper? Increase the reach of your titles
YUMPU automatically turns print PDFs into web optimized ePapers that Google loves.
44<br />
Part I: <strong>Wireless</strong> <strong>Networking</strong> Fundamentals<br />
Understanding Wi-Fi channels<br />
Now for a little talk about frequency bands used<br />
by the various Wi-Fi standards. In 1985, the FCC<br />
made changes to the radio spectrum regulation<br />
and assigned three bands designated as the<br />
industrial, scientific, and medical (ISM) bands.<br />
These frequency bands are<br />
� 902 MHz–928 MHz: A 26 MHz bandwidth<br />
� 2.4 GHz–2.4835 GHz: An 83.5 MHz bandwidth<br />
� 5.15–5.35 GHz and 5.725 GHz–5.825 GHz: A<br />
300 MHz bandwidth<br />
The FCC also opened some additional frequencies,<br />
known as Unlicensed National Information<br />
Infrastructure (U-NII), in the lower reaches <strong>of</strong><br />
the five GHz frequencies.<br />
The purpose <strong>of</strong> the FCC change was to encourage<br />
the development and use <strong>of</strong> wireless networking<br />
technology. The new regulation<br />
permits a user to operate, within certain guidelines,<br />
radio equipment that transmits a signal<br />
within each <strong>of</strong> these three ISM bands without<br />
obtaining an FCC license.<br />
<strong>Wireless</strong> networks use radio waves to send<br />
data around the network. 802.11a uses part <strong>of</strong><br />
the U-NII frequencies, and IEEE 802.11b and g<br />
use the ISM 2.4 GHz band. 802.11n can use<br />
either band, though not all 802.11n systems do<br />
(many use only the 2.4 GHz band).<br />
An important concept when talking about frequencies<br />
is the idea <strong>of</strong> overlapping and<br />
nonoverlapping channels. As we discuss in<br />
Chapter 18, signals from other APs can cause<br />
interference and poor performance <strong>of</strong> your<br />
wireless network. This happens specifically<br />
when the APs’ signals are transmitting on the<br />
same (or sometimes nearby) channels. Recall<br />
that the standards call for a number <strong>of</strong> channels<br />
within a specified frequency range.<br />
The frequency range <strong>of</strong> 802.11g, for example, is<br />
between 2.4 GHz and 2.4835 GHz, and it’s broken<br />
up into fourteen equal-sized channels. (Only<br />
eleven can be used in the United States — any<br />
equipment sold for use here allows you to<br />
access only these eleven channels.) The problem<br />
is that these channels are defined in such a<br />
way that many <strong>of</strong> the channels overlap with one<br />
another — and with 802.11g, there are only<br />
three nonoverlapping channels. Thus, you<br />
wouldn’t want to have channels 10 and 11 operating<br />
side by side because you would get signal<br />
degradation. You want noninterfering, nonoverlapping<br />
channels. So you find that people tend<br />
to use Channels 1, 6, and 11, or something similar.<br />
802.11a doesn’t have this problem because<br />
its eight channels, in the 5 GHz frequency band,<br />
don’t overlap; therefore, you can use contiguous<br />
channels. As with 802.11b and g, however,<br />
you don’t want to be on the same channel.<br />
Wi-Fi history: 802.11b and 802.11a<br />
In 1990, the IEEE adopted the document “IEEE Standards for Local and Metropolitan<br />
Area Networks,” which provides an overview <strong>of</strong> the networking technology<br />
standards used in virtually all computer networks now in prevalent<br />
use. The great majority <strong>of</strong> computer networks use one or more <strong>of</strong> the standards<br />
included in IEEE 802; the most widely adopted is IEEE 802.3, which<br />
covers Ethernet.<br />
IEEE 802.11 is the section that defines wireless networking standards and is<br />
<strong>of</strong>ten called wireless Ethernet. The first edition <strong>of</strong> the IEEE 802.11 standard,