802.11b Channels and Interference
The IEEE 802.11b
specification
details 11 possible overlapping frequencies on which communications can take place. Much like the
different channels on a cordless phone, changing the channel can help
eliminate noise that degrades network performance and can even allow
multiple networks to coexist in the same physical space without
interfering with each other.
Rather than attempting to set up a single central access point with a
high-gain omnidirectional antenna, you will probably find it more
effective to set up several low-range, overlapping
cells.
If you use access point hardware, and all of the APs are connected to
the same physical network segment, users can even roam seamlessly
between cells.
This spectrum's 11 overlapping channels are shown in
Table 2-1.
Table 2-1. 802.11b channel frequencies
|
1
|
2.412
|
|
2
|
2.417
|
|
3
|
2.422
|
|
4
|
2.427
|
|
5
|
2.432
|
|
6
|
2.437
|
|
7
|
2.442
|
|
8
|
2.447
|
|
9
|
2.452
|
|
10
|
2.457
|
|
11
|
2.462
|
The channels actually use 22MHz of signal bandwidth, so adjacent
radios will need to be separated by at least five channels to see
zero overlap. For example, channels 1, 6, and 11 have no overlap.
Neither do 2 and 7, 3 and 8, 4 and 9, or 5 and 10. While you will
ideally want to use non-overlapping channels for your access points,
in a crowded setting (such as a city apartment building or office
park) this is becoming less of an option. It is possible, although
not ideal, to use channels that are closer together. For example, you
can use channels 1 and 4 in the same space.
You stand a better chance at saturating your area with usable signal
from many low-power cells rather than a single tower with a high-gain
antenna. As your individual cells won't need a
tremendous range to cover a wide area, you can use lower gain (and
lower cost) antennas, further limiting the chances of interfering
with other gear in the band. As you can see in Figure 2-2, you could use as few as
three
channels to cover an infinitely large area, with no channel overlap
whatsoever.
The worst possible case would involve two separate, busy networks
trying to occupy the same channel, right next to each other. The
further you can get away from this nightmare of collisions, the
better. Realistically, a single channel can easily support 50 or more
simultaneous users, and a fair amount of channel overlap is
tolerable. The radios use the air only when they actually have
something to transmit, and retransmit automatically on error, so
heavy congestion feels more or less like ordinary net lag to the end
user. The sporadic nature of most network traffic helps to share the
air and avoid collisions, like playing cards shuffling together into
a pack.
To sum up: be a good neighbor, and think about what
you're doing before turning on your own gear. The
radio spectrum is a public resource and, with a little bit of
cooperation, can be used by everyone to gain greater access to
network resources.
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