IEEE 802.11 wireless nodes communicate
with each other using radio frequency signals in the
ISM (Industrial, Scientific, and
Medical) band between 2.4Ghz and 2.5Ghz. Neighboring
channels are 5Mhz apart. However,
due to spread spectrum effect of the signals, a node sending
signals using a particular channel
will utilize frequency spectrum12.5Mhz above and below the
center channel frequency. As a result,
two separate wireless networks using neighboring channels
(for example, channel 1 and channel
2) in the same general vicinity will interfere with each other.
Applying two channels that allow
the maximum channel separation will decrease the amount of
channel cross-talk, and provide
a noticeable performance increase over networks with minimal
channel separation.
802.11b Radio Frequency Channels
|
Channel
|
Center Frequency
|
Frequency Spread
|
|
1
|
2412Mhz
|
2399.5Mhz - 2424.5Mhz
|
|
2
|
2417Mhz
|
2404.5Mhz - 2429.5Mhz
|
|
3
|
2422Mhz
|
2409.5Mhz - 2434.5Mhz
|
|
4
|
2427Mhz
|
2414.5Mhz - 2439.5Mhz
|
|
5
|
2432Mhz
|
2419.5Mhz - 2444.5Mhz
|
|
6
|
2437Mhz
|
2424.5Mhz - 2449.5Mhz
|
|
7
|
2442Mhz
|
2429.5Mhz - 2454.5Mhz
|
|
8
|
2447Mhz
|
2434.5Mhz - 2459.5Mhz
|
|
9
|
2452Mhz
|
2439.5Mhz - 2464.5Mhz
|
|
10
|
2457Mhz
|
2444.5Mhz - 2469.5Mhz
|
|
11
|
2462Mhz
|
2449.5Mhz - 2474.5Mhz
|
|
12
|
2467Mhz
|
2454.5Mhz - 2479.5Mhz
|
|
13
|
2472Mhz
|
2459.5Mhz - 2484.5Mhz
|
802.11a Frequency band
The 5 GHz U-NII frequency band
is segmented into three 100 MHz bands for operation in the US. The lower
band ranges from 5.15 -5.25 GHz, the middle band ranges from 5.25-5.35
GHz and the upper band ranges from 5.725-5.825 GHz. The lower and middle
band, accommodate 8 channels in a total bandwidth of 200 MHz and the upper
band accommodates 4 channels in a 100 MHz bandwidth. The frequency channel
center frequencies are spaced 20 MHz apart. The outermost channels of the
lower and middle bands are centered 30 MHz from the outer edges. In the
upper band the outermost channel centers are 20 MHz from the outer edges.
In addition to the frequency and
channel allocations, transmit power is a key parameter regulated in the
5 GHz U-NII band. Three transmit power levels are specified: 40 mW, 200
mW and 800 mW. The upper band defines RF transmit power levels suitable
for bridging applications while the lower band specifies a transmit power
level suitable for short-range indoor home and small office environments.
The following table shows the operating
frequency and maximum power.
OFDM Operating
Bands and channels
|
BAND
|
CHANNEL
NUMBERS
|
FREQUENCY
(MHZ)
|
MAXIMUM
OUTPUT POWER
|
|
U-NII
lower band
5.15 to
5.25 MHz
|
36
|
5180
|
40mW
(2.5mW/MHz)
|
|
40
|
5200
|
|
44
|
5220
|
|
48
|
5240
|
|
U-NII
middle band
5.25 to
5.35 MHz
|
52
|
5260
|
200mW
(12.5mW/MHz)
|
|
56
|
5280
|
|
60
|
5300
|
|
64
|
5320
|
|
U-NII
upper band
5.725
to 5.825 MHz
|
149
|
5745
|
800mW
(50mW/MHz)
|
|
153
|
5765
|
|
157
|
5785
|
|
161
|
5805
|
