What is 2.4 GHz Band?
The
2.4 GHz ISM band is a commonly accepted band for worldwide operations.
Microwave ovens, cordless phones, medical diathermy machines, military radars
and industrial heaters are just some of the equipment that makes use of 2.4 GHz
ISM band.
There are total of
fourteen (14) channels defined for use by Wi-Fi 802.11 for the 2.4 GHz ISM
band. The 802.11 WLAN standards specify a bandwidth of 22 MHz and channels are
on a 5 MHz incremental step. The 22 MHz bandwidth and channel separation of 5
MHz means that adjacent channels overlaps and signals on adjacent channels will
interfere with each other.
The 22 MHz channel bandwidth holds for all standards even
though 802.11b WLAN standard can run at variety of speeds: 1, 2, 5.5, or 11
Mbps and the 802.11g standard can run at speeds up to 54 Mbps.
Fig-2: Graphical
representation of 2.4 GHz band channels overlapping
As depicted in fig-2, it is possible to find a maximum of three
non-overlapping channels i.e. 1,6 & 11 in 2.4 GHz band. A channel number 14
cannot opt as a non-overlapping because the range of this channel (2473 MHz to
2495 MHz) exceed the overall range of 2.4 GHz band (2400 MHz to 2483 MHz).
2.4 GHz
Technology Comparison
|
Technology
|
Data Rate
|
No. of Channel
|
Minimum Bandwidth Required
|
|
Wi-Fi (802.11b)
|
11 Mbps
|
14
|
22 MHz (Static)
|
|
Bluetooth
|
723 Kbps
|
79
|
15 MHz (Dynamic)
|
|
Wireless USB
|
62.5 Kbps
|
79
|
1 MHz (Dynamic)
|
|
Zigbee
|
128 Kbps
|
16
|
3 MHz (Static)
|
Table-1: 2.4 GHz Wi-Fi channel
frequencies
|
Channel Number
|
Range (in MHz)
from Lower Frequency to
upper frequency
|
Center Frequency (in MHz)
|
|
1
|
2401- 2423
|
2412
|
|
2
|
2406-2428
|
2417
|
|
3
|
2411-2433
|
2422
|
|
4
|
2416-2438
|
2427
|
|
5
|
2421-2443
|
2432
|
|
6
|
2426-2448
|
2437
|
|
7
|
2431-2453
|
2442
|
|
8
|
2436-2458
|
2447
|
|
9
|
2441-2463
|
2452
|
|
10
|
2446-2468
|
2457
|
|
11
|
2451-2473
|
2462
|
|
12
|
2456-2478
|
2467
|
|
13
|
2461-2483
|
2472
|
|
14
|
2473-2495
|
2484
|
What are
non-overlapping channels?
Each channel on the 2.4 GHz spectrum is 20 MHz wide. The channel centres
are separated by 5 MHz, and the entire spectrum is only 100 MHz wide total.
This means that the 11 channels have to squeeze into the 100 MHz available, and
in the end, overlap.
Fig-3: Graphical representation
of 2.4 GHz band non-overlapping Channels
Why 1,6
& 11 channels in 2.4 GHz band are most commonly used?
Fig-4: Graphical representation
of 2.4 GHz band non-overlapping Channels
There are five combinations of available non overlapping channels: -
i.
(1, 6, 11) or
ii.
(2,
7, 12) or
iii.
(3,
8, 13) or
iv.
(4,
9, 14; if allowed) or
v.
(5,
10 and possibly 14; if allowed)
Often Wi-Fi routers are
set to channel 6 as the default, and therefore the set of channels 1, 6 and 11
is possibly the most widely used.
40MHz channels in 2.4 GHz band –
Fig-5:
40 MHz Channel Separation in 2.4 GHz Band
With the use of IEEE 802.11n,
there is the possibility of using signal bandwidths of either 22 MHz or 40 MHz.
When 40 MHz bandwidth is used to gain the higher data throughput, this
obviously reduces the number of channels that can be used.
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