WiFi, 802.11B / G – 2,4GHz en
802.11A – 5GHz
There are a lot of dubious stories
about WiFi and especially about the 5GHz 802.11a standard. We have put some
facts together:
The
frequencies and maximum power output settings for Europe |
|
Frequency |
Maximum power
output |
2400 – 2483.5
MHz |
100 mW
e.i.r.p. (1) |
5150 – 5350
MHz (2) |
200 mW
e.i.r.p. (3) |
5470 – 5725
MHz (2) |
1 W e.i.r.p.
(3) |
1 ) For ‘Direct Sequence Spread
Spectrum’ the maximum spectral power density should be limited to -20 dBW/1
MHz. For ‘frequency hopping spread spectrum’ the maximum spectral power density
should be limited to -10 dBW/100 kHz. (100 mW e.i.r.p - Equivalent
isotropically radiated power - means 100mW send from the antenna; this is
typically measured on 10 meters from the center of the antenna.)
2 ) equipment working on the
frequencies 5470 to 5725 MHz and/or 5250 to 5350 MHz , must have the following
options: ‘Transmit Power Control’ and ‘Dynamic Frequency Selection’. (note:
This is the new 802.11h standard) Transmitter Power Control means that the
system must be able to lower the average output power with 3 dB when possible
and when other signals on the same frequency are detected. Dynamic Frequency Selection in this context
means that the device must be bale to select an other channel when encountering
a signal on the currently used channel.
3 ) This is the maximum averaged
e.i.r.p. (Equivalent isotropically radiated power). Averaged e.i.r.p means the
average of a burst transmission on highest setting of the transmitter. If a
transmitter can be hard-coded of set to a fixed maximum output setting, this
setting is regarded as the highest possible setting.
Channels vs. frequencies.
For the 2.4 GHz.
In Europe there are 13 channels
available, in the US 11 channels and in Japan 14 channels. Some countries have
other regulations, but the mentioned 3 are the most used. These channels
correspond with a frequency. The channel vs. frequency table is as follows:
Channel |
Freq. |
Usable |
1 |
2412 |
EU & US |
2 |
2417 |
|
3 |
2422 |
|
4 |
2427 |
|
5 |
2432 |
|
6 |
2437 |
US |
7 |
2442 |
EU |
8 |
2447 |
|
9 |
2452 |
|
10 |
2457 |
|
11 |
2462 |
US |
12 |
2467 |
|
13 |
2472 |
EU |
14 |
2484 |
|
When configuring the 2.4 GHz
channels/frequency’s keep in mind that no all channels can be used. The WiFi
radio’s are overlapping neighboring channels when they sending a signal.
Channels 1, 6 and 11 can be used for the US settings and Channel 1, 7 and 13 in
Europe and Japan. Do not use the channels in-between! These channels are
to be avoided, because you would be interfering two regularly used channels.
E.g. if you would use channel 3, you would interfere, not only on channel 1,
but also on channel 6. Effectively you would be scrambling 2 of the 3 usable
channels. (you would not be very polite towards your neighbors, so keep this in
mind!)
For the 5 GHz.
I will stick to Europe here. There
are lots of different regulations worldwide; I will not go into here.
For Europe there are two blocks of
frequency’s; from 5150 to 5350 MHz and from 5470 to 5725MHz.
# |
Freq. |
Channel |
# |
Freq. |
Channel |
|
5180 |
36 |
5 |
5500 |
100 |
1 |
5200 |
40 |
|
5520 |
104 |
|
5220 |
44 |
6 |
5540 |
108 |
2 |
5240 |
48 |
|
5560 |
112 |
|
5260 |
52 |
7 |
5580 |
116 |
3 |
5280 |
56 |
|
5600 |
120 |
|
5300 |
60 |
8 |
5620 |
124 |
4 |
5320 |
64 |
|
5640 |
128 |
|
|
|
9 |
5660 |
132 |
|
|
|
|
5680 |
134 |
|
|
|
10 |
5700 |
138 |
The channels in the 5 GHz are 20MHz
wide (much wider as the 2.4GHz: 5MHz). We do not need as much channels to
separate the radio’s here. 1 free channel between each used channel is enough
for the 5GHz. The picture below
explains very clearly how channel separation works.