jpr_88 wrote:
I basically understand what I did. What I dont understand is the setting '40hz/20hz'.
It did not say 20hz, or 40hz.
Read it again, once you know what it says I'll tell you what it does,
Or read this.
In addition to specifying the channel centre frequency, 802.11 also specifies (in Clause 17) a spectral mask defining the permitted power distribution across each channel. The mask requires the signal be attenuated a minimum of 20 dB from its peak amplitude at ±11 MHz from the centre frequency, the point at which a channel is effectively 22 MHz wide. One consequence is that stations can only use every fourth or fifth channel without overlap.
Availability of channels is regulated by country, constrained in part by how each country allocates radio spectrum to various services. At one extreme, Japan permits the use of all 14 channels for 802.11b, and 1–13 for 802.11g/n-2.4. Other countries such as Spain initially allowed only channels 10 and 11, and France only allowed 10, 11, 12 and 13; however, they now allow channels 1 through 13.[31][32] North America and some Central and South American countries allow only 1 through 11.
Spectral masks for 802.11g channels
1 – 14 in the
2.4 GHz band
Since the spectral mask only defines power output restrictions up to ±11 MHz from the center frequency to be attenuated by −50 dBr, it is often assumed that the energy of the channel extends no further than these limits. It is more correct to say that, given the separation between channels, the overlapping signal on any channel should be sufficiently attenuated to minimally interfere with a transmitter on any other channel. Due to the near-far problem a transmitter can impact (desense) a receiver on a "non-overlapping" channel, but only if it is close to the victim receiver (within a meter) or operating above allowed power levels.
Confusion often arises over the amount of channel separation required between transmitting devices. 802.11b was based on DSSS modulation and utilized a channel bandwidth of 22 MHz, resulting in three "non-overlapping" channels (1, 6, and 11). 802.11g was based on OFDM modulation and utilized a channel bandwidth of 20 MHz. This occasionally leads to the belief that four "non-overlapping" channels (1, 5, 9 and 13) exist under 802.11g, although this is not the case as per 17.4.6.3 Channel Numbering of operating channels of the IEEE Std 802.11 (2012) which states "In a multiple cell network topology, overlapping and/or adjacent cells using different channels can operate simultaneously without interference if the distance between the center frequencies is at least 25 MHz."[33] and section 18.3.9.3 and Figure 18-13.
This does not mean that the technical overlap of the channels recommends the non-use of overlapping channels. The amount of interference seen on a 1, 5, 9, and 13 channel configuration can have very small difference from a three channel configuration[34] and in the paper entitled "Effect of adjacent-channel interference in IEEE 802.11 WLANs" by Villegas this is also demonstrated.[35]
802.11 non-overlapping channels for 2.4GHz. Covers 802.11b,g,n
Although the statement that channels 1, 5, 9, and 13 are "non-overlapping" is limited to spacing or product density, the concept has some merit in limited circumstances. Special care must be taken to adequately space AP cells since overlap between the channels may cause unacceptable degradation of signal quality and throughput.[36] If more advanced equipment such as spectral analyzers are available, overlapping channels may be used under certain circumstances. This way, more channels are available.[37]
20Mhz is the normal channel width, 40 is excessive (unless you live far from anyone else there will be overlap between the chanls you use, and other channels that others may be using)
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