I will use a 802.11n router with 3 antennas that is able to operate
simultaneously in the 2,4 GHz and 5 GHz band, so it advertises "up to
900Mbps" (i.e. 450 Mbps in the 2,4 + 450 Mbps in the 5 GHz band) - I do
not know if it is able to use 80 MHz channels, but I read at wikipedia
that this is only available for the new 802.11ac generation and not for
the 11n that I own. Is that correct?
I suppose theoretically with 3 radios in the 2.4Ghz spectrum and 3 in
the 5Ghz spectrum (so 6 radios total) you could potentially push
higher speeds (possibly ~160Mbps total across both spectra).
Could I tweak an 11n to use 80 MHz channels, e.g. by using an
alternative firmware on the router such as dd-wrt?
I think with 3 radios, you could potentially use 60Mhz across 3
channels, though you will need to be very careful (especially at 5Ghz)
to make sure the frequencies you're using are legal - the 5Ghz
spectrum is complicated - bands A B and C have different regulations
and allowable power levels.
Not responding to all "explanations" here, there do seem to be some
misunderstandings:
With .11n you either use HT20 or HT40 (20Mhz or 40Mhz wide channel).
You don't have multiple radios per card. You have multiple RF-chains
which each "can" carry their own spatial stream.
The number of antennas most often (but not necessarily) correlate with
the number of RF-chains you have internally.
You "can" have one spatial stream per chain. However multiple spatial
streams only work if you are in an environment where reflections exist.
--> in a long-distance point-to-point link without reflections you can
have only a single spatial stream, limiting the bandwidth to 150Mbps
(MCS7, SGI and HT40).
The additional Antennas there only help the signal integrity (google
Space-Time-Block-Code).
For a list of what bandwidth is to be expected with which settings see:
http://mcsindex.com/
The claimed 450Mbps of WLAN usually refers to MCS23 --> 3 spatial
streams each with SGI and HT40.
So you have per radio (refering to a single WLAN-card):
one center-frequency (be it 2.4 or 5 GHz band)
multiple MCS-indices which change with an algorithm (google "wireless
minstel").
multiple bandwidths: 20Mhz or 40Mhz (with 11.ac 80MHz) which change with
minstrel too
Depending on the link quality the MCS index, the bandwidth and the guard
interval change controlled by minstrel.
The 450Mbit are only possible when both sides (client and AP) have a 3x3
radio (3 receive chains, 3 transmit chains), there are enough
reflections around for the spatial streams to be differentiated, the
signal strength of each stream is high enough that it can be decoded
correctly (consumer market devices usually require a signal greater than
-60dBm.)
If one side has only a 1x1 radio (usually the client), then you will be
limited to 72.2/150Mbps at MCS7.
I have yet to see a consumer-market device (besides the APs) actually
containing a 3x3 radio (sometimes 2x2).
So yes there are quite many things which can limit your bandwidth to
only 50-80Mbps, but they usually aren't a limitation of the
hardware/software, but simply of a misunderstanding what is actually
required to achieve higher bandwidths.
It's usually not the AP which is the problem, but the client.
Some real-world advice (which you probably already know):
Use two radios: one 2.4Ghz, one 5Ghz,
Use a frequency no-one uses if possible, allow HT40, allow SGI.
Minstrel will scale down to HT20 and no SGI when required.
There really isn't much more you can do other than using better hardware
which costs remarkably more.
Regards
Matthias May
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