Hello Matthias,
Thank you for your time!
On 2013-10-25 12:45, Matthias May wrote:
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).
Ok, that confirms what I read on wikipedia: 80 MHz comes with 801.11ac
only..
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.
Yes, as far as I know, each RF-chain of N-draft is 150 Mbps, so a
router/AP that is advertised as 450Mbps and that comes with 3 antennas,
should have 3 RF-chains, isn't it?
In contrast to that I have seen some routers with 3 antennas but only
300Mbps, so they seem to have only 2 RF-chains but for some reason come
with 3 antennas.
You "can" have one spatial stream per chain.
What exactly is a spatial stream and how do I initiate it?
However multiple spatial streams only work if you are in an
environment where reflections exist.
.. such as a WLAN inside a normal building, right?
--> 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.
I do not understand this section and the table at mscindex.com, since my
knowledge of WLAN seems to be too limited. Could you give me some
kick-off help to understand what and how I need to set my AP/router so
to achieve the highest bandwidth possible (i.e. as close to the
advertised 450 Mbps per band, as possible)?
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.
Ok, I will experiment with ministrel!
The 450Mbit are only possible when both sides (client and AP) have a
3x3 radio (3 receive chains, 3 transmit chains),
I will be using such clients so I guess I comply with that requirements...
... there are enough reflections around for the spatial streams to be
differentiated,
... the WLAN will be inside a normal office building with solid brick
walls and drywalls; is that what is needed to get those reflections, or
am I misunderstanding something? ...
.. 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.)
... the AP will be located in a closed room at the center of the floor
of the building that I want to provide with WLAN. The clients will be
arranged circularly around the AP in distances of 3-15 meters each, with
an average of approx 5 meters.
Do you think that this setup will be able to approximate the 450 Mbps or
will I need to take additional measures?
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).
How about:
http://www.tp-link.com/us/products/details/?model=TL-WDN4800
http://www.intel.com/content/www/us/en/wireless-products/ultimate-n-wifi-link-5300-brief.html
Aren't those such 3x3 radio interfaces that you meant?
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,
Ok, my AP is able of using both bands simultaneously and I will be using
them.
Use a frequency no-one uses if possible
ok, there is no other WLAN nearby anyway as far as i figured
, allow HT40, allow SGI.
what are those and how do I activate them?
Minstrel will scale down to HT20 and no SGI when required.
Ok. How exactly is ministrel implemented on a linux machine? Is it a
kind of add-on that I have to plug in between the wlan0 device and e.g.
network manager, or how is the general concept of ministrel?
There really isn't much more you can do other than using better
hardware which costs remarkably more.
Do you have any further ideas on how to improve? E.g. producing more
reflection, etc. or something else?
Regards
Matthias May
Thank you!!
Kind regards
Thinker Rix
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