On 25/10/13 13:56, Thinker Rix wrote:
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.
Yes.
Some manufacturers use a 2x2 radio and delay the signal of the 3rd
antenna slightly (and feed the delayed signal to the second input). In
complex reflecting environments this "might" improve the signal quality.
You "can" have one spatial stream per chain.
What exactly is a spatial stream and how do I initiate it?
http://en.wikipedia.org/wiki/Spatial_multiplexing
http://en.wikipedia.org/wiki/Transfer_function
You calculate the transfer function of the space the signal traverses
and multiply the inverse function with the received signal.
Since different spatial streams take different physical paths you get
different transfer functions and thus can calculate multiple distinct
signals out of the received signal in the timedomain.
However multiple spatial streams only work if you are in an
environment where reflections exist.
.. such as a WLAN inside a normal building, right?
Yes.
--> 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)?
This really depends on the router you are using.
Most consumer-grade users don't allow you to adjust these values.
If you have one where you can change this stuff:
The MCS index is usually what you can influence.
MCS0-7 define a single spatial stream.
MCS8-15 define two spatial streams.
etc...
If you want reliable connections it often makes sense to fix the MCS
index and don't allow it to be changed by minstrel.
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!
If you are interested in some background:
These are good starting points:
http://ecs.victoria.ac.nz/foswiki/pub/Courses/NWEN403_2013T1/LectureSchedule/Minstrel_slides.pdf
https://internetnz.net.nz/system/files/pages/2013/icc_13_final.pdf
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? ...
Yes, as long as stuff is around you get reflections.
.. 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?
To calculate the distance you always start at an attenuation of 20dB and
for 2.4GhZ a distance of about 12cm, for 5GHz about 6cm.
Double the distance, add 6dB to the attenuation. So:
25cm: 26 / 32
50cm: 32 / 38
1m: 38 / 44
2m: 44 / 50
4m: 50 / 56
8m: 56 / 62.
So already at 8m you go over the physically possible range to achieve
the highest MCS indices.
Add some more dBi to the link budget from the txpower of the transmitter
(above i calculated with txpower of 0 dBm) and the antennas, but at
higher speeds you will not get more than around +5~10dBm txpower from
consumer market devices. What is very important and most people don't
know, is that the listed txpower is only for the slowest speed. With
higher speeds the txpower goes down drastically.
The first card you linked has at this range a packet error rate of 10%.
The second one doesn't even list it's performance which is an indicator
that it's even worse.
You will never get 450Mbps *usable* data. I think the fastest we get
here in the lab is around 370/380 in a cabled environment.
The setup you describe could be improved by moving the AP from the floor
of this closed room to somewhere higher outside this room. (No walls
between clients and AP). And use good antennas.
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?
Yes these are 3x3 radios.
But have you ever seen such a card in an actual PC/Laptop/Mobiledevice
which you get from the 0815-store?
And as i wrote before, their performance isn't exactly stellar.
There are cards which have easily 10-15 dB better rx-sensitivity and
tx-powers over the full range close to the legally allowed limit.
But then we are in a price range multiples higher, only for the card,
than what you pay for the whole accesspoint (and i don't mean cheap
access-points).
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?
Enabling HT40 allows the AP to operate in 40MHz wide bands. SGI is the
short guard interval (400ns instead of 800ns, see the table in the link
before to see what difference it makes).
Again, depends on the AP you are using if you can enable/disable them
and it's default values.
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?
It's implemented in the driver. Unless you want to start messing with
the code not something you want to touch.
You can force the bitrates on linux with iw:
something along the line of
iw wlan0 set bitrates mcs-5 0
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?
Use dedicated 5GHz antennas and dedicated 2.4GHz antennas (No dualband).
Avoid APs with internal antennas.
Best would be an AP with detachable antennas and then connect your own.
Regards
Matthias May
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