Discussion for understanding N Channels

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redhawk0
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PostPosted: Thu May 21, 2009 20:10    Post subject: Reply with quote
Wow...so this not only can interfere with the adjacent channel...it can extend into channels above and below the adjacent channel also.

Great pic...that explains a lot

redhawk

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Eko
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PostPosted: Thu May 21, 2009 20:15    Post subject: Reply with quote
LOM wrote:
802.11n is a modulation technology which can be used on either a single 20MHz channel or a pair of 20Mhz channels (channel bonding).

RIGHT!
LOM wrote:

In the 2.4GHz band each of the channels 1..13 are 5Mhz wide so we need 4 of them to build a 20Mhz channel and 8 of them to build a 40Mhz 802.11n channel bond.

WRONG! channels are 5MHz appart, not 5MHz wide. Even normal old b-band or g-band channel is 20MHz wide
It takes space of 4 channels... but it's still only 1
LOM wrote:

The bond, being designated "upper" or "lower", must therefore be spaced 4 channels away from the control channel.

I would think so too.

Channel Bonding, also known as 40 MHz, is a second technology incorporated into 802.11n which can simultaneously use two separate non-overlapping channels to transmit data. Channel bonding increases the amount of data that can be transmitted. 40 MHz mode of operation uses 2 adjacent 20 MHz bands. This allows direct doubling of the PHY data rate from a single 20 MHz channel. (Note however that the MAC and user level throughput will not double.)



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Last edited by Eko on Thu May 21, 2009 20:35; edited 2 times in total
Eko
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PostPosted: Thu May 21, 2009 20:18    Post subject: Reply with quote
good reading http://www.wi-fi.org/files/kc/WFA_802_11n_Industry_June07.pdf
Eko
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PostPosted: Thu May 21, 2009 20:47    Post subject: Reply with quote
And the code from broadcom....

Code:
/* Get Ctrl SB for 40MHz channel */
      if (nbw == WL_CHANSPEC_BW_40) {
         str = nvram_safe_get(strcat_r(prefix, "nctrlsb", tmp));

         /* Adjust the channel to be center channel */
         if (!strcmp(str, "lower")) {
            nctrlsb = WL_CHANSPEC_CTL_SB_LOWER;
            channel = channel + 2;
         } else if (!strcmp(str, "upper")) {
            nctrlsb = WL_CHANSPEC_CTL_SB_UPPER;
            channel = channel - 2;
         }
      }
Eko
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PostPosted: Thu May 21, 2009 20:59    Post subject: Reply with quote
So I think maybe i figured this out...

let say we have displayed "1 + 3"... this means that first 20MHz channel is 1, second 20MHz channel is 5, but if we add both 20MHz together, we get 1 40MHz channel with center at 3.

Explains why when using "upper", max 1st channel is 9, so it's using 9 and 13, but displays "9 + 11". And for "lower" min is 5. So it's using 5 and 1; displays "5 + 3"... right??

Similar for A-band channels.

Any thoughts??
redhawk0
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PostPosted: Thu May 21, 2009 21:06    Post subject: Reply with quote
Eko wrote:
So I think maybe i figured this out...

let say we have displayed "1 + 3"... this means that first 20MHz channel is 1, second 20MHz channel is 5, but if we add both 20MHz together, we get 1 40MHz channel with center at 3.

Explains why when using "upper", max 1st channel is 9, so it's using 9 and 13, but displays "9 + 11". And for "lower" min is 5. So it's using 5 and 1; displays "5 + 3"... right??

Similar for A-band channels.

Any thoughts??


So you are saying that a 40MHz channel is only using 4 channels....two higher and two lower than the displayed channel...

I gotta think about this one some more...

redhawk

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Eko
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PostPosted: Thu May 21, 2009 21:12    Post subject: Reply with quote
No... it's using 2 channels, but spectra will cover also adjacent channels... so for "1 + 3" it will use spectra from -1 to 7 (8 x 5MHz), right?? So you can't have another non-overlapping 40MHz channel anywhere.

But you can use another 20MHz channel e.g. on 9 (7 - 11) 10, 11 ....
Macgyver
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PostPosted: Fri May 22, 2009 3:16    Post subject: Reply with quote
Eko wrote:
So I think maybe i figured this out...

let say we have displayed "1 + 3"... this means that first 20MHz channel is 1, second 20MHz channel is 5, but if we add both 20MHz together, we get 1 40MHz channel with center at 3.

Explains why when using "upper", max 1st channel is 9, so it's using 9 and 13, but displays "9 + 11". And for "lower" min is 5. So it's using 5 and 1; displays "5 + 3"... right??

Similar for A-band channels.

Any thoughts??


LOM I had a feeling that channels were working like that diagram..that picture explains what I was trying to say much easier Laughing Very Happy
using 'upper' max 1st channel is 9, but really using 11 as the primary, and 7 as the secondary, but showing 9 as the resulting channel?
and for 'lower' min being 5, (selectable?), that would be 3 in the resultant channel and 1 as the primary, 5 as secondary?
I know before things were changed around in the recent builds, to me it made more sense how 'lower' was the lower of the 2 bonded channels, and 'higher' was the higher one first (lower 52 = 52+56 displaying 54, higher 56 = 56+52= 54)
Basically, for 2.4GHz band it would be like this for 40MHz:
1+5
2+6
3+7
4+8
5+9
6+10
7+11

AND other combinations of:

11+7
10+6
9+5
8+4
7+3
6+2
5+1

Spec dictates that the bonded channel be released if there is interference to be a 'good neighbor', and also that a form of CTS protection is used in 40MHz mode
I typed out something for 5GHz band a while ago it's here somewhere....just can't find it,
but same thing..
36+40
52+56
etc....
not technical by any means just trying to keep it simple

The broadcom code is only showing the middle of the spectrum between the 2 channels as the 'channel' when really the channelS are -2/+2 of the indicated channel

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Macgyver
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PostPosted: Fri May 22, 2009 3:33    Post subject: sorry to c/p this again but seems relevant here Reply with quote
Just adding this post from greekstile:
Quote:
I'll start by quoting from this ticket:

http://svn.dd-wrt.com:8000/dd-wrt/ticket/815

Part of the issue, "Wide channel selection not working -N only mode results in -2 undefined channel when 40MHz option is selected".

Brainslayer's response, "channel selection in 5 ghz is something different. consider that 5 ghz is forced to auto selection due the FCC / ETSI regulation".

If I am understanding this right, it seems like the devs believe that the only way to 40mhz channel bonding and 270mbps is with auto channel selection? This is not correct. This only applies to the DFS channels listed below:

•52 (5260 MHz)

•56 (5280 MHz)

•60 (5300 MHz)

•64 (5320 MHz)

•100 (5500 MHz)

•104 (5520 MHz)

•108 (5540 MHz)

•112 (5560 MHz)

•116 (5580 MHz)

•120 (5600 MHz)

•124 (5620 MHz)

•128 (5640 MHz)

•132 (5660 MHz)

•136 (5680 MHz)

•140 (5700 MHz)

Due to sharing with military and weather radar, these channels have to auto switch if they are interfering. The remaning channels (34, 36, 40, 44, 48, 149, 153, 157, and 161) are not subject to DFS and can be set manually.


So we should be able to manually set 34, 36, 40, 44, 48, 149, 153, 157, and 161, and the other channels in the list should be the ones scanned when set on 'auto'?
Out of all the above channels, there are different channels available in north america, and europe/japan
edit:
still can't find my previous posts about channels but it had some useful info in it...been a long day and have the dumb Confused

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LOM
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PostPosted: Fri May 22, 2009 4:36    Post subject: Reply with quote
Eko wrote:

LOM wrote:

In the 2.4GHz band each of the channels 1..13 are 5Mhz wide so we need 4 of them to build a 20Mhz channel and 8 of them to build a 40Mhz 802.11n channel bond.

WRONG! channels are 5MHz appart, not 5MHz wide. Even normal old b-band or g-band channel is 20MHz wide
It takes space of 4 channels... but it's still only 1


Channels, according to the frequency plan, are always 5MHz apart and therefore also 5MHz wide.
The 5GHz band (802.11a) with it's 20MHz wide channels therefore uses channel numers which are 4 channels
apart (48, 52,... 64).

Let's take a look at the different standards, their bandwidth (channel usage) and their throughput:

802.11a 5GHz 4 x 5MHz channel, typical throughput 27Mbs, net bit rate 54Mbs.
802.11b 2.4GHz Single 5MHz channel, typical throughput 5Mbs, net bit rate 11Mbs.
802.11g 2.4GHz 4.5(=5) 5Mhz channels, typical throughput 22Mbs, net bit rate 54Mbs.

802.11n 4 x 5Mhz channels, net bit rate 65Mbs per spatial stream, 2 streams gives 130Mbs.
802.11n 8 x 5Mhz channels, net bit rate 135Mbs per spatial stream, 2 streams gives 270Mbs.

Eko wrote:
And the code from broadcom....

Code:
/* Get Ctrl SB for 40MHz channel */
      if (nbw == WL_CHANSPEC_BW_40) {
         str = nvram_safe_get(strcat_r(prefix, "nctrlsb", tmp));

         /* Adjust the channel to be center channel */
         if (!strcmp(str, "lower")) {
            nctrlsb = WL_CHANSPEC_CTL_SB_LOWER;
            channel = channel + 2;
         } else if (!strcmp(str, "upper")) {
            nctrlsb = WL_CHANSPEC_CTL_SB_UPPER;
            channel = channel - 2;
         }
      }


This makes sense but this code is only for adjusting the primary channel in a bond pair.

Ouch, this will become very technical..

Lets take an example of 40MHz operation in the 5GHz band:

User picks channel number 52 which is a valid number according to the channel plan.
He also chooses lower for the bonded channel.
Channel 52 consists of the 4 channels 52,53,54, and 55.
Now, the radio doesn't operate from beginning channel number and upwards, it operates from
the center and has a lower sideband of 10MHz and an upper sideband of 10MHz with the carrier
in the middle So the radio must be tuned to channel 54!

If user had chosen upper for the bond, then we subtract 2 and get 50 as the center frequency for the primary channel.
That is channel 48 (48,49,50,51) according to the channel plan.
In both of these cases, the bonded channels center frequency must be handled in another piece of code.


But, what do you want to display in the GUI?
The channel according to the channel plan or the adjusted value for the radios carrier?
You display 11 + 9 where 11 is the channel start for the primary channel but 9 is the center
for the bonded channel.
You'll have to decide one way or the other for displaying them but not both ways. Wink

I also believe that the code snippet above is intended for 5GHz only, in the 2.4GHz band you
specify your center frequency directly.

One question you have to ask yourself is if it should be allowed for the user to place
a 40Mhz (8 x 5MHz ) bond anywhere in the 2.4GHz band or if 40Mhz bonds should be locked
to certain channels and always leave the same 3 channels free.
If memory serves me right, Brainslayer had them fixed to 1,2,3,4 + 5,6,7,8 and did not
allow channel selection for 802.11n 40Mhz in the 2.4GHz band.
That would equal channel 3 upper.
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PostPosted: Fri May 22, 2009 5:17    Post subject: deja vu! Reply with quote
edit: this post does not contain spatial streams, but an attempt to simplify matters and clarify things.
I found the flashback..I knew it wasn't a dream, and it was driving me nuts not knowing where the info was because I typed it already before! Laughing
The previous thread/info is here regarding 5GHz channels:
http://www.dd-wrt.com/phpBB2/viewtopic.php?p=266495#266495

The long of the short of it is this:
Using 2.4GHz band as a reference

Channel 1 is 'called' 2.412GHz, but it is really the 2.401GHz - 2.423GHz spectrum and 2.412 is the peak of the spectrum, hence the confusion about 20mhz(22mhz) channel width...hope that is somewhat cleared up.
Channel 1 is technically 2.401GHz - 2.423GHz, 6 is 2.426-2.448GHz, 11 is 2.451GHz - 2.473GHz, and everything in between works the same way with the 'frequency' being the peak of the spectrum.
This is why channels 1, 6 and 11 are the only ones that don't overlap, and how the 2.4GHz spectrum works with regards to channels
now on to the 5GHz:
Quote:

The possible channels for 5GHz systems are: 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 149, 153, 157 and 161.

NOTE: Channels may be slightly different depending on the regulatory requirements of the country.
Supported combinations of 802.11n channels at 5GHz with 40MHz bandwidth are:
(36,1) (40,-1); (44,1) (48,-1); (52,1) (56,-1); (60,1) (64,-1); (100,1) (104,-1); (108,1) (112,-1); (116,1) (120,-1); (124,1) (128,-1); (132,1) (136,-1); (149,1) (153,-1); (157,1) (161,-1)

there is no need for it to be any more confusing than that Cool

OR just have a simple channel list like:

(36,1)
(40,-1)
(44,1)
(48,-1)
(52,1)
(56,-1)
(60,1)
(64,-1)
(149,1)
(153,-1)
(157,1)
(161,-1)

These channels don't work in north america:
(100,1) (104,-1); (108,1) (112,-1); (116,1) (120,-1); (124,1) (128,-1); (132,1) (136,-1)

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greekstile
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PostPosted: Fri May 22, 2009 10:40    Post subject: Reply with quote
This is about as simple an explanation as one will find. Especially in dealing with the 2.4GHz spectrum:

802.11 b/g devices use a block of spectrum in the 2.4 GHz range that is 20 MHz wide. Here is a break down of how the non-overlapping channels are allocated:

Channel 1 uses 2402 MHz to 2422 MHz.
Channel 6 uses 2427 MHz to 2447 MHz.
Channel 11 uses 2452 MHz to 2472 MHz.

The earlier 802.11n devices used double the amount of spectrum with 40 MHz wide bands centered around channel 6. Unfortunately there are only three of these non-overlapping channels in the 2.4 GHz space for North America. This meant that any device that is centered around channel 6 using 40 MHz of bandwidth would run all the way from 2417 MHz to 2457 MHz which is the almost the entire usable spectrum.

http://blogs.zdnet.com/Ou/?p=410

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PostPosted: Fri May 22, 2009 10:57    Post subject: Reply with quote
This is not 100% true. Non-overlapping channels are 1,5,9 and 13 (but you in usa can't use 13 - so it's recommended to use 1,6,11; but as well 1,5,9 or 2,6,10 or 3,7,11 or 1,6,10 or 1,7,11 or 2,7,11 or 1,6,10 or 2,6,11 or ...)
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