Discussion for understanding N Channels

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redhawk0
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PostPosted: Thu May 21, 2009 16:09    Post subject: Discussion for understanding N Channels Reply with quote
LOM wrote:
redhawk0 wrote:


Channel selected is 11 (Lower) and "11 + 9" is displayed correctly on the Status tab.

redhawk


Strange channel numbers for 2.4GHz channel bonding.
I noticed the +- 2 in Eko's changeset yesterday and do not agree with that math.. Confused



Help me understand this.....

So your saying that the channel selection isn't centered around a channel (in my understanding 7 to 11 with 9 being the center) but it is actually the lowest channel +4 meaning it should be channel 7 to 11 set to channel 7.

So...it should read

11 + 7

in the display on the status tab?

redhawk

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Last edited by redhawk0 on Sat May 23, 2009 12:46; edited 1 time in total
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LOM
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PostPosted: Thu May 21, 2009 16:30    Post subject: Reply with quote
802.11n is a modulation technology which can be used on either a single 20MHz channel or a pair of 20Mhz channels (channel bonding).

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.

The bond, being designated "upper" or "lower", must therefore be spaced 4 channels away from the control channel.
LOM
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PostPosted: Thu May 21, 2009 16:51    Post subject: Reply with quote
If the selected channel equals the center of a group of channels, then channel 1 should be an invalid choice
for 802.11g and only be allowed for 802.11b.

802.11 g does also use 4 channels to create one g-channel similar to 802.11n.
No bonding for 802.11g and a different modulation technology though.
So if channel 1 is the middle of four , what is then the lowest frequency? Channel -1 ? Cool
barryware
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PostPosted: Thu May 21, 2009 16:52    Post subject: Reply with quote
Some more information that may help.

http://www.dd-wrt.com/phpBB2/viewtopic.php?t=51139

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redhawk0
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PostPosted: Thu May 21, 2009 16:55    Post subject: Reply with quote
So in my case above.

Channel 11 Lower selected.

It should display channel 7 as the bonded channel but use channels 4-11 (8 channels wide)

Would this be equal to selecting channel 4 Upper?


redhawk

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LOM
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PostPosted: Thu May 21, 2009 17:17    Post subject: Reply with quote
redhawk0 wrote:
So in my case above.

Channel 11 Lower selected.

It should display channel 7 as the bonded channel but use channels 4-11 (8 channels wide)

Would this be equal to selecting channel 4 Upper?


redhawk


Well, then you assume that the selection 11 means that 11 is not the start or the center , but the highest channel number that should be included for use.
And consequently that 7 means the highest of the bond 4-7.

My understanding is that the selected channel should be the start of the group so 11 would be 11-14
with 7-10 as the bonded channel.

Upper and lower is an attribute for the extra (bonded) groups of channels.
Swapping setting between upper and lower does not change what channels get occupied it only decides which of the 2 groups the bond group is.
The upper lower selection will therefore also decide which 4 channels will be abandoned if the router decides to release the bond (neighbour friendly detection and channel release).
redhawk0
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PostPosted: Thu May 21, 2009 17:32    Post subject: Reply with quote
Ah...now I am starting to get it....I think.

So...channel 5 Upper would use

5-8 as the 4 channels used...but channels 1-4 would be bonded to the upper 5-8 channels.



Channel 5 Lower would use 2-5 as the 4 channels used...but channels 6-9 would be bonded to the 2-5.


Am I getting closer....because I selected a channel above 7 as my base channel...FW had to lower the channel base so it can include upper and lower from the base channel....like you said...it can't go above 11 in US.


redhawk

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greekstile
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PostPosted: Thu May 21, 2009 17:42    Post subject: Reply with quote
By the way, I hope you guys using 40MHz wide channels on the 2.4GHz spectrum don't have any neighbors with WiFi. If so, you may be absolutely killing their connections. There is a reason wide channels aren't recommended or even supported by many adapters on the 2.4GHz band. You basically hog all the channels and screw your neighbors.
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LOM
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PostPosted: Thu May 21, 2009 18:44    Post subject: Reply with quote
greekstile wrote:
By the way, I hope you guys using 40MHz wide channels on the 2.4GHz spectrum don't have any neighbors with WiFi. If so, you may be absolutely killing their connections. There is a reason wide channels aren't recommended or even supported by many adapters on the 2.4GHz band. You basically hog all the channels and screw your neighbors.


A very valid point!
I have just ordered a WRT610N, took a couple of weeks Smile to decide between that one or a WRT350N.
It will be the 4th router here and the others are 802.11g only so I figured that 5GHz was a must if I should be able to run full 802.11n speeds.
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PostPosted: Thu May 21, 2009 18:49    Post subject: Reply with quote
1 thing I don't get with this explanation of channels is that channels 1,6 and 11 are the only non-overlapping channels, so why can't 1 and 11 be used? Typically channel 11 has the best signal, and if it is using 20MHz G channel, I don't see how that is even possible in north america, same goes for channel 1.
So if 1, 6 and 11 don't overlap/interfere, why can't we use 1 or 11 then? The only theory (and I emphasize THEORY) I have is that it will use 20MHz channel band of that spectrum, but what do I know Confused Could someone explain how 1,6 and 11 don't overlap and at the same time 2 of those 3 can't be used?
Could there be a difference in what is selected and what is actually used?
I can attest to the fact that the best 40MHz channel I get is using lower 3 (3-7) which effectively kills the entire 2.4GHz range according to this explanation, but that still does not even make sense to me because 3-4= -1 if you know what I mean...the part of channel 7 using extra MHz to 11 makes sense, but that's about it.
Please explain!
P.S I am a nice neighbor...only using G mode in 2.4GHz on 1 channel...wide band is for 5GHz Very Happy

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redhawk0
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PostPosted: Thu May 21, 2009 18:57    Post subject: Reply with quote
Yeah...I thought about the whole "channel-hog" thing too. I inititally tested the unit on 40MHz but have since selected Channel 11 and Set it to 20MHz. I get 130M speeds and that is good enough for my purposes. I've lived with 54M for so long...to almost triple it is quite noticable...however, I don't do a lot of LAN-LAN transfers to really take advantage of it.

I may even drop it down to 10MHz just to see what advantage I gain over the old single channel 5MHz broadcast.

Still playing...and I appreciate your helping me understand N class a bit better.

redhawk

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Macgyver
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PostPosted: Thu May 21, 2009 19:03    Post subject: Reply with quote
redhawk your findings explain what I was trying to say...I get the best performance AND signal using 20MHz channel 11(mixed mode), so how is that even possible based on the explanation above? It just doesn't make any sense to me, unless the router uses the adjacent channels and they are non-selectable...now that makes SOME sense!
20MHz channel width should not even work on channel 11 (according to the explanation above), but it does, so why would channel 1 not work the same way by the same token????? Confused

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greekstile
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PostPosted: Thu May 21, 2009 19:12    Post subject: Reply with quote
Macgyver wrote:
P.S I am a nice neighbor...only using G mode in 2.4GHz on 1 channel...wide band is for 5GHz Very Happy


Well said! I'm a firm believer in keeping wideband out of the 2.4GHz range.

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redhawk0
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PostPosted: Thu May 21, 2009 19:24    Post subject: Reply with quote
Sorry...I see 10MHz is as low as I can select....It must be that +/-5MHz around the center frequency (for a 10MHz total)...that would explain the cross-overs in adjacent channels

Almost like FM radio...modulates above an below the Base Frequency....that is why all FM channels are an odd .X number

So Channel 10's high 5MHz is the same as Channel 11's low 5MHz

redhawk

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LOM
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PostPosted: Thu May 21, 2009 20:04    Post subject: Reply with quote
Macgyver, I just found out that they do actually use
frequency spectrum below channel 1 for 802.11g Embarassed

Here are the non overlapping 802.11g channels, each channel (group) being 22Mhz wide with a 3Mhz gap between them.

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