Stating the obvious?

Roy_M · 05-11-2006

Hi

I was capturing some packets in ethereal the otherday and something new about 2.4GHz channels occured to me. We all know that the ISM band is chopped into 11 or 14 (depending on where you live) different channels. Of these channels only 3 are usable because the rest overlap.

I always 'assumed' that some sort of a channel filter was included in the standard. I thought that if your neighbor was using channel 3 and you're using channel 6, despite physically interfering with each other, your AP's will not see each others traffic. However, (to my knowledge) there is no header in a 802.11 mesage that informs anyone what channel you are transmitting on. While there is a header in ethereal that tells you the channel the frame was captured on, it does not tell you the channel the frame was transmitted on.

Am I stating the obvious here or did other people know this? Also does anyone else get the idea that they really screwed up the ISM band by giving people the option to but devices on channels 1 - 11 instead of 1, 6 and 11? Is there a reason for this?

Cheers

Roy

wrzwaldo · 05-11-2006

Quote:

Originally Posted by Roy_M

Hi

I was capturing some packets in ethereal the otherday and something new about 2.4GHz channels occured to me. We all know that the ISM band is chopped into 11 or 14 (depending on where you live) different channels. Of these channels only 3 are usable because the rest overlap.

I always 'assumed' that some sort of a channel filter was included in the standard. I thought that if your neighbor was using channel 3 and you're using channel 6, despite physically interfering with each other, your AP's will not see each others traffic. However, (to my knowledge) there is no header in a 802.11 mesage that informs anyone what channel you are transmitting on. While there is a header in ethereal that tells you the channel the frame was captured on, it does not tell you the channel the frame was transmitted on.

Am I stating the obvious here or did other people know this? Also does anyone else get the idea that they really screwed up the ISM band by giving people the option to but devices on channels 1 - 11 instead of 1, 6 and 11? Is there a reason for this?

Cheers

Roy

I believe some of what you are talking about is handled at the RF level (IF Filtering).

Here is an interesting read on channel separation.

Some info here also.

Roy_M · 05-11-2006

Thanks for that wrzwaldo.

The first article you posted says that hardware does physical layer filtering when both transmitting and receiving. They are also saying that you can use this to maximise the limited spectrum space, and get 4 useable channels (1,4,8,11). Pretty cool stuff.

I had never thought of a physical layer filtering mechanism. I assumed it would be a MAC layer process similar to not recieving another users frame in any broadcast environment based on their MAC address.

The reason this has come to my attention is because my tests are in a indoor environment with two AP's in close proximity. I am noticing an AP on channel 11 responding to management frame broadcasts (which I belive are) on channel 8, 9, and 10 when the client is in close proximity to the AP.

Cheers

Roy

Thorn · 05-11-2006

Quote:

Originally Posted by Roy_M

...
Also does anyone else get the idea that they really screwed up the ISM band by giving people the option to but devices on channels 1 - 11 instead of 1, 6 and 11? Is there a reason for this?

It isn't quite as "screwed up" as you might think. Channel overlap is fairly common in a lot of bands, and it allows the users to get more usage out of the same bandwidth. While the conventional wisdom is that "you MUST use only 1-6-11 (or 1-4-8-11)", that only really applies when the APs are in close proximity. Sure, mainly it's those three (or four) but, I've used all eleven US channels at one time or another. It all depends on the results of the site survey.

05-11-2006	#1 (permalink)
Roy_M Registered Member Join Date: Jun 2005 Posts: 72	Stating the obvious? Hi I was capturing some packets in ethereal the otherday and something new about 2.4GHz channels occured to me. We all know that the ISM band is chopped into 11 or 14 (depending on where you live) different channels. Of these channels only 3 are usable because the rest overlap. I always 'assumed' that some sort of a channel filter was included in the standard. I thought that if your neighbor was using channel 3 and you're using channel 6, despite physically interfering with each other, your AP's will not see each others traffic. However, (to my knowledge) there is no header in a 802.11 mesage that informs anyone what channel you are transmitting on. While there is a header in ethereal that tells you the channel the frame was captured on, it does not tell you the channel the frame was transmitted on. Am I stating the obvious here or did other people know this? Also does anyone else get the idea that they really screwed up the ISM band by giving people the option to but devices on channels 1 - 11 instead of 1, 6 and 11? Is there a reason for this? Cheers Roy

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05-11-2006	#3 (permalink)
Roy_M Registered Member Join Date: Jun 2005 Posts: 72	Thanks for that wrzwaldo. The first article you posted says that hardware does physical layer filtering when both transmitting and receiving. They are also saying that you can use this to maximise the limited spectrum space, and get 4 useable channels (1,4,8,11). Pretty cool stuff. I had never thought of a physical layer filtering mechanism. I assumed it would be a MAC layer process similar to not recieving another users frame in any broadcast environment based on their MAC address. The reason this has come to my attention is because my tests are in a indoor environment with two AP's in close proximity. I am noticing an AP on channel 11 responding to management frame broadcasts (which I belive are) on channel 8, 9, and 10 when the client is in close proximity to the AP. Cheers Roy