Non-overlapping channels - high density - need clarification....

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Hello,

Read the High-Density Design Principles doc, along with a few other things and I am still not clear on co-channel interference and non-overlapping channels.

1) What channels for 802.11a are really non-overlapping? 8 or 12?

2) in regards to 802.11a, channels 149 - 165 are considered for outdoor use so indoor availability is only 8 channels, correct?

3) What would be the non-overlapping channels for 802.11n? Indoor use only.

4) What happens when an 802.11n/a device and a traditional 802.11a device connect to the same network?

Thank you,
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jacob600

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Posted 4 years ago

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Brian Powers, Champ

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So, to answer them in order:

Co-channel interference is in its simplest forms, any two APs that are using the same channel that can hear each other.

Non-overlapping channels, again in its simplest form, are channels that are far enough apart that they both aren't using the same frequency space. So each channel represents a frequency number (in GHz). Channel 1 is 2.412 and the go up in increments of 5 MHz for each channel (2 is 2.417, 3 is 2.422, etc.). Each 802.11 transmission technically uses 16 MHz of frequency space. So since each channel is 5 MHz wide, we'd need 4 channels to completely separate transmissions. 1 and 6 work because there are 4 channels between them. Same with 6 and 11.

The 5 GHz frequency channel layout is slightly different, but not. Each channel still represents a frequency (36 is 5.180 GHz, 40 is 5.200 Ghz). So again, needing 20 MHz of separation, 5200 - 5180 = 20. So none of the common 802.11a channels overlap.

1.) Each usable channel in the 5 Ghz frequency space is 20 Mhz wide, so none of them overlap. This changes if you do channel bonding.

2.) 149-157 are UNII-3 channels that were originally designated for outdoor use, this is no longer the case and they can be used freely indoor or outdoors. I believe even 165 can be used indoors now. See here, http://goo.gl/lqSFSi

3.) 802.11n uses the same channels as 802.11a/g use. There are just enhancements that give the increased speed. Again, channel bonding changes this some.

4.) Since both 802.11a and all 802.11n devices use OFDM, there would be no implications. The 802.11n standard is completely backwards compatible with previous 802.11 standards. The only thing you would most likely see would be slower throughput from the 802.11n devices (in a similar way that 802.11b devices slowed down 802.11g networks)

Tons of knowledge from Andrew over @ his blog. http://www.revolutionwifi.net/

Keiths too. http://wirelesslanprofessionals.com/

These books are invaluable here. http://www.cwnp.com/
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jacob600

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Thank you. Definitely clears most of it up. Last question/clarification.

If my goal is to use 802.11n exclusively or predominantly, so this assumes channel bonding ala 40mhz, this reduces it to 4 non-overlapping channels, correct? What would these 4 channels be???

Thank you,
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Brian Powers, Champ

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So in the radio profile, you have to set it to either 40-MHz above or 40-MHz below. This will somewhat dictate which channels are bonded.

Since 36 is the lowest usable channel, if you wish to bond it, it would be 36+40 (40 MHz above). You could technically bond 40+44, but that would leave 36 and 48 out in the cold and stuck @ 20 MHz.

OR you do 40-36 (40 MHz below). Theres no difference that I'm aware of.

So, yes, if you are only able to use UNII-1 and 3, you'd have 4 non overlapping 40 MHZ wide channels. If your client devices are capable, UNII-2 and 2e open up many more options for 40 MHZ wide channels.