Non-overlapping channel (1/6/11) isn't always optimal

Well, sure, if there's significant interference from the optimal set of non-overlapping channels, then go for the non-optimal midpoints.

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My point is that 1, 6, and 11 aren't necessarily optimal.

In fact 1, 6, and 11 are only really optimal in terms of minimizing interference when you control multiple overlapping access points, and even then they aren't necessarily optimal -- 4 channel spacing (e.g., 1, 4, 8, and

11) will allow 4 access points (instead of just 3) to overlap with minimal interference.

In an uncontrolled environment an overlapping channel might be optimal, as it is in my case that started this thread, and in some other cases I know of.

It nonetheless makes sense to recommend first trying the non-overlapping channels (1, 6, 11) even in an uncontrolled environment because (a) they are the most likely to be optimal and (b) they are the most "neighbor friendly". But it nonetheless also makes sense to try other channels, particularly the midpoints between the non-overlapping channels, when the non-overlapping channels don't provide optimal results.

~ >>Just found another case where a non-overlapping channel isn't optimal: ~ >

~ >>* Interference from neighbors on channels 1, 6, and 10 ~ >

~ >>could get a reliable connection on channel 3. ~ >

~ >Well, sure, if there's significant interference from the optimal set ~ >of non-overlapping channels, then go for the non-optimal midpoints. ~ ~ My point is that 1, 6, and 11 aren't necessarily optimal. ~ ~ In fact 1, 6, and 11 are only really optimal in terms of minimizing ~ interference when you control multiple overlapping access points, and even ~ then they aren't necessarily optimal -- 4 channel spacing (e.g., 1, 4, 8, and ~ 11) will allow 4 access points (instead of just 3) to overlap with minimal ~ interference.

Our test results do not concur (assuming that you control your RF environment). The interference created by using 1/4/8/11 rather than 1/6/11 channel spacing will give you less aggregate throughput throughput the 2.4GHz band. With 4 APs, you are better off having two on the same channel.

This is because, with multiple BSSIDs on a single channel, the 802.11 MAC can (potentially) recognize the signals from the other BSSID as being valid

802.11 frames, and can hence do an informed backoff/transmission. With multiple BSSIDs on adjacent overlapping channels, the other BSSID's transmissions are just perceived as noise.

having said that ...

~ In an uncontrolled environment an overlapping channel might be optimal, as it ~ is in my case that started this thread, and in some other cases I know of. ~ ~ It nonetheless makes sense to recommend first trying the non-overlapping ~ channels (1, 6, 11) even in an uncontrolled environment because (a) they are ~ the most likely to be optimal and (b) they are the most "neighbor friendly". ~ But it nonetheless also makes sense to try other channels, particularly the ~ midpoints between the non-overlapping channels, when the non-overlapping ~ channels don't provide optimal results.

agreed - if you don't control your RF environment, then do whatever works best. (Within the bound of the law, of course.)

Aaron

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That's not my experience, which is consistent with these documents:

4 Simultaneous Channels Okay For 802.11b

The Overlapping Channel Problem

WiFi was *designed* to support many interfering, overlapping networks, handling the resultant packet collisions with minimum fuss.

Channel Overlap Calculations for 802.11b Networks

As it notes, that paper is at odds with the other authorities. In addition, the only supporting test was both limited and highly artificial.

It may be that what works best is different in different types of environments.

~ [POSTED TO alt.internet.wireless - REPLY ON USENET PLEASE] ~ ~ In on Thu, 30 Mar 2006 14:32:48 ~ -0700, Aaron Leonard wrote: ~ ~ >~ My point is that 1, 6, and 11 aren't necessarily optimal. ~ >~ ~ >~ In fact 1, 6, and 11 are only really optimal in terms of minimizing ~ >~ interference when you control multiple overlapping access points, and even ~ >~ then they aren't necessarily optimal -- 4 channel spacing (e.g., 1, 4, 8, and ~ >~ 11) will allow 4 access points (instead of just 3) to overlap with minimal ~ >~ interference. ~ >

~ >Our test results do not concur (assuming that you control your RF ~ >environment). The interference created by using 1/4/8/11 rather ~ >than 1/6/11 channel spacing will give you less aggregate throughput ~ >throughput the 2.4GHz band. With 4 APs, you are better off having two on ~ >the same channel. ~ ~ That's not my experience, which is consistent with these documents: ~ ~ 4 Simultaneous Channels Okay For 802.11b ~ ~ ~ The Overlapping Channel Problem ~

~ ~ WiFi was *designed* to support many interfering, overlapping networks, ~ handling the resultant packet collisions with minimum fuss. ~ ~ Channel Overlap Calculations for 802.11b Networks ~ ~ ~ >This is because, with multiple BSSIDs on a single channel, the 802.11 MAC ~ >can (potentially) recognize the signals from the other BSSID as being valid ~ >802.11 frames, and can hence do an informed backoff/transmission. With ~ >multiple BSSIDs on adjacent overlapping channels, the other BSSID's ~ >transmissions are just perceived as noise. ~ >

~

~ As it notes, that paper is at odds with the other authorities. In addition, ~ the only supporting test was both limited and highly artificial. ~ ~ It may be that what works best is different in different types of ~ environments.

I've glanced at your references, and have this comment: ALL of them reference 802.11b not 802.11g, and all of them point back to the original '02 Mitch Burton paper. Note that Burton concedes that the 4-channel allocation scheme is not well suited to 802.11g OFDM, which uses considerably more energy at the edges of the channel.

Surely more and better testing using 802.11g is in order.

Regards,

Aaron

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I would agree.