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Hi Everyone,
 
I have a question about deploying neighboring APs on the same channel.
Here is the setup:
- Multi-storey (4) building, 2 APs per floor, placed on opposite corners on each floors, staggered across floors.
- All APs: 802.11g/n in the 2.4Ghz range.
- Same SSID and settings for all APs (they are in a cluster).
- About 5 to 10 users per AP, used only for internet connectivity (web browsing).
- VLAN completely isolated from the rest of the network.

Here is how the channels are now setup:
- APs on each floor on the same channel (ie. floor 1, 2 APs on channel 1; floor 2, 2 APs on channel 6; floor 3, 2 APs on channel 11; floor 4, 2 APs on channel 1).This is based on the assumption that it is better to use the same channel rather than having an adjacent-channel interference (considering the location where the APs are placed, there would eventually be such interference using a 3 channels scheme across all floors). My assumption was derived from the fact that it is better to have the APs operating on the same channel, with the down side of having more contention for airtime access (but this is acceptable because of the low utilization of the wireless) but with the plus side of having cleaner interference-free (almost) channels.

I hope the explanation was clear!
My question to you guys is: am I right in my assumptions? Could this setup introduce hidden nodes issues?
 
Thanks!

Adjacent channel interference will occur only if you choose overlapping channels and not when choosing 1 , 6 and 11. All these channels are spaced 25 Mhz apart and will not cause Adjacent channel interference. 

Using the same channel can cause hidden node issues but more than that the channel access will be more and will reduce the performance a lot. 

If this was my design I would just stagger the channels 1,6,11 on each AP between the floors

e.g   f1   1         6
        f2  11        1
        f3  6        11
        f4   1        6

so APs on the same channel are not directly upon each other and given the distance, anttenuation, they wont interfere greatly. Gets rid of any hidden node issue plus improves performance. Job done, sleep easy! :)

Best practice is to do a proper channel plan, which means you minimize co-channel interference where possible.  In your case, you would probably be better off putting APs on the same floor on different channels, something like:

Floor 4: 1 -- 6

Floor 3: 11 -- 1

Floor 2: 6 -- 11

Floor 1: 1 -- 6

I'd also recommend using 5GHz, if possible.
Hidden node issues can occur in many scenarios.  By enabling RTS/CTS you can mitigate this, but I wouldn't be overzealous about it unless you see evidence that it's actually occurring.
--@@ron

Additionally, you could test with higher gain omni antennas (if using APs with external antennas) on each floor to diminish vertical spread of the RF. HOwever, with many indoor APs today, this is just not a practical option.

Looks like Alan and I were on the same frequency that night!

--@@ron

Thanks to everyone for the replies.

@alan, @ @@ron: The channel planning you suggest would end up having some co-channel interference anyway, (a 4 non-verlapping channel scheme would be a life saver here), e.g. on floor 1 & 2 the two APs on channel 1 would interfere with eachother (APs are located at opposite corners on each floor); this is what led me to set them up with the same channel for each floor, as the following:

 Floor 1: 1 - 1

Floor 2: 6 - 6

Floor 3: 11 - 11

Floor 4: 1 - 1

So I guess co-channel interference is less with the channels planned this way?

The APs do not serve many clients, and they are mostly phones/tablets.

APs are dual-band, but can't use them at the same time, it is either one or the other. 5GHz is not an option since many users connect from their phones, which only supports 2.4GHz.

@Tom: Higher gain antennas may help indeed, APs have external antennas.

Thanks

Yes, there could be overlap between cells on the same channel with the proposed channel plan.  I guess it all depends on building layout and AP placement, but separating the cells with walls and a floor would usually do a better job of containing the signal than putting them on the same floor.

As Ravi explained, adjacent channel interference isn't much of a problem when the channels are sufficiently separated, which is why using 1, 6, and 11 is better than putting APs on the same channel in close proximity.
Not knowing the building floor plan, construction, and other factors, the channel plan that Alan and I suggested is the preferred approach because it maximizes distance and attenuation between APs on the same channel.  You can test this assumption by doing an active survey with your current setup, then another after changing the channel plan.  If you're able to do so, please share the results with us -- best of luck!
--@@ron

Thanks @@ron. I will perform a site survey with the two different channel planning and check out the results.

Cheers!

@@ron, great minds :)