Wireless channels and roaming

BradyAU used Ask the Experts™
Hi all,

I currently have 3 Linksys E4200 routers that run a roaming wireless network. Authentication is WPA2 Enterprise and of late, there have been some authentication issues with EAP timeouts etc...

From what I can see, all was working fine up until about a week ago when I decided to change the wireless channels on all 3 devices to auto (on both 2.4 and 5.8 bands). Now some devices (ranging from mobile phones to laptops) will see the single SSID, attempt to connect and will fail. Laptops will give the certificate prompt, then say it was unable to connect.

2 of the 3 devices are relatively close together and will sometimes 'argue' with each other while the 3rd device is far enough away not to cause interference.

What I'm thinking is I should change the channels on all 3 devices back to manual.

There is a person in the vicinity who uses 2.4 channel 1. What channels would I best be using?

So far:

Device 1: 2.4 channel 6, 5.8 channel 44
Device 2: 2.4 channel 11, 5.8 channel 36 (this device is the closest to the other persons device that uses channel 2.4 channel 1)
Device 3: 2.4 channel 1, 4.8 channel 48 (this device is the furtherest away from all devices including the other persons device that uses 2.4 channel 1)
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Attached is a site survey which shows device 1 and device 2.
You didn't say how many clients are using each AP.

Try using channel 11 on all 3 for the 2.4GHz. Even if they're in range of each other, they should take turns gracefully.

Try channels 149 through 165 (if available) on the 5.8GHz.

I would set the 2.4GHz side to 20MHz only.
But the 5.8GHz I would allow 40MHz-wide channels... e.g. Auto (20 MHz or 40 Mhz).

The 5.8GHz lower band (which you are currently using) is restricted to indoor use in most countries, but that doesn't mean those are the only channels you can use indoors.

Typically the middle band on the 5.8GHz radio will allow only auto-channel-select, to prevent interference with airport radar, but that's ok because the 5.8GHz channels do not overlap as they do in the 2.4GHz band. The middle band can use 5x as much power as the lower band.

The upper band (containing 149 through 165) should let you pick the channels, plus they can use up to 20x as much power as the lower band, and though the Linksys firmware doesn't allow you to set those levels it should adjust them automatically and you ought to see 'less negative' dBm numbers with those channels in your site scanner.

Using 40MHz-wide channels does still use 2 adjacent frequencies, of course, and since there are only 5 channels available in the upper band (in most enforcement zones), you can either set two units to use 20/40 and one to use 20-only in the upper band, or set one unit to use Auto 20/40 in the DFS middle band (channel 52 to 140), and the other two using, say, 149+153 and 157+161.


Hi Darr,

The E4200 only lists 4 options for the 5.8GHZ channels:

Auto (DFS)
36 - 5.180GHz
40 - 5.200GHz
44 - 5.220GHz
48 - 5.240GHz
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Top Expert 2014
It's worth noting - Bands A (Indoor ONLY) and Band B are the ONLY frequencies you should be using indoors.  These bands are license-free (subject to territory)..

Band C (5.725 - 5.850) is only for use in Fixed Point-to-Point external installations and MUST be licensed on a light basis (subject to territory).

Darr is referring to DFS in the middle band (Band B), but that is only a regulation in some countries so you'll need to check that your kit is compliant and that if so, the feature is turned ON if your territory requires it.

Also, you should NOT put adjacent APs on the same channel in the 2.4 or 5GHz bands.  This will cause co-channel interference and will almost definitely give you problems.  It won't cause the problem you're seeing with authentication though.  If you're in the UK you have around 15 channels to play with at 5GHz so you shouldn't ever need to put APs on the same channel.
> Band C (5.725 - 5.850) is only for use in Fixed Point-to-Point external
installations and MUST be licensed on a light basis (subject to territory).

Not here in FCC land. I have a few different brands and models of indoor dual band routers/APs with channels 149-165 available. You must be in Europe, under the ETSI enforcement zone, or Japan under the MKK, which are the only zones that don't allow using those channels.

WorldWide 5GHz Frequency Map
Of course, the asker could be in Japan or Europe, too... there's no way for us to tell since EE stopped putting the asker's time zone in the question.  :-|

> Also, you should NOT put adjacent APs on the same channel
in the 2.4 or 5GHz bands.  This will cause co-channel
interference and will almost definitely give you problems

I disagree. It does not cause interference - they take turns gracefully - if all units are using the same SSID, and I don't know any way to setup fast roaming using different SSIDs.

It's my understanding that some Apple products get confused if you use the same SSID on both 2.4 and 5.8GHz, so it doesn't hurt anything to use different a different SSID on each band.


I'm in Australia :)
Profile will be updated shortly.

Our version the FCC is ACMA - Australian Communications and Media Authority (www.acma.gov.au)


It won't cause the problem you're seeing with authentication though.

Since I've switched to different channels manually, there have been no connection/authentication problems. The main problem was the connecting device would timeout (I think) maybe due to some sort of interference.
It was already locked before I could edit out the extra "different" in the last sentence... and I was going to add:

I'm not exactly sure what the name of Japan's broadcast regulating agency is. The only expansion of the acronym MKK I can find related to Japan is Mitsubishi Kakoki Kaisha.
Top Expert 2014

Darr, I hate to argue, but RF in WiFi doesn't behave like that.  If you have two devices on the same channel they will not wait for the other radio with the same SSID to finish transmitting.  They use CSMA/CA which works on a random timeout.  No conversation between APs takes place to tell each when the other has finished sending so at some point you WILL interfere with the other AP.

You may be talking about a SCA solution such as Meru's offering, but that uses an airtime-fairness algorithm which is proprietary to Meru.
Yes they will, it's part of the 802.11 specification - carrier sense multiple access with collision avoidance (CSMA/CA).
They listen first to see if the channel is in use... usually they can just calculate by the duration field from the last frame heard... if it's the same SSID. But if they're not on the same SSID, they don't recognize it's in use and will just start sending, which causes interference. The only time that doesn't work is you're not broadcasting the SSID (just another reason not to bother with trying to hide the SSID).

Normally, for maximum throughput you would put them on channels 1, 6 and 11. However, in this case the site survey shows another nearby radio using channel 1, and the default used by most manufacturers is channel 6 so I prefer not to even use it... any new AP installed nearby will likely start up on channel 6, causing instant interference. That's why I recommended putting them all on channel 11.

It does look like Oz requires a license for those upper 5GHz channels, too, by the way...
#19a on http://www.comlaw.gov.au/Details/F2011C00543
Top Expert 2014

You're wrong Darr!  I'm a Cisco Advanced WLAN Specialist, and I've NEVER heard of this behaviour.
I can't help that you've never heard of it, but I'm surprised Cisco doesn't teach it before certifying someone on WLANs...  Still, I'll wager if you search their site for CSMA/CA you'll find it's as I described. As I said, it's part of the IEEE 802.11 specification, just as CSMA/CD is part of their 802 wired specs.



et al, et cetera.

Note that if they were Cisco APs run by a WLC, the WLC would automatically set them to all use the same channel if that provided the least amount of interference.
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Thanks for clarifying the fact that the WLC would set the APs to the same channel if that provided the 'least' amount of interference.

Next time I install a wireless LAN I'll make sure I set everything to the same channel, and also tell the vendor that they only need one channel on their kit ;-)

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