WiFi Jedi.com

I received a following set of questions via email and thought that rather than only respond to the one individual, but post it on WiFiJedi.com for general consumption.

Here’s what I received:

I have recently done a deployment where the customer is using 802.11n in both 2.4 and 5 GHz range.  I have configured a WLan with that is providing coverage for all the radios including a/b/g/n on a 4402 controller in WiSM Module.  The clients are also unable to run on all the radios.  Everything is working fine, but we are seeing clients that have been connected to either a 2.4ghz radio or 5ghz radio, tend to switch between them when running for a while.  I am trying to understand this scenario, since there is no setting on the controller that I can find would allow the preference of the radio.  we are currently running the latest 6.0 code on the WLCs.  Secondly the data rates are set to 24Mbps or higher for both radios, that means 802.11b is not allowed.  The client machine was sitting at one position for 24hrs that means the user was not moving around hence the roaming should not be involved I think.  Lastly we did the survey for 802.11a coverage and I don’t think there is any issue with the coverage, since it connects to 802.11a at excellent.  The question really is that once the client connects to 802.11a on excellent and is running on that radio for an hour perfectly and the user is not moving at all why is it that it switches itself to 802.11g and then again to 802.11a??  I wanted to get your opinion as if do you know any bugs or vulnerabilities by having both radios enable?

And here is my response:

Thank you for your message.  Unfortunately, at this time most/all of the roaming decisions are made by the stations (laptops, etc.) and not the infrastructure (Access Points and WLAN Controllers).  The IEEE is devising a standard to change this, but it will take some time.

What type of stations are you using?  Are they laptops, or something else such as handheld scanners, Voice-over-WiFi phones, etc?  If they are laptops, are they Windows or Macintosh?   If you are using Windows based laptops, you may be able to set a preference for 5 GHz within the client driver.  For example I have an Intel 4965 AGN adapter — if I right-click on my wireless adapter and select “properties”, then click “configure” I can select the Wireless mode to be 802.11a/n only.  I can also set the roaming aggressiveness so that it roams less frequently.  Macintosh computers have a natural preference for 2.4 GHz and it is more difficult to encourage them to connect to something at 5 GHz – in a case like that, you may consider adding a 5 GHz SSID on its own VLAN.  Again, these roaming decisions are made by the station, so your best bet is to look at the laptop settings to see what you can tweak.

One other potential “gotcha” that came to mind was how you enabled 802.11g only.  The data rates have to be carefully managed for full interoperability between the station and the infrastructure.  There are two types of data rates – the “basic” rates and the “supported” rates.  The basic rates should include all 802.11g rates, even below 24 Mbps (therefore you should double-check that the basic rates include 6,9,12, and 18 Mbps).  The requirements for supported rates are less strict.