Email Questions: 802.11n Deployment Using 2.4GHz and 5GHz Simultaneously

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.

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5 comments so far

  1. Keith R Parsons on

    Douglas,

    Another issue. For any distance from an AP, the 2.4GHz signal will always be stronger than the 5GHz signal, so without some sort of client side configuration (like a distinct SSID for 5GHz) the clients will all eventually move toward the 2.4GHz side.

    Some vendors also support forms of ‘Band-Steering’ trying to influence the client’s decisions on where to go, but it is the clients’ responsibility to choose which AP to associate to.

    Also need to know if the clients support 802.11n, and if so, which feature sets, what MCS, support SGI, etc.

    I like all your additional questions, we’d need to know those other items to help diagnose the situation.

    Though I have a question as to why you suggest ALL 802.11g data rates need to be supported?

  2. Antonio Serdar on

    I have both a dual-band wireless router (Linksys WRT610n) as well as a dual-band wireless network card Linksys WMP600n) in a computer located in a location to which it is impractical to run cable.

    With this card and router is it possible – or even desirable – for this station to receive/transmit on BOTH 2.4 and 5GHz bands simultaneously?

    In other words, is it possible to “double” the potential bandwidth/speed by using both channels at the same time?

    I’m not sure if I’ve posted this in the right spot, so apologies go out in advance if I’ve flouted convention.

  3. wifijedi on

    Antonio,

    Thanks for your question. I believe the Linksys card that you have has a single radio chipset in it, although it is software tunable to either frequency band (either 2.4 or 5 GHz). Therefore, it can only build one association at a time.

    Said another way, no, I don’t think you can connect to both bands simultaneously.

    My suggestion is to utilize channel bonding in 5 GHz to maximize the available bandwidth and avoid interfering devices.

    Thanks for reading.

    Respectfully,
    Douglas

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