Using Bluetooth to Debunk the Dual 5 GHz Myth

What does Bluetooth have to do with 5 GHz Wi-Fi?  Nothing.  Bluetooth operates in the 2.4 GHz frequency band.

If you look in the right spot, however, Bluetooth can teach you something about 5 GHz Wi-Fi.  Specifically, the Bluetooth 5.0 specification can teach you that Dual 5 GHz access points are a bad idea.

This blog's opposition to Dual 5 GHz access points (APs) is not news to long time readers.  The 'Two Radios Are Better Than One (Unless They're Both 5 GHz)' blog post, which details how Dual 5 GHz APs (like Cisco 3800 Series APs, for example) make enterprise Wi-Fi less stable, is nearly three years old. 

Today's re-stating of the pitfalls of dual 5 GHz radio APs is due to something I uncovered while reading about Bluetooth.  I was reading the Bluetooth 5.0 specification in search of anything that might affect Wi-Fi when I happened upon this table:


The middle rows of the table is most dramatic, showing that out-of-band RF activity can interfere with the 2.4 GHz band at signal levels as low as -27 dBm.  

It is the top and bottom rows of the table that relate to Dual 5 GHz APs.  Those rows show that RF signals further away from the 2.4 GHz band may interfere, if those interfering signals are above -10 dBm.

The problem with Dual 5 GHz APs is that two 5 GHz radios are within the same AP, and thus very close to one another.  With two 5 GHz radios so close to one another, the chances of signals reaching levels above -10 dBm are likely; perhaps assured.  

5 GHz channels 36 and 149 may seem far apart.  In some respects, they are: Channel 36 is centered at 5.180 GHz and channel 149 is centered at 5.745 GHz, which means 565 MHz of frequency space between channels.  The problem is that, when Dual 5 GHz APs are used, the close proximity of the two radios makes it so that busy Wi-Fi networks will be unstable.  And nobody -- not users, not support folks, and not admins -- likes unstable Wi-Fi.

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Comments

  1. Interesting topic and article ! Aruba did an interesting benchmark on a Cisco dual 5GHz AP vs a dual band Aruba AP a while back. At that time it supported the claim of dimishing bandwith returns using dual 5GHz
    http://community.arubanetworks.com/t5/Technology-Blog/Does-the-Dual-5GHz-Story-Stand/ba-p/271720

    I was some years back doing conference solutions using the large Xirrus arrays with 8 and 16 sectorized antennas. Their secret sauce was the alleged effective isolation between multiple sectors in the same housing. Having a central high capacity AP mounted high above the clients is however no longer the recommeded solution

    ReplyDelete
    Replies
    1. I would agree. Definitely best to avoid having multiple AP radios in one "box".

      Delete
  2. That chart is relative to 2.4 Ghz devices; it can't be extrapolated out to 5.8 GHz devices as you're using it. As best I can tell, you're reading those signal levels backwards. -10 dBm is a much stronger received signal than -27 dBm, though both are whoppingly strong. That chart perfectly illustrates the effect of band-pass filtering. Any 2.4 GHz device will be designed to reject signals outside of its operating range which is why that chart shows it takes a *stronger* signal above and below the 2.4 band to cause problems.

    Whatever signal rejection characteristics a device such as a 5.8 GHz AP might have are specific to itself alone, not other equipment, not the depicted chart.

    ReplyDelete

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