Do we need Spectrum Analysis Capable APs to see non-WiFi Energy?

Many WiFi-focused people have seen some disruptive non-WiFi interference on WiFi bands. Those can be on 2.4Ghz and 5Ghz but the 2.4Ghz is especially crowded in terms of both WiFi and non-WiFi energy levels on those bands.

Enterprise and Telco grade access points should be able to detect/understand and measure the energy levels, not only coming from WiFi based sources, but also non-WiFi based sources.

There can be numerous non-WiFi energy sources hitting to 2.4Ghz band, such as:

  • Non-data-transmitting devices, such as Microwave ovens or other magnetron based devices.
  • Data transmitting, in-band non-WiFi devices, such as video bridges.
  • Data transmitting, out-of-band non-WiFi devices, such as LTE or 3G multi-band equipment, whose -probably- third harmonics hit 2.4Ghz WiFi band.

On the other hands, there are basically two types of Access Points, in the enterprise/telco market.

  • Spectrum Capable Aps, which have integrated spectrum analyzer to see the raw energy in the environment.
  • Non-spectrum capable Aps, which have no spectrum visibility for non-WiFi energy.

However, there is some confusion here. What are the capability levels that we can expect from APs?

  • Capability Level1 – An AP which is doing basic WiFi stuff, but no capability to see non-WiFi energy on the channel. If the Ap is on channel1 and there is a non-WiFi energy on the same frequency, AP cannot see that energy, cannot report it, cannot change channels to jump to a “cleaner” channel.
  • Capability Level2 – An AP which broadcast WiFi signal and sees the channel metrics including WiFi and non-WiFi energy levels on the channel it operates. If there is a non-WiFi energy on the channel, AP can see the existence of non-WiFi energy, can report the level the signal occupies in the given channel/spectrum space and the AP is capable of changing the channel to get better channel with less non-WiFi interference.
  • Capability Level3 – An AP which broadcast WiFi signal and it has correct hardware/software components to see the actual FFT from the channel and it has correct algorithm to see duty cycle of the channel to understand (or tries to understand) the source of the signal whether it is coming from a microwave oven, or from a Bluetooth device or from a Video bridge etc. So, this type of APs can report the “reason” and “source” of the non-WiFi energy.

Some vendors are offering spectrum capable APs, which are on “Capability Level3” and offering non-spectrum capable APs which are on “Capability Level1”

On the other hand, some vendors are offer spectrum capable APs which are Level3 but offering non-spectrum capable APs, interestingly, which are on “Level2”

Now, let’s look at Aruba’s AP207 which does NOT support spectrum analysis capability and it is an entry-level AP.

We know that AP207 has no Level3, and we’ll try to test if AP207 has Capability Level1 or Level2.

For this testing, I used a non-WiFi signal generator from a brand called “Next” which is actually a video transmitter device works on 2.4Ghz band, without obeying any 802.11 transmission rules. No IFS, no contention window, no CSMA/CA, no back-off, no EDT, no NAV. This device just shouts out, without caring about anybody in the band.

next-interferer

I wanted to test Aruba AP207 for this. My aim was to understand which level it is on, in terms of spectrum visibility.

Screen Shot 2017-12-06 at 21.52.10

First test is to define the baseline.

Here is the empty channel values from a 3rd party spectrum analyzer. Video bridge is not working and all 2.4Ghz spectrum seems OK.

interference-no

Aruba AP207 broadcasts a test SSID, with regular load on the channel, and 207 reports that the below channel metrics:

channel-normal

Here, the AP207 reports the following values:

  • %85 of the channel (in this instance, this is channel1) is free.
  • Remaining %15 of the channel is used according to the below values:
  • %2 of the channel is busy, because of this APs “transmitting” frames.
  • %1 of the channel is consumed by non-WiFi or “cannot-be-decoded-WiFi” signals.
  • %12 of the channel is consumed by this APs “receiving” frames. Actually, in this sampling interval, 14559 frame just have hit to the radio Rx, and 14366 of 14559 is actually destined for another WiFi STA in the same channel but also hit to this APs Rx. 193 frames out of 14559 destined for this AP and hit to this AP’s Rx.

Now, it is time to enable video bridge to block all the transmissions on channel 1.

interference-yes

As you can see from the photo above, video device is shouting like crazy in the channel 1..

Now, it is impossible to Tx any 802.11 frame on channel 1 because the WiFi STA tries to Tx but hits energy detect threshold (which is -62dbm), defers access and reports that the channel is busy. Because all of the trials are not successful and the transmitter cannot send even one 802.11 packet, it reports high channel utilization with all of them comes from a non-wifi source, which means the Rx cannot decode the incoming energy, so it is reported as non-WiFi. Sometimes, some distant WiFi transmissions which cannot be decoded at the Rx, can be reported as non-WiFi energy.

So, here are the channel metrics after the interferer device has been turned on:

channel-interfere

As we can see from the above output, the Rx of the AP cannot receive any frame because of the high level of non-WiFi interference seen on the channel.

So, it reports zero Tx and zero Rx and high level of interference. So it can see the non-WiFi interference. It also reports high level of noise.

After a couple of seconds or minutes, the AP selects another channel and jumps to Channel 6, to broadcast its SSID in a fairly clean environment. So, the AP saw the non-WiFi interference and changed the channel to recover from that interference.

arm-channel-change

So, even though AP207 has no spectrum analysis capability, it saw the non-WiFi interference and managed to change the channel to recover itself. The “Reason: N” means that the AP has changed the channel because of “Noise Threshold Exceeded”. This confirms that AP207 is on the capability level2, based on the capability levels explained above.

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