My foil hat - will it provide a RF shelter?

Well, to start off we must make some measurements of how much of the RF-energy that is absorbed and/or reflected off by the foil and thus not permitted into our brain where it can be absorbed instead. I got this beautiful top hat from my father in law that I thought would be the excellent object for the measurement.

Regarding the foil we'll use standard household aluminum foil in a layer or two and then measure the difference with or without foil in 2.4Ghz with Airmagnet Survey Pro. We'll also have a look at the RF spectrum how this might change. I'll use my Cisco Meraki MR26 located in a closet about 8 meters away as the access point and RF emitter. The MR26 operates at channel 6 in 2.4Ghz at full power which should mean about 23dBm EIRP for the beacons transmitted at the lowest basic rate which is at 6Mbit/s OFDM modulation (since the legacy data rates are disabled in the Meraki Dashboard).

 

The results in Airmagnet Survey with the NIC Proxim 8494 were as follows:

Hat without foil: -66dBm

Hat with foil:      -74dBm

I'd say that the variance during the measurements were about +/- 2dB and I guess it can be explained by the kids runnings around in our house playing seek and hide. Anyway we can be pretty sure that the attenuation in this case is roughly 8dB.

It was possible to (maybe..) notice a slight change in the spectrum during the two tests:

Hat without foil:

Hat with foil: 

So will the hat protect me? Should I perhaps wear it? Personally I doubt that the risks of RF from access points and clients can result in cancer or other health issues. The Swedish Radiation Safety Authority state that "no health risks are posed by exposure to base stations for mobile telephony, wireless networks nor similar transmitters" 

But they also state...

"However, when it comes to long-term use of mobile phones, there is a slight suspicion that it has an impact on human health. This factor combined with people’s widespread and regular use of mobile phones today has led to the Swedish Radiation Safety Authority’s conclusion that reducing unnecessary exposure is justified"

Alright - if we live long-term enough we'll know ;-)

BTW - there are some vendors that produce RF tents and shelters, here we get about 50-70 dB of attenuation, have a look here. 

/Fredrik

Channel Switch Announcements in CUWN

In this post we'll have a look at how the Channel Switch Announcement (CSA) IE could be present in beacons and actions frames from radios in a CUWN platform. In most of the literature (including CWAP from CWNP) and CUWN-stuff at CCO you get the impression that CSA, either as an IE in a beacon frame or an action frame is only relevant in a 802.11h radar detection and avoidance context but is actually also implemented when the system does dynamic channel assignments (DCA) if configured. And not only for the DFS-channels we have in UNII-2 and UNII-2e, we can also notice the CSA IE in UNII-1 during DCA events.

To test this I have a WLC 2504 running code 7.6.130 and some APs like 1142, 2602 and 3702 in the lab. To provoke a channel change I've set two radios statically to the same channel (ch40) and before RRM and DCA decides and adjusts new channels I set one of the radios to "Global" which in the CUWN-language means that the WLC is in charge of the channels via DCA and automatically should choose the best channel. A packet capture is started on the relevant channel to collect the frames and stopped after a change of channels can be verified in the WLC GUI.

To "enable" Channel Switch Announcements we must also enable it in the WLC since it is disabled by default and an optional feature according to the 802.11h-2003 amendment.

So after collecting the frames during a channel change due to DCA we can open it in a packet analyzer and have a look at the frames on a high level when one of the AP radios (10:BD:18:31:DF:DC) leave channel 40.

Here we can see that AP DF:DC stops the transmission of beacons but we can also notice an action frame. This action frame contain the CSA IE that informs the STA's (the ones that understand 802.11h) that the new channel will be 64 and this change is about to happen any time after this frame.

In the last five beacon frames from AP DF:DC we can also notice the same CSA IE:

Alright - so what is the take on this?

I guess it is generally nice to inform the STAs in the cell that there is a change of operating channel coming up, if the STAs then are capable of interpreting this IE and also take some kind of action like scanning only on the announced channel to reduce roaming time is another question that is more kind of open.

Enabling Channel Switch Announcements is not included in Cisco's "WLC Configuration Best Practices" found here, perhaps it should be? However in Willam Jones excellent guide "Tuning Cisco WLC for High Density Deployments" found here he suggest it is a good idea to enable CSA to inform the clients about an upcoming change.

/Fredrik