In a recent post, I described what I thought to be odd behavior of an iPhone probing on channel 52. Channel 52 requires DFS, and a client device shouldn't probe unless it can hear an AP on the channel. I wasn't seeing anything on the channel but probes, and it was quite a mystery.
I removed the post, because I now know that wasn't what was happening. To summarize:
- I saw probes on channels 52 and 56, but not 60 or 64.
- No other traffic on 52 or 54.
- iPhone was right next to IAP-315 I was capturing with.
- When I captured on channel 48, the probes were about 40 dB stronger than the probes I saw on 52. Probes on 56 were only about 1-2 dB weaker than those on 52.
- It wasn't just the iPhone; it was my Moto G4, and the laptop I was running Wireshark on.
Here's a picture of the test setup:
|OFDM Spectral Mask|
This was the most plausible explanation of what I was seeing. The probe requests that I captured on 52 were actually transmitted on 48. The phone and AP were so close to one another that there was enough energy on the adjacent channel, 20 MHz away, to be decoded on channel 52. Looking at the spectral mask, it explains the 40 dB drop in power, and why I saw not only the iPhone, but also my laptop probe on 52.
To test further, I started capturing on 52, with the iPhone right next to the AP. I saw probe requests at -75 dBm. I left Wireshark running, and switched the capture channel from 52 to 48. I picked up the iPhone and moved it about 4 meters away from the AP. I saw probe requests at -61 dBm. Even though the phone was much farther away, the signal was received 14 dB stronger. To confirm things, I switched the capture back to channel 52 with the phone still 4 meters away. I saw no frames at all.
The first frame is from my laptop, right next to the AP. Note the receive power. The next frames are from the iPhone, which uses a randomized MAC when probing. The phone was placed next to the AP when probes were seen on 52, and 4 meters away when seen on 48.
There's been some discussion lately about APs that have dual-5 GHz radios, and why that can be a bad thing. After what I experienced, I tend to believe it. It's also a cautionary tale on how you setup your captures.
Thank you to Ben and all who viewed the blog and commented on Twitter. Ultimately, I was wrong, but I learned a lot.