Re: [Hackrf-dev] Decode the Signal

Wed, 14 Oct 2015 07:11:27 -0700

Some (all?) WiFi access points "poll", that is, if they currently have no active connections, they go through a sleep/wake cycle where you will see the signal appear and disappear in cycles.

I saw this on the Comcast/Xfinity Technicolor router that they started shipping to their customers earlier this year in certain US markets.  Their router has three different signals running, one of which has no SSID.  The other two are your locally-defined wireless network, and the much-maligned "xfinity wireless" which Comcast claims is to provide "free" WiFi to the world (or at least to your neighbors).  It's really not free in the respect that you have to be a Comcast customer with a valid login to use it, and it obviously is using someone else's (not Comcast's to be sure) electricity to host the signal (and your bandwidth).  But I digress.

The 3rd signal turns out to belong to Comcast/Xfinity's wireless home security system, and is there whether or not you've paid for that option.  If you use a simple app like WiFi Analyzer on an Android phone, you can see this signal come and go in a few second cycle, essentially polling for its home security devices.  Since there were none at my place (that belonged to Comcast) it instead managed to effectively interfere with my local wireless network.  Apparently a number of wireless clients are befuddled with having a strong signal come and go on the channel they are trying to use.  The result was a number of devices kept complaining about getting disconnected, or downloads and other streaming would halt and start, and other annoying things.

It also turned out that I couldn't move either of the other two signals off of my chosen channel for my local network.  Whenever I changed the channel for my local network, the other two signals would follow it (?!), thus guaranteeing that anything running on the other signals would slow down access to my local network.  What's particularly maddening about this is that Technicolor's implementation of what have should have been a simple VLAN exercise is flawed.  It should have worked, but apparently they can't program their way out of a paper bag.  At my workplace, we use access points that are running up to 16 VLANS (hosting such things as a guest network, employee network, visiting employee network, supplier network, etc, etc), and they perform flawlessly.

I put up with this for about a week, and took the router back to the local Comcast office and told them I didn't want it.  I went out and bought an Arris/Motorola Surfboard 6121 cable router which does not have wireless in it, and a Cisco wireless firewall/router.  Everything now runs fine, and I'm no longer fighting with two other signals I didn't want and did not intend to use.

73,

-- Dave, N8SBE
 
-------- Original Message --------
Subject: Re: [Hackrf-dev] Decode the Signal
From: "Srinivasan T" <[email protected]>
Date: Wed, October 14, 2015 9:11 am
To: "'Mike Walters'" <[email protected]>
Cc: 'hackrf-dev' <[email protected]>

Thanks.
I can see that there is one signal interfere at 2.4 GHz.
 
Do you want me to capture .iq file using 20 M ?
 
From: Mike Walters [mailto:[email protected]]
Sent: Wednesday, 14 October 2015 7:14 PM
To: Srinivasan T
Cc: hackrf-dev
Subject: Re: [Hackrf-dev] Decode the Signal
 
The signal in the first and second videos is Wi-Fi - if you use a sample rate of 20M you can see it much clearer.
 
On 14 October 2015 at 07:28, Srinivasan T <[email protected]> wrote:
 
Dear All,
 
I am managed to do some RND with unknown Signal at 2.4 GHz. This Signal works almost everywhere in Singapore, Indonesia and Malaysia.
Here some video for reference :
 
Here is the video :
 
Ø  FFT at 2.4 GHz ( we can see sudden fluctuation and power in db is really high - in Singapore at home )
        Detail : On this video, we can see peak hold in green color that leave a trace something comes and go
 
Ø  Spectrum Density of RF at 2.4 GHz ( in Medan - Indonesia )
        http://sendvid.com/mu9m2jeg
        Details : I have taken this video at place where there was no devices that operate at 2.4 GHz ( near plantations ). We can see noise floor in blue color comes and go.
                         This external interference cause Wi-Fi at 2.4 GHz to fluctuate and sudden fluctuate at different power level will increase of SNR of Wi-Fi ( signal to noise ratio )
 
Ø  AirMagnet XT ( in Singapore at home )
      Attached video file, file name ;   https://sendvid.com/tkq3xz0s
      Details :    This software one of the best software available in the market for detection interference Wi-Fi at 2.4 GHz with different options. We can see FFT picture at real
                        time in leave suddenly max hold around -10 dBm. ( We can spectrogram picture as well ).  There are 2 rules to be remembered :
·         If power level increase suddenly to 10 dBm then the source should be near and visible to me but on this case we dont see the source
·         Any RF Transmitter at 2.4 GHz will transmit at constant power and sudden increase of power at middle of transmission really no possible.
 
      Wi-Fi Fluctuations at Siloam Hospital in Medan - Indonesia
 
I would like to request on how decode header information or others of the signal. I have captured .iq and .cfile using HackRF :
https://www.dropbox.com/s/wj77dm4n7fj7rcl/Signal%20Capture.7z?dl=0
 
 
Please advise on  decode the signal.
 
Regards
 
 
Srinivasan T
 

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