Connect with us

Wi-Fi Beacon Photo or Plot?

Discussion in 'Radio and Wireless' started by epsolutions, Sep 30, 2019.

  1. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019
    Can anyone provide an oscilloscope screenshot, or plotted output, that clearly shows several cycles of the 10Hz beacon as embedded within a Wi-Fi transmission?
     
  2. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019
    I searched the net yet again and can find nothing in the way of published images.

    My oscilloscope will not read that high. Can anyone please help?
     
  3. davenn

    davenn Moderator

    13,294
    1,758
    Sep 5, 2009
    You DONT use oscilloscopes to look at a RF signal, you use a spectrum analyser
     
  4. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019
    Sorry. You've lost me there. I am interested in a screenshot of the actual Wi-Fi signal not a plot of its frequency spectrum.
     
  5. davenn

    davenn Moderator

    13,294
    1,758
    Sep 5, 2009

    good luck
     
  6. Nanren888

    Nanren888

    116
    22
    Nov 8, 2015
    Several cycles?
    .
    Can you clarify? You want the time waveform? As SS carrier with modulation, or individual cycles of RF showing QPSK?
    .
    You want several beacon packets? I guess as you suggest, they are usually sent every 100mS. But, I think they are just wifi packets, from memory, being beacons by data content.
    .
    What sort of scale? I don't remember the packet structure, so can't remember the packet length I'd be trying to capture.
    I guess one of the scopes will decode capture, but would need something to decode quickly to trigger the scope to hold the retrospectively captured signal, to know whether I have captured the right packet, as they are all just data packets, right? and there's like lots of other traffic.
    I guess one of the signal generators might produce a packet, or do you need one that's been through the air?
    From memory it is QPSK, so do you not want the IQ demodulated output, that is looking like a constellation diagram?
    .
    Sorry, as you can see, I'm struggling rather with what you want as can't really see what use any of these is.
    There might be Matlab,octave, python code to simulate wifi that can produce you a signal.
    .
     
    davenn likes this.
  7. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019
    Please excuse the late reply.

    I am trying to understand the Wi-Fi signal structure, in particular the "beacon", and thought a screenshot of an actual signal would help.

    Does the beacon take the form of a 1) 100mS packet of data or 2) phase shift in data that recurs every 100mS?

    Would it look similar to the second plot down on the left in this uploaded file?
     

    Attached Files:

  8. Nanren888

    Nanren888

    116
    22
    Nov 8, 2015
    You can check the specs, but from memory it's just another wifi packet. It is sent, by default, once every 100mS. So apart from it having a small payload, hence brief in time, I presume that it just looks like other wifi packets. With QPSK the data is represented by phase shifts on the carrier signal.
     
  9. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019

    Attached Files:

  10. Harald Kapp

    Harald Kapp Moderator Moderator

    9,306
    1,889
    Nov 17, 2011
    That is your interpretation, but it is incorrect. This image shows a veeeery symbolic representation of the signalling and the beacons are higher only to accomodate the text.
    Read the text on page 32, chapter "Establishing Contact" in the pdf you linked. It tells you that "A Beacon is transmitted from an AP and contains information about the AP along with a timing reference." so it is just anther WiFi frame (see @Nanren888 's post #8).
     
    davenn likes this.
  11. hevans1944

    hevans1944 Hop - AC8NS

    4,231
    1,995
    Jun 21, 2012
    I am sure we have "lost" you since you seem to have absolutely no idea what "the actual Wi-Fi signal" is. Why don't you get up to speed on modern communications theory and modulation methods before wasting our time here in this forum? Here is a Wi-Fi Primer (a beginners text) from Texas Instruments to get you started. You have already linked to this document in your post #9 but now would be a good time to read and understand the entire document before trying to, for example, interpret graphs therein out of context. Read ALL of the Primer! Don't just cherry-pick things you have no understanding about and bring them up here for our consideration. Those are things are for your consideration, but we are more than willing to discuss them with you after you have demonstrated some rudiments of understanding and self-education, You definitely need more than an oscilloscope and a spectrum analyzer (although those will help further down the road) to receive and interpret a Wi-Fi signal. Howsabout telling us what access point you are going to use and what its protocol is? Also, it would help if you would tell us what you are going to do with the beacon signal after you have found it, and how you think this will help you to understand Wi-FI signal structure. Of course, if if you have found the beacon signal, that pretty much says you now understand Wi-Fi signal structure... so good luck with that!
     
  12. epsolutions

    epsolutions

    18
    0
    Sep 7, 2019
    I have no practical application for this. I simply wanted to know, in general terms, how the beacon was integrated into the Wi-Fi physical layer. I thought a screenshot image might help. I would have liked to find one online, but could not.

    If I understand correctly, the beacon is a Wi-Fi frame of fixed shorter duration that recurs once every 100mS, preceded by an inter-frame space (PIFS).

    It helped to be advised here of the correct terminology so I could continue researching this online.

    https://www.rfwireless-world.com/Terminology/WLAN-SIFS-vs-PIFS-vs-DIFS-vs-EIFS-vs-AIFS.html

    https://www.researchgate.net/profil...censed-spectrum.pdf?origin=publication_detail
     
  13. hevans1944

    hevans1944 Hop - AC8NS

    4,231
    1,995
    Jun 21, 2012
    Looks like your education is coming along nicely. The Beacon is just one indication that a rabbit-hole exists. Now you must buy (or build) hardware that will inspect ALL the packets streaming through your Access Point and decide what to do with them. This is essentially what a firewall does.

    Most packets are quite innocent and can be simply passed along to their destination. Others may contain nasty "payloads" that will transfer control of your network, and the computers to which it connects, to somewhere and someone else on the Internet. Hopefully, your firewall recognizes, blocks, and logs such attempts so remedial measures can be taken. You will need specific hardware and software to explore how to spoof and bypass most "weak" firewalls.

    Tools to play with packets in real time might be a little hard to find on the Internet, but for the truly brave (or foolhardy) there is also the Dark Web, which (I have heard) while totally lawless and quite dangerous to explore does offer some "interesting" hacker tools. Beware of "free" software: you get what you pay for and maybe a bunch of bits you definitely don't want. Monetary transactions on the Dark Web are generally exchanges of digital currency, such as BitCoin. Caveat emptor.

    I don't know about any of this from personal experience: never been there, never done that, have no intention of ever trying to explore the Dark Web.

    Good luck with your research!
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-