signal numbers off WPC54G make sense?

Discussion in 'Wireless Internet' started by rob, Mar 4, 2007.

  1. rob

    rob Guest

    I'm wondering if these numbers mean anything to you guys in
    the know about wireless stuff.

    With a standard antenna on my WAP54G I see these numbers on
    the screen of a laptop connected to the AP using a WPC54G PC
    Card: noise -68dBm, signal -78dBm, link quality (I think
    it's called) 27%.

    Then I put a small corner reflector on one of the WAP54G
    antennas and it bumped up the lightbars and I got
    noise -58dBm, signal -68dBm, and quality 47%.

    I was surprised at several things. I was a bit surprised
    that the S/N was so low to begin but I can accept that. But
    why the heck wouldn't the S/N improve with a stronger signal
    from the AP? These numbers show both S and N going up 10dB.

    Finally, especially at low S/N I would have expected any
    measure of data quality would do more than double if S/N
    went up 10dB.

    Clearly I'm not up on this wireless system. I have looked
    around at some technical places but have never seen this
    subject discussed. Could you guys in the know point me where
    I can make sense of all this? Thanks.
     
    rob, Mar 4, 2007
    #1
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  2. Hang on a minute. I gotta put on my wizards hat.
    I think you have the signal and noise backwards. The noise cannot be
    more than the signal or you wouldn't hear anything. Assuming your
    numbers are backwards, that yields:

    signal = -68dBm
    noise = -78dBm
    S/N = 10dB (which sucks)

    The link quality number varies with the chipset used. There are
    different algorithms all of which are based on the percentage of the
    S/N ratio required to get a perfect BER (bit error rate) at whatever
    speed it's running at. I have the numbers buried here somewhere if
    you really want them. More commonly, the link quality is a measure of
    the actual BER. The more receive errors, the worse the link quality.
    That causes some rather bizarre results in the presense of
    interference. See below.
    Again backwards. I'll assume:

    signal = -58dBm
    noise = -68dBm
    S/N = 10dB (which sucks)

    Note that BOTH the signal and noise levels were increased by the
    corner reflector. That's what happens when you have a source of
    interference in the line of sight. The chipset treat everything that
    is not 802.11 or the desired access point as "noise".

    The reason the link quality went from 27% to 47% is that the strong
    signal made it easier to demodulate the data. In theory, the link
    quality should exactly follow the S/N ratio, but different types of
    intereference cause different types of data reception errors.
    Please check your numbers and try again?
    Nope. My guess(tm) is that your WPC54G uses the data error rate to
    determine the link quality. You could simply add a noise or
    interference source and the signal levels will not change. The noise
    level should change with the added interference. However, if you are
    at some threshold (as you probably are with such a lousy SNR), then
    the effects will be proportionally larger due to threhold effects.
    Not exactly. It's in the data sheets and application notes for the
    chipset, which often require an NDA or sacrifice of the first born to
    obtain. 2nd best are articles on the topic. Try searching:
    <http://www.commsdesign.com>
    <http://www.atheros.com/pt/papers.html>
    There are others but these are a good start. If I find anything
    useful, I'll post.

    I sorta blundered across these articles that looks useful:
    <http://www.cisco.com/en/US/products...iguration_guide_chapter09186a0080380e69.html>
    <http://lists.shmoo.com/pipermail/hostap/2006-December/014685.html>
     
    Jeff Liebermann, Mar 5, 2007
    #2
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  3. rob

    rob Guest

    Believe me, the numbers I gave were what is clearly seen on
    the screen, checked numerous times. I am really confused as
    to why the noise level would go up with the signal. I'm
    wondering if the guy at Linksys that formatted the output
    was thinking more about lunch than what he was doing.

    Thanks for the links, I'll check them and see what I can
    learn about this business. Rob
     
    rob, Mar 5, 2007
    #3
  4. Try the same measurements with:
    1. Using the Linksys client instead of Wireless Zero Config
    2. Netstumbler
    3. A different laptop

    It doesn't make any sense to have more noise than signal. Something
    is weird on your WPC54G client. If you change your setup from Windoze
    Wireless Zero Config, and switch to the Linksys supplied client, you
    can get S/N info from the "Link Information" tab, and not get
    entangled in the % link quality conversion. Hopefully, you'll see
    better numbers. Also, check that you have the latest WPC54G drivers
    installed.
     
    Jeff Liebermann, Mar 5, 2007
    #4
  5. rob

    rob Guest

    The only setup I know of that I used to configure my PC Card
    on the laptop was what came with the card. Don't know what
    Win Zero Config is but I'll try to find out. I have read
    about netstumbler but it seemed like a lot of stuff to wade
    into and since I finally got the laptop connecting, I didn't
    bother with it. I'll look it up again and see what it is.
    Don't have another laptop.

    I have not yet seen any numbers for what S/N these wireless
    links operate at (and I have been looking thru the links you
    gave out). But the telemetry links I have played with run at
    S/N below one to optimize bit rate/power which our friend
    Shannon pointed out. As I understand it, thruput is linear
    with BW but logrithmic in S/N.

    Thanks

    Jeff Liebermann wrote:
    Try the same measurements with:
    1. Using the Linksys client instead of Wireless Zero Config
    2. Netstumbler
    3. A different laptop
    <snip>
     
    rob, Mar 5, 2007
    #5
  6. rob

    John Navas Guest

    That's using Windows XP to manage wireless instead of the card vendor's
    utility. Check for an option to do that.
    It's actually pretty easy to use -- install and give it a try.
     
    John Navas, Mar 5, 2007
    #6
  7. rob

    rob Guest

    John, I got netstumbler and installed it on the laptop which
    has a WPC54G talking nicely to a WAP54G. But after about 30
    minutes of trying everything I could see, it continued to
    say "no wireless adapter found" (or close to that). I found
    at least one place that indicated that the Linksys PC Card
    adaptor is supported but I never found "WPC54G" explicitly
    given. So, if I could get past this one problem, perhaps I
    would find it easy to use.

    I never saw it mentioned but I presumed that the Laptop with
    the "client" adapter (I think it is called) is the place to
    use netstumbler. Just in case I put it on the machine wired
    to the WAP54G and got the same response - no adaptor found.

    I'll play with it some more and maybe I'll "stumble" into
    how to make it work. Thanks.
     
    rob, Mar 6, 2007
    #7
  8. rob

    John Navas Guest

    Check the available adapters under Device in Network Stumbler -- I have
    to specify the NDIS driver to get it to work with my Atheros-based
    wireless adapter.
     
    John Navas, Mar 6, 2007
    #8
  9. Find the check box under the properties for the wireless device with
    something like "Let Windoze Manage the Connection". With it checked,
    you get Windoze Wireless Zero Config. Unchecked and it goes to the
    vendor supplied client. If you uncheck the box, you may need to
    reboot as the Windoze Wireless Zero Config service is not
    automatically killed until the next reboot. If you're in a hurry,
    just go to:
    Control Panel -> Admin Tools -> Services
    and find the line for Windoze Wireless Zero Config. Hit "Stop".
    <http://www.netstumbler.com/downloads/>
    Install and operation is quite simple. If your wireless uses an NDIS
    5.1 driver (most do), then it should work.
    Borrow one. I the opposite problem.... too many laptops. It's handy
    for testing, but keeping them up to date is a time burner. You're
    probably better off with just one laptop.
    Ask and ye shall receive. The minimum Eb/No is the minimum signal to
    noise ratio that 802.11 will operate at a given speed. Note that it
    varies radically with speed.

    Speed Min Modulation Typical Noise
    Eb/No Sensit Floor
    mb/sec dB dBm dBm
    11 7.0 CCK -82 -89
    5.5 6.0 CCK -85 -91
    2 1.6 DQPSK -86 -87.6
    1 -3.0 DBPSK -89 -86
    54 24.6 64QAM/OFDM -71 -95.6
    48 24.1 64QAM/OFDM -71 -95
    36 18.8 16QAM/OFDM -78 -96.8
    24 17.0 16QAM/OFDM -79 -96
    18 10.8 QPSK/OFDM -82 -92.8
    12 9.0 QPSK/OFDM -84 -93
    9 7.8 BPSK/OFDM -87 -94.8
    6 6.0 BPSK/OFDM -88 -94

    What happens is that the your router does its best to maintain a
    constant BER (bit error rate). If it climbs, the router slows down
    the wireless connection speed until the BER improves. Actually, it
    uses PER (packet error rate) which is easier to calculate but the
    effect is the same. What the user sees is a constant S/N ratio which
    is directly proportional to the error rate.

    If you want to see the SNR change, dive into the routers wireless
    setup and change the connection speed from "auto" to 54Mbits/sec. That
    will yield a radically varying SNR with signal strength.
    Yep, that's the way it works for everything else remaining constant.
    In this case, the varying connection rate and modulation method, and
    the routers attempt to maintain a constant BER/PER, is what's causing
    an almost constant SNR. Even so, the noise level should NOT be higher
    than the signal level, so something odd is happening.
     
    Jeff Liebermann, Mar 6, 2007
    #9
  10. rob

    rob Guest

    Jeff, thanks for all the info. I'll definitely try again
    with the netstumbler and look for a way to make it use NDIS.
     
    rob, Mar 6, 2007
    #10
  11. rob

    rob Guest

    Jeff, I just tried netstumbler again. In the device menu the
    only option is "use any suitable device". One line above
    that is an Intel something but it is greyed out. Also I did
    find a site that said my specific Linksys WPC54G card is
    supported. So, while that very card is nicely linked to my
    AP, netstubler just won't do anything at all. The only
    version available that I can find is 0.4. Doesn't sound like
    it's been around too long.

    I looked around and found a Kismet but it must be for Macs.
    I also found and tried a UfaSoft product freebie. Damn, it
    finds the IP's of all my wireless components but there's no
    way I can get it to sniff the area. And as expected for a
    freebie, there's no documentation that really tells
    anything. I'll keep looking around; this sniffing sounds
    like a neat feature and maybe eventually I'll get it.

    Thanks again, Rob

    Jeff Liebermann wrote:
     
    rob, Mar 6, 2007
    #11
  12. I don't think you have to search for NDIS. Just install an run
    Netstumbler 0.40. If it works, you have an NDIS driver. Just about
    everything for XP and w2K use NDIS drivers.
    The modulation method varies with the connection speed. Get the
    operating wireless speed from your WPC54G connection manager, and just
    look up the corresponding details in the chart supplied.
     
    Jeff Liebermann, Mar 7, 2007
    #12
  13. That could be your ethernet. Your WPC54G should be recognized, which
    apparently it's not. No clue why it's failing.
    That's the current version of Netstumbler. Why update something that
    works just fine?
    There is a MacIntosh version, but Kismet is primarly written for
    Linux.
    Well, if you're shopping for a connection manager, try:
    <http://wifihopper.com>
    Works fine for me, with one gotcha. It installs a WiFiHopper service
    which seems to interfere with other Wi-Fi sniffers. Try it, but be
    prepared to disable the service if you try others:
    Control Panel -> Admin Tools -> Services
     
    Jeff Liebermann, Mar 7, 2007
    #13
  14. rob

    rob Guest

    I tried it and amazingly it does see my AP. But it's a
    freebie and not much else works.

    The signal level it shows stays at -64dBm(/Mhz I presume)
    even though I cover my antenna with my hand. My Linksys
    measurement shows signal dropping but hopper level never
    changes. Clearly it isn't finding the actual level.

    It doesn't find my neighbor's AP, no filters on. Linksys
    monitor does find it. I guess my Linksys monitor is a form
    of sniffer but all it does is list the ssid and strength, no
    other info is given. But it gives more than hopper.

    Apparently the hopper didn't interfere with the Linksys
    monitor, it's still working the same.

    Also the HELP menu item doesn't work so I can't really study
    how to use the thing. Maybe if I pay for it they will give
    me the help files.

    So I still don't have anything that does much sniffing but
    I'll keep looking. Thanks for your input. Rob
     
    rob, Mar 8, 2007
    #14
  15. It? Oh, WiFiHopper. It's not exactly free. The sniffer part of the
    puzzle is essentially free. The connection manager part times out
    after 15 or 30 days. The connection manager part is actually quite
    handy (for me) as it will allow me to connect by MAC address instead
    of SSID. That's useful when I have to deal with a network of access
    points, all with the same SSID.
    Use the graph, not the table to get your numbers. The table does not
    update in real time.
    Are they broadcasting their SSID?
    The Linksys monitor is nifty. It shows access points that don't
    broadcast their SSID.
    Nope. It's an active monitor and uses probe requests just like
    Netstumbler and WiFiHopper.
    Actually, it does interfere. When you have WiFiHopper running, it
    shuts down any other connection manager including Windoze Wireless
    Zero Config, and takes over using its own connection manager. Please
    read the docs. This is fine except that after the connection manager
    of the program expires after a month, it still kills the Windoze WZC
    or any other connection manager when running. There are other
    interactions that bug me. Every time it probes a live connection will
    stall. The beta version had it probeing every 3 seconds which
    resulted in a download practically stopping. Even set to the current
    default (10 sec???) it will still slow down downloads. In short, you
    can't really have it probeing for access points while trying to do
    useful web surfing or downloading. Also, when exiting the program, it
    will drop the connection. That's fine, but some drivers don't recover
    very gracefully. I find myself rebooting a bit too often. Still,
    it's a useful program if you don't try to do everything at once.
    Help works for me. However, I'm still running some old beta version.
    I'll try a later version when my laptop isn't busy.
    Well, another possibility is to use Kismet running on a Linux LiveCD.
    Nothing to install, just boot the CD. See:
    <http://www.remote-exploit.org/backtrack.html>
    707MBytes to download. Ugh.
     
    Jeff Liebermann, Mar 8, 2007
    #15
  16. rob

    rob Guest

    The graph doesn't work either. It does bump along updating
    "hits" on horiz axis but no graph line is seen. I presumed
    it was just plotting constant -64dBm, which is on the lower
    end of the vert axis. In any case, nothing interesting is
    seen on the graph.

    If SSID is something like "my Linksys", then yes, it is
    being broadcast by my neighbor.

    I just found some info on the wifihopper site, maybe that
    will help me figure out how to use it -- since the program's
    HELP doesn't exist.

    I'm beginning to think something is weird with my laptop
    system since I can't get netstumbler to do anything, and a
    lot of others are able to use it OK. But at least it does
    connect with the AP and hasn't broken down yet.

    Thanks again.
     
    rob, Mar 8, 2007
    #16
  17. rob

    John Navas Guest

    You do have a _unique_ SSID set on your own network, right?
    That's very important.
     
    John Navas, Mar 10, 2007
    #17
  18. rob

    rob Guest

    Yes sir. Out of the box I had the default but now it's quite
    unique -- after reading postings on this ng. Thanks.
     
    rob, Mar 10, 2007
    #18
  19. rob

    kev Guest

    If you are interested they have just released BT2 Final. The
    auto-configure for Kismet works for my WG511T(Atheros) and Ralink2500
    cards, in fact if both cards are fitted you get a choice of which one to
    use. They have included a new sniffer called Wicrawl ,which I haven't
    got to work with my cards yet.
    http://midnightresearch.com/projects/wicrawl/
     
    kev, Mar 10, 2007
    #19
  20. rob

    kev Guest

    Having had a further play I still can't get it to work with my Ralink
    card, however I have got it to work with the WG511T.

    1.Open console.
    2.Type airmon-ng start wifi0 press enter.
    3.This will report an ath in monitor mode, call it ath1 (due to playing
    I was at ath3 for monitor mode).
    4.Type ifconfig ath1 up press enter (It is important that you
    use the one designated in monitor mode)
    5.Start wicrawl ,go to Interfaces and make sure ath1 monitor mode is
    ticked.

    There seems to be a bug in the copy I have as the Channel reporting was
    incorrect.
     
    kev, Mar 10, 2007
    #20
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