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Thread: Wifi - strong signal but fluctuating speed‏

  1. #1

    Default Wifi - strong signal but fluctuating speed‏

    I know that this is often asked but I am not managing to solve it despite reading dozens of forums.
    AP: Linksys WRT54GSv4 - latest drivers + 2x 6db antenna
    WUSB54GSv2 - latest drivers
    Dell running XP Pro SP3
    Also Asus eee running XP SP3 with Ralink wireless card.
    No problem with the Asus - speed ranges 48 mbps to 54 mpbs.
    Dell starts out at 54 mpbs and soon drops to 11 mbps and even to 1 mbps.
    Have run Netstumbler and inSSIDer and no other wifi clashes shown. There should be no interference from dect phones etc. Changing channels has made no difference. Have run TCP optimizer etc and played with CTS RTS fragmentation etc all to no avail.
    The really odd thing though is that when I have the Asus on then the Dell has a stable WiFi speed. Soon as the Asus is shutdown the Dell speed drops.
    Any help would be much appreciated.

  2. #2

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    I felt this thread was better suited in the inSSIDer forum, as it did not relate to Wi-Spy/Wi-Spy recordings.

  3. #3

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    Not sure why it would be more stable with the asus running (I assume you meant the dell was better with the asus connected to the ap and not the ap was better).....

    If the dell is setup to "look for a better ap" then it may be spending time "off" your channel to see what is on the other channels. This channel hop and search of beacons/ap will mean no-data to/from your connected ap until it returns.

  4. #4

    Default Wifi - strong signal but fluctuating speed‏

    Yes, I do mean that it is the Dell has a stable speed when the Asus is running.

    I currently have the roam tendency set to conservative (+30 db) although I have tried the other two settings, aggressive (+10 db) and moderate (+ 20 db) with no discernible difference.

    My usual RSSI is -65 and the highest strength neighbours get is -90 and usually -95 or worse.

    BTW inSSIDer measures all speeds at 54 although my windows monitor currently shows my connection at varying between 2 and 5.5. Netstumbler also seemed to show the potential (G) rather than the actual.

    It really is bizarre. As soon as I boot up the Asus and it's connected the Dell speed jumps back to 54. I disable the Asus' connection and the Dell speed drops. Reconnect the Asus the Dell jumps back to 54 - and the Dell and Asus are 20 feet and two solid walls apart.

  5. #5

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    What happens to the speed on the dell with no Asus but with a big file transfer?

    My thought here is the speed may be dropping back as its not needed (eg: Back to a B speed not at the G), then when the Asus connectes it insists it run at G speeds so the AP goes into G mode, thus the dell sees the higher speed.

    If this holds true, then you should see the speed increase with a big file transfer.

  6. #6

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    Tried a large download and the speed hovered between 11 and 18.

    I don't know if this is relevant but I also noticed that on the Asus the AP I'm connected to will be shown on inSSIDer with a marginally fluctuating RSSI whereas on the Dell it is unchanging. If I disconnect from the AP on the Dell then the RSSI fluctuates. Both are running 1.2.3.1014.

  7. #7

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    I still think the Asus is setting the AP speed. Someone can correct me if I'm wrong, but I think the soho APs will operate at the slowest speed needed. Then up the speed as required, but locked to the max speed of the slowest talking device.

    So, if the asus, told that AP that it needed 54Meg (ie: it only Supports 54Mbps), and the Dell was happy to run at that speed, then that is what it would be set to.

    If you could capature the raw 802.11 packets
    The Beacon from the ap would show its Supported Rates
    eg: 1.0 2.0 5.0 11.0 18.0 24.0 36.0 54.0
    Then it may also have some extened rates
    eg: 6.0 9.0 12.0 and 48.0

    Next the client will send a Probe Request
    My Dell had
    Supported Rates: 1.0 2.0 5.5 11.0 6.0 9.0 12.0 18.0
    Exteneded Rates: 24.0 36.0 48.0 54.0

    The Probe response show that the AP now support the 5.5 to match my request.

    Next, the Association Request to the AP
    Rates : 1.0 2.0 5.5 11.0 6.0 12.0 18.0
    Extended : 24.0 36.0 48.0 54.0

    The Association Response from the AP
    Rates 1.0 2.0 5.5 11.0 18.0 24.0 54.0
    Extended : 6.0 9.0 12.0 48.0

    While all that was going on the actual speeds in the radio headers
    Beacon 1.0
    Probe Request 1.0
    Probe Response 1.0

    A few things back and forth unto the EAP (Radius) Auth; This was done at 54Mbps
    I then downloaded a file from my NAS to my notebook via the AP. The date rate to Me was 54Mbps, the ACKS went back at 24Mbps.

    So what I was seeing is the actual speeds kept changing to match what was needed.
    Again, if the Asus said it could only support 54 (or that was the only one in common), then it may be forced to work at the faster speeds.

    The key reason for the dynamic speed changes is to be as robust and friendy as they can.
    If you run at 54Mbps, you may find the re-transmissions go up. This means you just get errors quicker not actual data, but a slower speed may have little to no errors, thus a faster tx speed. Also the slower speeds means there is more time for other devices to tx/rx on the same channels.

    Just my 2 cents worth, but a sniff with a promisc WiFi card or an AirPCap would show you.
    You really need to get the Radio Headers to see it.

  8. #8

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    Quote Originally Posted by saxophone View Post
    Tried a large download and the speed hovered between 11 and 18.

    I don't know if this is relevant but I also noticed that on the Asus the AP I'm connected to will be shown on inSSIDer with a marginally fluctuating RSSI whereas on the Dell it is unchanging. If I disconnect from the AP on the Dell then the RSSI fluctuates. Both are running 1.2.3.1014.
    btw: was the d/l on you lan, thus should be able to run as fast as the wifi will let it?
    You could try a d/l on the dell with only the dell and then with the asus on and the dell and see if the actual d/l time is better, or just the ap speed says it is.

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