• Hey guys,

    I have a real life issue here. I have an AP indicating a High Utilization. It's because of the retransmissions at 31%. There are 3 clients connected to the AP. We have great SNR.

    Here is the info that I have connected in this scenario.

    Retries % : 31%
    Noise Floor: -93
    RSSI: -52
    SNR: 43
    # of Clients: 3

    I have two SSID but only one is being used by wireless users. The other SSID hardly is used.

    I have check the area for Co-Channel interfernce and that isn't the case is this situation. Could it be a hidden node or multipath cause all the retries. How can I detect Multipath ?


  • What is the other SSID being used by and how many devices? Are both SSID's being broadcast from the same AP/channel or do you have dual 2.4 & 5GHz deployment?

    What type of building is this? Warehouse/Office space/retail etc

    Hidden node can be partially addressed if you enable RTS/CTS on the client devices if possible, its a protocol solution to a physical layer problem.

  • Thanks Mike for the reply.

    Both SSID are being broadcasted on both radios. Come to think about it, the hidden/secured SSID that is used to managed door lock only operate on the b/g radio. So I could disable that SSID on the 'a' radio. The hidden/secured SSID doesn't have that many clients and isn't used that often. As for the multipath, we a device has throughout issue, this is one of the thing I check. As you said, some laptops has this option to set and some don't. This is a university residence hall.

    Here is another example in a different building.

    Retries Percent 16
    Noise Floor -95
    RSSI -66
    Radio Channel 11
    Technology Type 802.11g
    TX Power 15
    No. of Clients 2
    SNR 29

    I know retries should be less than 10%.


  • First... hidden node. Hidden node really doesn't happen all that often anymore, at least not the traditional hidden node. Anyway, unless your devices are doing a lot of upstream traffic hidden node doesn't matter.

    Here is an easy way to detect hidden node. It appears that you have a frame analysis tool.

    - Find out what the MAC addresses are of all of the clients on the AP. That's pretty easy.
    - Stand next to one of the clients and run a packet capture while the other clients are running.
    - Check the capture for any frames that have the client's MAC address is the transmitter field.
    - If you see the frames and the CRCs are good, you don't have hidden node problem.

    You may have to repeat this at different client locations.


  • GT,

    I don't understand how listening next to [u]the[/u] client will help. Do you mean listening for one of the [u]other[/u] clients ?

    Or near the AP?

  • I'll take a stab at this answer for GT, as he helped me out in the past with some of my troubles. You want to listen next to one of the clients for other clients. You then want to compare this to the client list from your AP. If you notice clients that are "audible" from the client location that are not listed on the AP client table, you have the possiblility of Hidden Node.

    I would imagine, GT is having you test it this way as a very quick Hidden Node issue test. As he said, it tends to be on the way out due to newer wireless technologies. Again I'd imagine he will post here and let us know as well.

    Also I see that you have the following types of clients: a,b/g,n. With the assumption that your n is only 5GHz n, I think there may be a problem there.

    My thought would be to put the security SSID as you described it on a. Then check the air for b clients. If it is a dorm building, I'd hope the students had moved to g radios at the very least by now. But just in case, check the air for b clients. If there aren't any, I would disable b. You still run the risk of a b device coming in and having/causing some problems, but that is a risk/benefit analysis that you'll need to do and have the discussions regarding.

    These changes should leave you with sufficient a and 5GHz n data rates, as well as a relatively clear g. Again, I think a full survey would give us some more of this information, and possibly help more than just the RSSI and SNR values, but that's what I see at first blush.

    And as has been my norm recently, understand I have been a tad removed from the wireless specific tech recently, so take what I say with a grain of salt.

  • As you said, "....for the other clients".

    That is what I wanted to clarify.

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