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Thread: retransmissions/retry rate

  1. #1

    Default retransmissions/retry rate

    Digging deeper into 802.11 analyzing; I understand a certain amount of retries is to be expected, especially in high-density environments with a lot of noise.
    I'm trying to get a better understanding of what indicates a problem, or 'too much' retry rate?
    Seems even when not in high-density area's, theirs always at least a client or two with extremely high (>50% retry rate).
    I guess my question, to be precise, is: what should I be analyzing to determine if these retry rates are symptoms of an RF issue, or if they are expected behavior? By RF issue, I'm referring to excessive CCI, or similiar...not non-802.11 interference (I've got Chanalyzer for that).

    I'm familiar with CCI, data-rates, (what I call 'infrastructure' part of wifi design), but am now starting to dig deeper into packet analytics. My goal is to better understand how I can use packets to determine optimal (or sub-optimal) RF design.
    Last edited by mike909; 10-16-2013 at 11:23 PM.

  2. #2
    Join Date
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    Default

    i think the way to get the understanding its to do a lot of self tests in a controlled environment, do spectrum analysis and packet capture in different conditions to get a clearer image.

    i am doing so because i got my wi-spy a week ago and inspecting the stats and captures you can see a lot or retransmission and frames received with crc problems, that is normal and the tecnology its designed to deal with that.

    one advantage of doing this learning its to understand the fragility of radio data transmission.

    I think the key factors of wifi behavior are:
    signal coverage: always try to maintain good signal to expect good results if not its a matter of luck
    rf interference: key factor to discard any wifi problem
    throughput saturation: even in an ideal conditions a saturated wifi network will show a poor performance, coverage
    and interference problems maximize under saturation conditions
    faulty devices: some times certain devices show specific problems that can be only mitigated with some tricks (lowering data-rates or setting a lower spec only network) but only can be solved completely by replacing the faulty device, software and drivers updating can help, but if the device its fault or has a compatibility problem the problem will persist

  3. #3

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    Quote Originally Posted by chechito View Post
    i think the way to get the understanding its to do a lot of self tests in a controlled environment, do spectrum analysis and packet capture in different conditions to get a clearer image.
    Thanks for the suggestions. I have been doing this in different environments, and I can say that it is possible to achieve extremely low re-transmissions (even 0%).
    However, in higher density (ie classroom) environments, I still regularly see devices with 50-80% re-transmissions. I'm beginning to think this is normal, for an environment with 100 mixed devices (higher-ed).

  4. #4

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    I personally start thinking excessive around the 30-40 percent retry rate mark. At that point, I start looking into interference issues, because there has to be a reason why those frames are getting corrupted. Is it adjacent channel interference (use inSSIDer to check)? Non-Wi-Fi interference (Use Chanalyzer + Wi-Spy to check)? Is a hidden node talking and interrupting the transmission? A few things to think about when you see high retransmission rates.

    Here at MetaGeek, I see around 30 percent, due to the adjacent channel interference.
    Joel, Mobility+, ECSE, CWNE #233
    Technical Trainer
    MetaGeek

  5. #5

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    Thanks Joel, that you see 30% due to ACI tells me a lot.
    In my classroom buildings, ACI/CCI is inevitable...even with patch antenna's. What started me down this path is actually trying to determine how many 2.4GHz radio's to disable, to reduce ACI/CCI...or more specifically, how to see the end-result of too much CCI, in the packets/re-transmissions. I know the 'rule of thumb' is to not be closer than -85.

    If I could pick your brain while I have your attention, how many AP's (on same channel) at stronger than -80 RSSI before you typically start disabling radio's? Or do you just simply keep disabling radio's until neighbor AP's on same channel are all above -80 (or -85)?

    I know this is sort of a "it depends" answer, so to give some more info...:
    I've set 54Mb lowest mandatory data-rate, and lowest TxPower, but in certain buildings theirs just too much client density. I've essentially surveyed/deployed a great 5GHz network, and am trying to make the 2.4GHz "acceptable". The worst-case environment is a 3-story classroom building, stacked...each 800sq.ft. room holding 60 seats, mixed clients (higher-ed)...no real "target" application per se. With no radio's disabled, I have -50, -47 and -74 neighbors on same channel. I know the Rx sensitivity on my AP's is higher than a client, but my current thought is to disable radio's until the closest Rx neighbor on same-channel is at least -80 away.

    EDIT: Forgot to mention an important note re: disabling radio's. When I do so, I ensure that clients in the room who's 2.4 Radio is disabled, can still connect to neighboring room AP's on 2.4, to ensure I'm not cutting off any clients. What this boils down to is an increase in client count per AP, but my assumption here is that I'd prefer a single AP to handle the client count, rather than 2 AP's sharing a channel. I believe the single ap scenario reduces overhead of mgmnt/control traffic.

    Thanks for your input, and I have been using Chanalyzer to identify, locate & learn about non-802.11 interference. Next step is to upgrade to the CleanAir version.
    Last edited by mike909; 10-22-2013 at 11:25 PM.

  6. #6

    Default

    Great thread Mike.

    I am watching with great interest as this is a problem I see a lot, but there is not a lot of guidance or information on this issue. I also do a lot of surveying for RTLS systems and I just can't seem to find away of surveying 2.4GHz radio without causing a lot of CCI. With Ekahau RTLS you must have all areas covered with a lease 3 APs at -75 or better!!! It makes planning a good 2.4GHz network extremely difficult

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