• Hey Guys,

    Hoping for a bit of help. When doing a wireless packet capture, I noticed that when I get a lot of retries, my retransmission data rate is 6  Mb/s . I am right next to my AP, and almost of all of my packets are at very high data rates.

    Does this happen because once the collision occurs it drops the data rate back to retransmit? even though I have -30 signal? Why does it not try 12 Mb/s?

    Thanks in advance.

  • What are all of the rates, in your beacons, that are set to Basic (i.e. required) and which rates are optional ?

    Do you mean -30 dBm ?    That is an awful lot of signal.  If truly -30 dBm, you may be saturating the radio - it might have poor dynamic range (in a sensitivity sense).

    I usually consider anything stronger than -45 dBm superfluous.

  • Hi Howard, thanks for replying.

    In the beacon frame, I see supported data rates of 6,9,12,18,24,36,48,54 Mbit/Sec.

    The SSI of the packet is -30 dBm.

  • By JCottrell - edited: November 10, 2014

    Sorry, the Basic rates are 6, 12, 24.

    I might have found the issue:

    The wireless device always attempts to transmit at the highest data rate set to Basic, also called Require on the browser-based interface. If there are obstacles or interference, the wireless device steps down to the highest rate that allows data transmission.

  • By Howard - edited: November 10, 2014

    I would have thought that, because of the physical range, your original frames would have been sent at 54 Mbps.   Since your highest basic rate is 24 Mbps, that is what I would have expected for re-transmissions.  

    Maybe your radio always sends all retransmissions at the 6Mbps rate - perhaps as part of its "secret sauce".   After all, it has the best chance of getting through since it has the lowest SNR requirement.

    If not, I suspect a PHY level problem - maybe bad transmitter EVM, or similar level issue.   But then I work in the PHY level all day, so I'm suspicious of things like that. 

    What brand/models of hardware do you have on each end ?

    If Cisco AP's, do you have RRM enabled?

  • Radios are much like us in how we behave. 

    If you are having a problem speaking with someone due to something noisy in the background what do you do ? You speak louder and you speak slower. Radios do this as well. If I'm next to a radio where I should be able to TX at 54 PHY. I trigger a frame. If I don't get a response me (being a radio) will step down in PHY and speak slower. Technically whats going on - you're client is dropping down to simpler modulations sending less bits in the process. Some drivers may step down from 54, 48, 36 etc. I have seen most go from 54 to 6. Then if transmissions are successful they start to step back up in PHY. This is all driver magic of course. 

    Something to note -- 54 PHY mean 54 million bits (not bytes) per transmission. When you step down to 6 PHY in your example. You are send 6 million bits per transmission. You're allowing the receiving radio to digest less bits in the process in hopes the noise floor doesn't impact them. 

  • By Howard - edited: December 31, 2014


    I think you meant to say 54 Mb means 54 Mbps (per second), not 54 Mb per transmission.


    A a  term sometimes used is "Headline Rate", which I think is a better description of the PHY rate.  802.11 is lucky if it gets even 50% of the headline rate for the true throughput.

  • To be clear PHY is represented with Mbps (mega bits per second). 

    MB = Unit of measure typically give to storage. MegaBytes (8 bits in a byte)

    Mbps = Unit of measure typically give to transmission. MegaBits are individual 0's and 1s. 

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