• For a 802.11b environment, if station A is within 30ft of the ap, and connects at 11Mbps, and station B is a 300 ft, and connects at 1Mbps, as they both contend for the RF channel, will station A transmit/recieve at 11Mbps, or 1Mbps?


  • A quick and basic answer is that STA A will transmit data at 11Mbps. I'll try and post more in a bit.

    GT Hill

  • that's correct. each STA will xmit at it's own data rate, however, in this scenario, the overall throughput of the BSS has been reduced down to approximately 1 Mbps. Frames in the downstream (toward the client) are typically sent round-robin, so if you can imagine a 1500-byte frame being sent SLOOOOOOOOWLY followed by a frame being sent quickly, followed immediately by another frame sent SLOOOOOOOOWLY....well, you get the picture. The 1 Mbps STA dominates the airtime and prevents the 11 Mbps STA from having any throughput.

  • Thanks GTHill and Devinator.

    As a follow-up, since station A established his 11Mbps rate, I assume that the AP, when it has frames for A, will also send at the 11mpbs rate? Assuming yes, then station B, is still able to properly discern info to allow him to enter contention properly - correct.

    Thanks so much.

  • that's correct. The AP will send frames to each STA at the rate of the current connection of each STA. Yes, both units will be able to hear transmissions to/from other stations because everyone is using DSSS-modulated PHY and MAC headers.



  • Devin,
    I wanted to take this thought further to see if I am thinking about this correctly.
    When you say "hear" I interpret that as being able to detect the RF signal (CCA) since the modulation is correct, but STA B (the 1Mbs one) won't be able to hear the MAC Header (duration), so it won't be able to set it's NAV timer. After the transmisssion is done, STA B will begin waiting a DIFS (because it's NAV is 0). Now, STA B will be interrupted because of the shorter SIFS and subsequent ACK from the AP. Now everyone is waiting a DIFS and the process starts over.
    Does that sound correct? Thanks!

    GT Hill

  • Hi GHTill,

    If the 11 Mbps STA and 1 Mbps STA can detect each other's transmissions through a CCA (meaning they can "hear" each other), everything in this scenario will work fine. The STA transmitting at 1 Mbps is still an 802.11b STA capable of modulating and demodulating DSSS signals - regardless of whether they use BPSK, QPSK, or CCK. Just because it may be using BPSK to transmit doesn't mean it can't understand a CCK-modulated signal and thereby read the PHY and MAC headers (including the MAC header duration field).

    By being able to hear (and understand) each other's transmissions, the virtual carrier sense mechanism (duration & NAV) works fine.


  • Thanks a lot as that makes total sense. Just because you are speaking one language at a time doesn't mean you can't understand all of the languages you know. (I always try to put things into an analogy). Thanks again!

    GT Hill

  • that's exactly right.

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