17 posts by 6 authors in: Forums > CWNA - Enterprise Wi-Fi Admin
Last Post: January 1, 2006:
  • Hi Sriss

    Thanks for replying that fast.
    So Now, To summarize, all ather station, that will notice that there is an infected frame is being transmitted, will wait for the EIFS timer before contending. and the same for the Station that has transmitted the frame, it would wait for the Eifs, then it will start contending on the medium trying to resend the same frame. So now, All the stations are having the same probability to allocate the medium, and the random backoff will be the tie breaker in this case. And of course, the station trying to retransmit will have slightly smaller prob. coz of the exponential increase in its CWmax due to retransmission retries.

    Am I wrong in this suggestion ??

  • By the way, I forgot to say merry christmas

    Thanks criss

  • Rafik,
    You are correct, a station that hears a bad frame will wait an EIFS until the EIFS expires OR it hears a good frame (It doesn't matter who the good frame was destined to).

    However, the transmitting STA will not wait an EIFS. Rememeber, a transmitting STA doesn't know that the frame that it transmitted was bad. A STA can't receive while transmitting.

    The reason for the EIFS is to give the transmitting STA extra time to retransmit (after not receiving an ACK) the frame that was corrupted. In real life it doesn't work this pretty, but in theory this is how it is supposed to work.

    Good luck to you and you also have a Merry Christmas.

  • Hi GTHill

    But I am thinking of it in another way,
    Now a STA that was transmitting a bad frame, as u said it would be given another chance to retransmit by making all the other STAs wait the EIFS. I am thinking that it's the contrary, this STA should be punished, that's why any STA during retransmission would have a larger CWmax (higher limit in the backoff timer). so all the STAs, including the STA that has transmitted the bad frame should start contending at the same moment. I mean ALL the STA will wait for the EIFS before contending.

  • Hi Rafik:

    A frame is not "bad" because the transmitter did anything wrong and needs to be punished. A frame may be bad for some receivers and good for others.

    Think of EIFS as a precaution taken by stations with frames queued for transmission and waiting for a current reception to finish before resuming its own countdown and eventual transmission attempt. If the current reception succeeds then wait a DIFS. If the current reception fails then wait a longer EIFS.

    The longer wait covers the possibility that the frame that arrived badly here, arrived intact at the intended receiver, and that intended receiver is even now transmitting an ACK, and that ACK is beyond the receiving range of this station. The EIFS is designed to be just long enough to cover this possibility.

    EIFS has nothing to do with giving the transmitter that does not receive a timely ACK, an extra chance at retransmitting. It must get in line again with all contending transmitters.

    I hope this helps. Thanks. /criss

  • Hi:

    It is true that IEEE 802.11 stations turn off their receiver circuitry whenever they ramp up their transmission circuitry. If it were otherwise the effort would be wasted.

    If IEEE 802.11 was redesigned such that stations listen while they transmit, what they heard would be no efficient basis upon which to decide if the intended receivers successfully received frames. Transmitters would still have to rely on ACKs from the intended receivers to operate efficiently in the confusion of RF space.

    For one example, a transmitter may be in the presence of a strong signal from a third station for some part of its transmission, while the intended receiver is out of range of that same third station, receives the frame correctly, and follows with an ACK.

    I hope this helps. Thanks. /criss

  • Criss,

    Thanks for pointing out my error concerning EIFS. I appreciate the correction. One day, I'll know something that you don't. Well... maybe. :)

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