Changes between Version 3 and Version 4 of 802.11/Benchmarks/IFS


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Timestamp:
Nov 24, 2013, 12:49:45 PM (10 years ago)
Author:
chunter
Comment:

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  • 802.11/Benchmarks/IFS

    v3 v4  
    77Honoring the interframe spacings (IFS) of 802.11 is fundamental to the interaction of different 802.11 devices in a shared wireless medium. For this reason, we have thoroughly calibrated and characterized the IFS values of the 802.11 Reference Design. For a comparison baseline, we have also characterized a [http://en.wikipedia.org/wiki/Linksys_WRT54G_series#WRT54GL Linksys WRT54GL] running the [http://en.wikipedia.org/wiki/Tomato_(firmware) Tomato firmware].
    88
    9 == SIFS ==
     9== Short Interframe Space (SIFS) ==
    1010
    11 The Short Interframe Space defines the gap in time between:
     11The SIFS defines the gap in time between:
    1212
    13131. A data frame and its corresponding ACK
    … …  
    1919||  '''Comparison of SIFS interval with commercial 802.11 device'''  ||
    2020
    21 The above figure shows the monitor's thresholded RSSI signal in addition to signals from the perspective of the WARP AP that correspond to the PHY's start of TX processing and RX processing. The WARP implementation of 802.11 adheres to the standard's specification of a SIFS interval of 16µs.
     21The above figure shows the monitor's thresholded RSSI signal in addition to signals from the perspective of the WARP AP that correspond to the PHY's start of TX processing and RX processing. The 802.11 Reference Design implementation adheres to the standard's specification of a SIFS interval of 16µs.
    2222
    23 == DIFS / Slot ==
     23== DCF Interframe Space (DIFS) and Slot ==
     24
     25Two other critical interframe spacings are the DCF Interframe Space (DIFS) and slot time. The DIFS corresponds to the minimum amount of time between busy medium event and the transmission of another frame (excluding the cases covered by SIFS). The slot time corresponds to the amount of time between two different contention window (CW) values. To test our implementation's understanding these intervals, we have reversed the flow of traffic relative to the SIFS test above. Here, the DUT generates a data frame and the WARP AP is responsive for sending an ACK for that packet. In the case that the DUT chooses a CW of zero, the observed gap in time where the medium is idle corresponds to a DIFS. By increasing persistence on the oscilloscope to view multiple transmissions, other intervals corresponding to nonzero CW values can be observed. Comparing the relative start times of these other transmissions allows us to measure the slot time.
    2426
    2527||  [[Image(wiki:802.11/files:difs_bench.png, width=600)]]  ||
    2628||  '''Comparison of DIFS interval and slot time with commercial 802.11 device'''  ||
     29
     30The above figure shows that the 802.11 Reference Design implementation adheres to the standard's specification of a slot interval of 9µs. Furthermore, it shows that the implementation adheres to a DIFS interval of 34µs (SIFS + 2*slot time = 16µs + 2*9µs = 34µs).