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I have questions on two MAC figures in Draft P802.11REVmc_D5.0 (and early versions). Any explanation is appreciated. 1) Figure 10-4: Some IFS relationships In statement "Immediate access when Medium is idle >= DIFS or AIFS[i]", should >= be >? Because >= indicates two cases: > or =. The case > is no problem, but case = may violate backoff rule. Because with =, a STA can wait for DIFS or AIFS[i], then immediately access medium even after busy medium (since medium is
already idle for DIFS or AIFS[i] time). [Adrian P Stephens] There are two different uses for DIFS (and likewise AIFS): 1.
If an MSDU arrives for transmission when the medium has been idle for >= DIFS 2.
To precede a backoff time of random() x slot duration “Immediate access”, IMHO, applies to the first case, and the equals sign is appropriate. See 1289.43 (REVmc D5.0). This is distinct from backoff. The rules on the first case are not at all clear, and CIDs 7086 and 7789 in the current round are about trying to fix this. 2) Figure 10-13: Example of exponential increase of CW Should the initial CW value (i.e., CWmin value) be 8 instead of 7? Because if CW = 7 for initial attempt, it means CWmin = 7. Then for the first retransmission, CW should be CWmin x 2 - 1 = 7 x 2 - 1 = 13, and for the second retransmission, CW should be 7x2x2-1 = 27, ... [Adrian P Stephens] Please see 1288.62: “The set of CW values shall be sequentially ascending integer powers of 2, minus 1” This is not particularly clear, though. Perhaps we could clarify it when we resolve CIDs 7086, 7789 and 7788. The EDCA version of this, 1350.63, is much clearer: If CW[AC] is less than CWmax[AC], CW[AC] shall be set to the value (CW[AC] + 1)×2 – 1. If CW[AC] is equal to CWmax[AC], CW[AC] shall be left unchanged. Also note, incidentally, that CW is not the backoff counter or even the initial value of the backoff counter. It is the range of the initial value of the backoff counter when the backoff procedure is performed (but see CID 7541 about the fact that "the backoff procedure" is not a clearly defined concept). Mark --
Mark RISON, Standards Architect, WLAN English/Esperanto/Français Samsung Cambridge Solution Centre Tel: +44 1223 434600 Innovation Park, Cambridge CB4 0DS Fax: +44 1223 434601 ROYAUME UNI WWW:
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