CWmaxContention Window Maximum
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Contention window become CWmin when the transmission completed successfully which is 7, while collision occur, it was doubled till CWmax (1024) was reached.
If (successful transmission) CW[left arrow]CWmin; else CW[left arrow]min(2*CW, CWmax) 3.
Figures 32 and 33 show the default and optimized values, respectively, for CWmin and CWmax. Results are drawn for default values of CWmin and CWmax which are (-1) for IEEE 802.11e standard.
Default EDCA MAC parameters values TXOPLIMIT (ms) Extended DS-CCK Rate/OFDM Other AC AIFSN CWmin CWmax (11b) (.11a/g) PHYs VO 2 (aCWmin +1) (aCWmin +1) 3.264 1.504 0 4-1 2-1 VI 2 (aCWmin +1) aCWmin 6.016 3.008 0 2-1 BE 3 aCWmin aCWmax 0 0 0 BK 7 aCWmin aCWmax 0 0 0 Table 2.
CW may increase to the maximum congestion window size limitation (CWmax) when the erasure rate is low.
Service differentiation between ACs is achieved by setting different Contention Window (CWmin, CWmax,) sizes, Arbitrary Interframe Space (AIFS) values, and Transmission Opportunity duration limit (TXOP limit).
Table 3 shows the CW value of the jth retransmission of different priority nodes and from the fifth retransmission nodes employ the CWmax as the CW value to obtain contended allocation intervals.
MAC and PHY definitions for IEEE 802.11 Parameter Value Mac/802_ 11 set dataRate 6.0e6 Mac/802_ 11 set basicRate 6.0e6 Mac/802_ 11 set CCATime 0.000004 Mac/802_ 11 set CWMax 1023 Mac/802_ 11 set CWMin 15 Mac/802_ 11 set PLCPDataRate 6.0e6 Mac/802_ 11 set PLCPHeaderLength 50 Mac/802_ 11 set PreambleLength 16 Mac/802_ 11 set SIFS 0.000016 Mac/802_ 11 set SlotTime 0.000009 Phy/WirelessPhy set RXThresh 6.72923e-11 Phy/WirelessPhy set freq 5.15e9 Phy/WirelessPhy set Pt 0.281838 Table 2.