Changes between Version 19 and Version 20 of 802.11/MAC/Lower/Retransmissions
- Timestamp:
- May 5, 2015, 3:52:29 PM (9 years ago)
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802.11/MAC/Lower/Retransmissions
v19 v20 65 65 '''''Excerpt A of Section 9.3.4.4 of 802.11-2012''''' 66 66 67 '' After transmitting a frame that requires acknowledgment, the STA shall perform the ACK procedure, as defined in 9.3.2.8.The SRC ... and the SSRC shall be incremented every time transmission of a MAC frame of length less than or equal to dot11RTSThreshold fails ... . This SRC and the SSRC shall be reset when a MAC frame of length less than or equal to dot11RTSThreshold succeeds ... .''67 ''The SRC ... and the SSRC shall be incremented every time transmission of a MAC frame of length less than or equal to dot11RTSThreshold fails ... . This SRC and the SSRC shall be reset when a MAC frame of length less than or equal to dot11RTSThreshold succeeds ... .'' 68 68 69 69 In other words, the SRC increments because of the MPDU transmission failure. The SSRC increments because the SRC increments. The CW increases because the SSRC increments. When an ACK is received in the second attempt of MPDU 1, Section 9.3.4.4 dictates that the success of that MPDU causes the SRC and the SSRC to be reset. Section 9.3.3 dictates that the successful delivery of the MPDU also resets the CW. … … 120 120 }}} 121 121 122 This example shows the first type of failure that can occur in the transmission of a long MPDU. A failure is inferred if a CTS is not received after the transmission of an RTS.122 This example shows a type of failure that can occur in the attempted transmission of a long MPDU. If an ACK is not received in response to the transmission of the long MPDU, then the following excerpts of the 802.11 standard applies: 123 123 124 '''''Excerpt C from 9.3.4.4 of 802.11-2012'''' 124 '''''Excerpt D of Section 9.3.3 of 802.11-2012''''' 125 126 ''Every STA shall maintain a ... STA long retry count (SLRC), ... which shall take an initial value of 0. ... The SLRC shall be incremented when any long retry count (LRC) ... is incremented. The CW shall take the next value in the series every time an unsuccessful attempt to transmit an MPDU causes either STA retry counter to increment, until the CW reaches the value of aCWmax.'' 127 128 '''''Excerpt C from 9.3.4.4 of 802.11-2012''''' 129 130 ''The LRC for an MPDU ... and the SLRC shall be incremented every time transmission of a MAC frame of length greater than dot11RTSThreshold fails ... . This LRC and the SLRC shall be reset when a MAC frame of length greater than dot11RTSThreshold succeeds ... .'' 131 132 The failure to receive an ACK for the long MPDU causes both the LRC and and the SLRC to increment. The increment operation on the SLRC causes the CW to increase to its next value. When the ACK is received in the second attempt, the CW is reset to its minimum value according to '''''Excerpt B of Section 9.3.3 of 802.11-2012''''' from [#ExampleS.1 Example S.1]. 133 134 == Example L.3 == 135 136 {{{ 137 #!div style="overflow-x:scroll; border:1pt solid" 138 [[Image(ex_l_3.png)]] 139 }}} 140 141 This example shows a type of failure that can occur in the attempted transmission of a long MPDU. A failure is inferred if a CTS is not received after the transmission of an RTS. 142 143 '''''Excerpt D from 9.3.4.4 of 802.11-2012''''' 125 144 126 145 ''After an RTS frame is transmitted, the STA shall perform the CTS procedure, as defined in 9.3.2.6. If the RTS transmission fails, the SRC for the MSDU or MMPDU and the SSRC are incremented.'' … … 128 147 Furthermore, '''''Excerpt A from 9.3.3 of 802.11-2012'''' in [#ExampleS.1 Example S.1] applies here as well, so the contention window immediately increases to 31 after the post-RTS timeout. The second attempt of the first MPDU begins with a new RTS transmission. When a CTS is received in response to that RTS, the following excerpt instructs our behavior: 129 148 130 '''''Excerpt Dfrom 9.3.3 of 802.11-2012'''''149 '''''Excerpt E from 9.3.3 of 802.11-2012''''' 131 150 132 151 '' The SSRC shall be reset to 0 when a CTS frame is received in response to an RTS frame ... .''