Changes between Version 12 and Version 13 of 802.11/MAC/Lower/Retransmissions

Timestamp:

May 5, 2015, 11:35:53 AM (9 years ago)

Author:

chunter

Comment:

--

802.11/MAC/Lower/Retransmissions

@@ -1 @@
-[[TracNav(802.11/TOC)]]
+ [[TracNav(802.11/TOC)]]
 
 = 802.11 Reference Design: Recovery Procedures and Retransmit Limits =
@@ -12 +12 @@
 * ''dot11ShortRetryLimit'' = 7
 * ''dot11LongRetryLimit'' = 4
+* ''aCWmin'' = 15
+* ''aCWmax'' = 1023
 
 === Reading the Figures ===
@@ -61 +63 @@
   ''The CW shall be reset to aCWmin after every successful attempt to transmit a frame containing all or part of an MSDU or MMPDU, ... .''
 
-  '''''Excerpt of Section 9.3.4.4 of 802.11-2012'''''
+  '''''Excerpt A of Section 9.3.4.4 of 802.11-2012'''''
 
   ''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 ... .''
@@ -74 +76 @@
 }}}
 
-The above examples shows a scenario where the no MPDU transmission ever succeeds. As in [#ExampleS.2 ex]
+The above examples shows a scenario where the no MPDU transmission ever succeeds. As in [#ExampleS.2 Example S.2], '''''Excerpt A of Section 9.3.3 of 802.11-2012''''' dictates that each failure to delivery the transmission causes an increase in the SRC, SSRC, and the CW. The termination of retransmissions is dictated by the following excerpt from the 802.11 standard:
+
+  '''''Excerpt B of Section 9.3.4.4 of 802.11-2012'''''
+
+  ''Retries for failed transmission attempts shall continue until the SRC ... is equal to dot11ShortRetryLimit ... . When [this limit] is reached, retry attempts shall cease, and the MPDU ... shall be discarded.''
+
+On the 7th failure, the SRC increments from 6 to 7. At this moment, the SRC equals the ''dot11ShortRetryLimit', causing the MPDU to be discarded. Simultaneously, '''''Excerpt A of Section 9.3.3 of 802.11-2012''''' from [#ExampleS.2 Example S.2] dictates that the SSRC increase from 6 to 7 due to the increment operation of the SRC. At this point in time, the SSRC equals the ''dot11ShortRetryLimit'' as well. This event triggers the following behavior:
+
+  '''''Excerpt C of Section 9.3.3 of 802.11-2012'''''
+
+  ''The CW shall be reset to aCWmin ... when SSRC reaches dot11ShortRetryLimit.''
+
+As such, the 2nd MPDU begins with a fresh CW minimum. However, it is important to note that the SSRC itself is not reset, so the next MPDU begins with an SSRC of 8.
+
+== Example S.4 ==
+
+{{{
+#!div style="overflow-x:scroll; border:1pt solid"
+[[Image(ex_s_4.png)]]
+}}}