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= 802.11 Reference Design: Frame Retries =

An important role of the 802.11 DCF is to detect the presence of transmission failures and properly recover from those errors through the use of a retransmission mechanism. This behavior is subtle and often misunderstood. This document serves two roles:

1. To describe the behavior implemented in the Mango 802.11 Reference Design.
1. To defend the behavior with references to the 802.11 standard.

The organization of this document is a series of examples that build on one another and show corner cases in the behavior. The retransmission mechanism in 802.11 is further complicated by the presence of RTS/CTS medium reservation handshakes prior to the transmission of an MPDU, so examples are broken up into sections depending on whether RTS/CTS is not used (i.e. an MPDU is "short") or RTS/CTS is used (i.e. an MPDU is "long"). Without loss of generality, the examples in this document assume the following parameters:

* ''dot11ShortRetryLimit'' = 7
* ''dot11LongRetryLimit'' = 4

=== Glossary of Terms ===

||= '''Term''' =||=  '''Definition'''  =||
||  '''SRC'''  ||  Short Retry Count  ||
||  '''LRC'''  ||  Long Retry Count  ||
||  '''SSRC'''  ||  Station Short Retry Count  ||
||  '''SLRC'''  ||  Station Long Retry Count  ||
||  '''CW'''  ||  Contention Window  ||

= Short MPDUs (MPDU length ≤ ''dot11RTSThreshold'') =

== Example S.1 ==

[[Image(ex_s_1.png)]]

The above example shows the ideal scenario the transmission of 2 MPDUs. The reception of an ACK after each MPDU transmission keeps all the associated retry counts in the station at their minimum along as well as the contention window.

== Example S.2 ==

[[Image(ex_s_2.png)]]

The above example shows a scenario where 2 MPDUs are ultimately successful. However, the first transmission attempt of the first MPDU does not result in the successful reception of an ACK. Instead, a timeout occurs and the originator infers a transmission failure.