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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.
  2. 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

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

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.

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