If the Retry Field bit is set to 0, an original transmission of the frame is occurring. If the Retry Field bit is set to 1 in either a management or data frame, the transmitting radio is indicating that the frame being sent is a retransmission.
above the picture from CWAP
In MAC Layer service, if the 802.11 frame is received properly and CRC of the FCS passes, the 802.11 radio that received the frame will reply with ACK frame.
IF the ACK is received, the original station knows that the frame transfer was successful. Almost all unicast 802.11 frames must be acknowledged for delivery verification purposes. Broadcast and multicast frames do not require an acknowledgment.
wlan.fc.retry == 0
If any unicast frame is corrupted, the CRC will fail, and the receiver will not send ACK from to the sender. If an ACK frame is not received by the sender, the sender will retransmit the data frame. Im most cases, unicast frames must be a acknowledged by an ACK or Block ACK frame.
wlan.fc.retry == 1
Excessive layer 2 retransmission affect the WLAN int two ways.
- Layer 2 retransmissions increase overhead and therefore decrease throughput
- If application data has to be retransmitted at layer 2, the timely delivery of application traffic becomes delayed or inconsistent
In the following picture, we track retry statistics.
Layer 2 retransmissions are result of many possible problems, such as, Multipath, RF interference, and low SNR. Also hidden node, near/far, mismatched power settings, and adjacent cell interference
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