MAC header - Frame Control - Retry Field

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 

MAC header - Frame Control - More Fragments

The More Fragment filed is 1 bit in length. a

above the picture from CWAP

The More Fragments field is set to 1 if another fragment of the same MSDU follows in a subsequence frame. It is set to 0 in all other frames. The following packet capture show More Frame is 1

Notice that MAC layer never fragment broadcast and multicast frames because frames sent to a multicast are never acknowledged and retransmitted in any event. The following packet capture show More Frame is 0

MAC header - Frame Control - "To DS" and "From DS"

The "To DS" and "From DS" indicate the flow of the 802.11 data frames between a WLAN environment and the distribution system (DS).

Above the picture from CWAP

There are four possible combinations for "To DS" and "From DS" as summary in the following table.

To DS = 0 and From DS = 1
It indicates that an 802.11 data frame is being sent downstream from an access point to a client. For example, a DHCP offer packet through an AP to a client station.

To DS = 1 and Frome DS = 0
It indicates that an 802.11 data frame is being sent upstream from a client station to an access point. For example a client station sending DHCP request packet through an AP to DHCP server.