802.11 Protocol Architecture
receive the corresponding Clear-to-Send (CTS). Or, station A may
send a data frame and never receive an acknowledgement. Because of
these problems, the MAC Layer has error recovery mechanisms.
Stations initiating frame exchange sequences have the responsibility of
error recovery. The station or access point receiving a data frame will
respond with an acknowledgement frame if no errors in the frame are
found. The error control function involves the retransmission of the
data frame after a period of time if no acknowledgement frame is
received from the destination station. This also takes into account that
bit errors could have made the acknowledgement frame
unrecognizable.
The MAC layer has limits on how many times it will retry an
apparently failed frame exchange sequence. The relevant MIB
variables are dot11RTSThreshold, dot11ShortRetryLimit and
dot11LongRetryLimit, whose default values are 2347, 7 and 4
respectively. If a frame is less than or equal to the dot11RTSThreshold
value in length (too short to be preceded by RTS/CTS control frames),
then the MAC layer will transmit it dot11ShortRetryLimit times before
discarding it. If a frame is long enough to be preceded by RTS/CTS
control frames, then the MAC layer will transmit it
dot11LongRetryLimit times before discarding it. Default values for
short and long try limits are seven and four respectively.¹
Unless the RTS threshold is lower than the default, all frames are
"short." And although called "retry" limits they are really "try" limits.
Data Rate Shifting
The 802.11 standard recognizes that vendors may incorporate an
automatic data rate shifting process that enables the station to adjust the
data rate of the frame transmission to make efficient use of the medium.
If the station senses too many retransmissions or the received signal
strength decreases below a specific threshold, then the station will shift
down to a lower data rate. This enables the station to maintain
1
IEEE 802.11 - 1999 (R2003) – Section 9.2.5.3
