Control Frames

Control Frames
After establishing association and authentication between stations and access points, control
frames provide functionality to assist in the deliver of data frames. A common flow of control
frames is shown in Figure 4.11.
The following defines the structure of each control frame subtype:
• Request to Send (RTS) A station sends an RTS frame to a particular receiving station
to negotiate the sending of a data frame. Through the aRTSThreshold attribute stored in
the MIB, you can configure a station to initiate an RTS frame sequence always, never, or
only on frames longer than a specified length.
Figure 4.12 illustrates the format of an RTS frame. The value of the Duration field, in
microseconds, is the amount of time the sending station needs to transmit the frame, plus
one CTS frame, plus one ACK frame, plus three short interframe space (SIFS) intervals.  • Clear to Send (CTS) After receiving an RTS, the station sends a CTS frame to
acknowledge the right for the sending station to send data frames. Stations will always
pay attention to the duration information and respond to an RTS frame, even if the station
was not set up to initiate RTS frame sequences.
Figure 4.13 illustrates the format of a CTS frame. The value of the Duration field, in
microseconds, is the amount of time from the Duration field of the previous RTS frame,
minus the time required to transmit the CTS frame and its SIFS interval.
• Acknowledgement (ACK) A station receiving an error-free frame must send an ACK
frame to the sending station to acknowledge the successful reception of the frame. Figure
4.13 illustrates the format of an ACK frame.
The value of the Duration field, in microseconds, is equal to 0 if the More Fragment bit
in the Frame Control field of the previous data or management frame is set to 0. If the
More Fragment bit of the previous data or management frame is set to 1, then the
Duration field is the amount of time from the Duration field of the previous data or management
frame minus the time required to transmit the ACK frame and its SIFS interval.
• Power-Save Poll (PS Poll) If a station receives a PS Poll frame, the station updates its
network allocation vector (NAV), which is an indication of time periods during which a
station will not initiate a transmission. The NAV contains a prediction of future traffic on
the medium. Figure 4.14 illustrates the format of a PS Poll frame.
• Contention–Free End (CF End) CF End designates the end of a contention period
that is part of the point coordination function. Figure 4.15 illustrates the format of a CF
End frame. In these frames, the Duration field is always set to 0, and the receiver address
(RA) contains the broadcast group address.
• CF End + CF Ack This frame acknowledges the contention-free end announcement of
a CF End frame. Figure 4.15 illustrates the format of a CF End + CF Ack frame. In these
frames, the Duration field is always set to 0, and the receiver address (RA) contains the
broadcast group address. In the network depicted in Figure 4.16, station A and station B can both communicate directly
with the access point; however, the barrier that represents lack of connectivity prevents stations
A and B from communicating directly with each other. The problem is that a collision will
occur when station A attempts to access the medium, because it will not be able to detect that
station B is already transmitting To guard against collisions based on hidden nodes and high utilization, the transmitting station
B should send an RTS frame to the access point, requesting service for a certain amount of
time. If the access point approves, it will broadcast a CTS frame announcing this time to all
stations that hear the frame transmission. As a result, all stations, including station A, will not
attempt to access the medium for the specified amount of time. The RTS/CTS exchange also performs both a type of fast collision inference and a transmission
path check. If the return CTS is not detected by the station originating the RTS, the originating
station may repeat the process (after observing the other medium-use rules) more
quickly than if the long data frame had been transmitted and a return ACK frame had not been
detected. The RTS/CTS mechanism need not be used for every data frame transmission.
Because the additional RTS and CTS frames add overhead inefficiency, the mechanism is not
always justified, especially for short data frames