Appendix A Wireless LANs
WAP6906 User’s Guide
73
Figure 44
RTS/CTS
Note: Stations cannot hear each other. They can hear the AP.
When station
A
sends data to the AP, it might not know that the station
B
is already using the channel. If
these two stations send data at the same time, collisions may occur when both sets of data arrive at the
AP at the same time, resulting in a loss of messages for both stations.
RTS/CTS
is designed to prevent collisions due to hidden nodes. An
RTS/CTS
defines the biggest size data
frame you can send before an RTS (Request To Send)/CTS (Clear to Send) handshake is invoked.
When a data frame exceeds the
RTS/CTS
value you set (between 0 to 2432 bytes), the station that wants
to transmit this frame must first send an RTS (Request To Send) message to the AP for permission to send
it. The AP then responds with a CTS (Clear to Send) message to all other stations within its range to notify
them to defer their transmission. It also reserves and confirms with the requesting station the time frame
for the requested transmission.
Stations can send frames smaller than the specified
RTS/CTS
directly to the AP without the RTS (Request
To Send)/CTS (Clear to Send) handshake.
You should only configure
RTS/CTS
if the possibility of hidden nodes exists on your network and the "cost"
of resending large frames is more than the extra network overhead involved in the RTS (Request To
Send)/CTS (Clear to Send) handshake.
If the
RTS/CTS
value is greater than the
Fragmentation Threshold
value (see next), then the RTS (Request
To Send)/CTS (Clear to Send) handshake will never occur as data frames will be fragmented before
they reach
RTS/CTS
size.
Note: Enabling the RTS Threshold causes redundant network overhead that could negatively
affect the throughput performance instead of providing a remedy.
Fragmentation Threshold
A
Fragmentation Threshold
is the maximum data fragment size (between 256 and 2432 bytes) that can
be sent in the wireless network before the AP will fragment the packet into smaller data frames.
A large
Fragmentation Threshold
is recommended for networks not prone to interference while you
should set a smaller threshold for busy networks or networks that are prone to interference.
