ZyXEL Communications NBG6503 User Manual page 216

Appendix D Wireless LANs
RTS/CTS
Figure 138
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, 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 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.
If the Fragmentation Threshold value is smaller than the RTS/CTS value (see previously) you
set 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.
NBG6503 User's Guide
216