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Appendix E Wireless LANs
NWA1000 Series User’s Guide
181
cannot "hear" each other, that is they do not know if the channel is currently being used. Therefore,
they are considered hidden from each other.
Figure 92 RTS/CTS
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.
Note: Enabling the RTS Threshold causes redundant network overhead that could
negatively affect the throughput performance instead of providing a remedy.
Preamble Type
Preamble is used to signal that data is coming to the receiver. Short and long refer to the length of
the synchronization field in a packet.
Short preamble increases performance as less time sending preamble means more time for sending
data. All IEEE 802.11 compliant wireless adapters support long preamble, but not all support short
preamble.
Use long preamble if you are unsure what preamble mode other wireless devices on the network
support, and to provide more reliable communications in busy wireless networks.