ZyXEL Communications ZYAIR B-4000, User Manual

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ZyAIR B-4000 Spot Gateway
10-4 Wireless LAN
Figure 10-4 RTS/CTS
When station A sends data to the ZyAIR, it might not know that 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.