Appendix D Wireless LANs

NWA1300-NJ User’s Guide

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wireless gateway, but out-of-range of each other, so they 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 51   

 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. 

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.