ZyXEL Communications USG-300 - V2.20 ED 2, Manual

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Appendix E Wireless LANs
ZyWALL USG 300 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 670 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.