Fragmentation Threshold; Figure 20 Rts/Cts - ZyXEL Communications ZyAIR G-3000 User Manual

ZyAIR G-3000 User's Guide

Figure 20 RTS/CTS

When station A sends data to the ZyAIR, 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.

5.2.2 Fragmentation Threshold

A Fragmentation Threshold is the maximum data fragment size (between 256 and 2432
bytes) that can be sent in the wireless network before the ZyAIR will fragment the packet into
smaller data frames.
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Note: Enabling the RTS Threshold causes redundant network
overhead that could negatively affect the throughput
performance instead of providing a remedy.
Chapter 5 Wireless Configuration and Roaming