Rts/Cts; Figure 32 Rts Threshold - ZyXEL Communications ZyWall 35 User Manual

ZyWALL 35 User's Guide

6.2.3 RTS/CTS

A hidden node occurs when two stations are within range of the same access point, but are not
within range of each other. The following figure illustrates a hidden node. Both stations (STA)
are within range of the access point (AP) or 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 32 RTS Threshold

When station A sends data to the ZyWALL, 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.
Enabling the RTS Threshold causes redundant network overhead that could negatively affect
the throughput performance instead of providing a remedy.
114 Chapter 6 Wireless LAN and Authentication Server