ZyXEL Communications Prestige 653HWI series, User Manual

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Prestige 653HWI Series User’s Guide
6-2 Wireless LAN Setup
6.1.4 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 6-1 RTS Threshold
When station A sends data to the Prestige, 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.
A and B do not hear
each other. They
can hear the AP.