ZyXEL Communications P-662HW-63, User Manual

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Prestige 662HW Series User’s Guide
91 Chapter 8 Wireless LAN Setup
8.1.3 ESS ID
An Extended Service Set (ESS) is a group of access points or wireless gateways connected to
a wired LAN on the same subnet. An ESS ID uniquely identifies each set. All access points or
wireless gateways and their associated wireless stations in the same set must have the same
ESSID.
8.1.4 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 24 RTS/CTS
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.