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244 Chapter 9 –Troubleshooting Wireless LAN Installations
users connected to the wireless LAN detecting an unusual sluggishness of the network.
Throughput may be decreased by up to 40% because of a hidden node problem. Since
wireless LANs use the CSMA/CA protocol, they already have an approximate overhead
of 50%, but, during a hidden node problem, it is possible to lose almost half of the
remaining throughput on the system.
Because the nature of a wireless LAN increases mobility, you may encounter a hidden
node at any time, despite a flawless design of your wireless LAN. If a user moves his
computer to a conference room, another office, or into a data room, the new location of
that node can potentially be hidden from the rest of the nodes connected to your wireless
LAN.
To proactively troubleshoot a hidden node, you must test for degraded throughput and
also find as many potential locations for a hidden node as possible during the initial and
any subsequent site surveys.
Solutions for Hidden Node
Once you have done the troubleshooting and discovered that there is a hidden node
problem, the problem node(s) must be located. Finding the node(s) will include a manual
search for nodes that might be out of reach of the main cluster of nodes. This process is
usually trial and error at best. Once these nodes are located, there are several remedies
and workarounds for the problem.
Use RTS/CTS
Increase power to the nodes
Remove obstacles
Move the node
Use RTS/CTS
The RTS/CTS protocol is not necessarily a solution to the hidden node problem. Instead,
it is a method of reducing the negative impact that hidden nodes have on the network.
Hidden nodes cause excessive collisions, which have a severely detrimental impact on
network throughput. The RTS/CTS (request-to-send/clear-to-send) protocol involves
sending a small packet (RTS) to the intended recipient to prompt it to send back a packet
(CTS) clearing the medium for data transmission before sending the data payload. This
process informs any nearby stations that data is about to be sent, having them delay
transmissions (and thereby avoiding collisions). Both the RTS and the CTS contain the
length of the impending data transmission so that stations overhearing either the RTS or
CTS frames know how long the transmission will take and when they can start to
transmit again.
There are three settings for RTS/CTS on most access points and clients: On, Off, and On
with Threshold. The network administrator must manually configure RTS/CTS settings.
The Off setting is the default in order to reduce unnecessary network overhead caused by
the RTS/CTS protocol. The threshold refers directly to the packet size that will trigger