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RTS/CTS Protocol
A shorter packet is less likely to have a collision than a longer packet.
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When the packet exceeds the threshold, the device sends an RTS message and waits
until the receiving device responds with a CTS message.
The RTS message includes the length of the frame that the device wishes to transmit. The
receiving device includes this information as a radio-silence time indicator in its CTS
response message. The CTS message announces to all the devices in the wireless network
which device is allowed to transmit its message. All other devices defer their transmissions
for the radio-silence time identified in the CTS message.
Access Point - RTS Threshold
The RTS Threshold on a RoamAbout Access Point specifies the packet size of
transmissions, where messages larger than the specified size must use the RTS/CTS
protocol. The default value, 2347, effectively turns off the RTS Threshold.
A lower RTS Threshold is useful when collisions frequently occur at the Access Point. This
can be caused when the Access Point and a client (or Access Point in a LAN-to-LAN
configuration) transmit data to each other simultaneously. A lower RTS Threshold forces
the Access Point to send an RTS to the device before transmitting a packet that exceeds the
threshold. The Access Point waits until the device responds with a CTS message.
Lowering the RTS Threshold imposes additional network overhead that could negatively
affect the throughput performance. You should only lower the RTS Threshold when the
wireless network experiences frame collisions and lost messages.
Wireless Client - Medium Reservation
Use Medium Reservation to resolve a hidden station problem. A wireless device is a hidden
station when its transmissions cannot be sensed by another wireless device in the same
network. Therefore, multiple devices could transmit at the same time. This problem can
occur with clients located at opposite ends of an Access Point coverage area.
Figure 2-2 illustrates a hidden station example. Clients A and B are within range of the
Access Point. However, Client B cannot sense transmissions from Client A, since Client A
is outside of Client B's coverage area (shown in gray). Client B could transmit while Client
A is transmitting. Therefore, messages of both Client A and B collide when arriving
simultaneously at the Access Point. The collision results in a loss of messages for both
clients. Figure 2-2 also illustrates that Client C is not hidden from the other clients.
2-8 Understanding Wireless Network Characteristics
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