WESM zl Management and Configuration Guide WT.01.28 and greater

3-21
Radio Port Configuration
Configuring Radio Settings
There are two types of preambles: a long preamble and a short preamble. It takes a
maximum of 192 ms to process the long preamble and 96 ms to process the short
preamble. Because the short preamble yields about 50 percent savings in frame
overhead, it can improve the throughput of a network, particularly one transmitting
traffic such as VoWLAN and streaming video frames.
However, 802.11b devices do not support the short preamble, and by default, RP
802.11bg radios allow stations to use either the short or the long preamble. You can
configure these radios to require the short preamble—raising overall throughput in
wireless cells. But you should never require the short preamble in an environment
with any 802.11b stations; this configuration prevents your wireless devices from
avoiding interference with those stations.
The short preamble is part of the 802.11a standard. So you cannot configure this
option for radios of that type: the short preamble is always required.
To configure 802.11bg radios to support a short preamble, complete these steps:
1. Select Network Setup > Radio Adoption Defaults and click the
Configuration tab.
2. Select the radio type and click the Edit button.
3. Check the Short Preambles only box.
4. Click the OK button.
Setting the RTS Threshold. Your wireless network may have “hidden” stations:
two stations that can each hear RP beacons but cannot hear each other’s transmissions
because of a wall or other barrier between them. Because these stations cannot detect
contention from each other, their frames may collide; valuable transmission time is
spent in resending data.
Stations can avoid transmitting at the same time by exchanging RTS and Clear to
Send (CTS) packets with the RP. A wireless station sends an RTS packet to notify
the radio that it would like to transmit. If the channel is clear, the radio sends a CTS
packet to the requesting station. This procedure clears the air for a specific transmis-
sion when many stations may be contending for transmission time.
Employing RTS/CTS exchanges can result in fewer data collisions and better
communication with hidden or obscured nodes. However, the RTS/CTS exchange
itself consumes bandwidth and can increase latency and reduce data-frame through-
put.