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LM821 User Manual
Wireless LAN (WLAN)
A wireless LAN does not use cable to transmit signals, but rather uses radio
or infrared to transmit packets through the air. Radio Frequency
(RF) and infrared are the commonly used types of wireless transmission. Most
wireless LANs use spread spectrum technology. It offers limited bandwidth,
usually under 11Mbps, and users share the bandwidth with other devices in the
spectrum; however, users can operate a spread spectrum device without
licensing from the Federal Communications Commission (FCC).
Fragment Threshold
The proposed protocol uses the frame fragmentation mechanism defined in IEEE
802.11 to achieve parallel transmissions. A large data frame is fragmented into
several fragments each of size equal to fragment threshold.
By tuning the fragment threshold value, we can get varying fragment sizes.
The determination of an efficient fragment threshold is an important issue in
this scheme. If the fragment threshold is small, the overlap part of the master
and parallel transmissions is large. This means the spatial reuse ratio of parallel
transmissions is high. In contrast, with a large fragment threshold, the overlap is small
and the spatial reuse ratio is low. However high fragment threshold leads to low
fragment overhead. Hence there is a trade-off between spatial re-use and fragment
overhead.
Fragment threshold is the maximum packet size used for fragmentation.
Packets larger than the size programmed in this field will be fragmented If you find that
your corrupted packets or asymmetric packet reception (all send packets, for
example). You may want to try lowering your fragmentation threshold. This will cause
packets to be broken into smaller fragments. These small fragments, if corrupted, can
be resent faster than a larger fragment. Fragmentation increases overhead, so you'll
want to keep this value as close
to the maximum value as possible.
RTS (Request To Send) Threshold
The RTS threshold is the packet size at which packet transmission is governed
by the RTS/CTS transaction. The IEEE 802.11-1997 standard allows for short packets to
be transmitted without RTS/CTS transactions. Each station can have a different RTS
threshold. RTS/CTS is used when the data packet size exceeds the defined RTS
threshold. With the CSMA/CA transmission mechanism, the transmitting station sends
out an RTS packet to the receiving station, and waits for the receiving station to send
back a CTS (Clear to Send) packet before sending the actual packet data. This setting
is useful for networks with many clients. With many clients, and a high network load,
there will be many more collisions. By lowering the RTS threshold, there may fewer
collisions, and performance should improve. Basically, with a faster RTS threshold, the
