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3.10
What
is
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.
3.11 What is 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 be fewer collisions, and performance should improve.
