AirLive WN-370USB - Manual (Page 39)

5. Appendix
AirLive WN-370USB User’s Manua 35
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 IEEE802.11-1997 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. Basically, with a faster RTS threshold, the system can recover from problems
faster. RTS packets consume valuable bandwidth, however, so setting this value too low
will limit performance.
Beacon Interval
In addition to data frames that carry information from higher layers, 802.11 include
management and control frames that support data transfer. The beacon frame, which is a
type of management frame, provides the "heartbeat" of a wireless LAN, enabling stations to
establish and maintain communications in an orderly fashion. Beacon Interval represents
the amount of time between beacon transmissions. Before a station enters power save
mode, the station needs the beacon interval to know when to wake up to receive the
beacon (and learn whether there are buffered frames at the access point).