5.10 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.
5.11 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. 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.
5.12 What is 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).
5.13 What is Preamble Type?
There are two preamble types defined in IEEE 802.11 specification. A long preamble basically gives
the decoder more time to process the preamble. All 802.11 devices support a long preamble. The short
preamble is designed to improve efficiency (for example, for VoIP systems). The difference between
the two is in the Synchronization field. The long preamble is 128 bits, and the short is 56 bits.
5.14 What is SSID Broadcast?
Broadcast of SSID is done in access points by the beacon. This announces your access point (including
various bits of information about it) to the wireless world around it. By disabling that feature, the SSID
configured in the client must match the SSID of the access point.
Some wireless devices don't work properly if SSID isn't broadcast (for example the D-link DWL-120
USB 802.11b adapter). Generally if your client hardware supports operation with SSID disabled, it's
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