User manual

MWGAR/MWGARB User Manual

Version: 2.0

4.10 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.

4.11 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