Change the Fragmentation Threshold

Fireware allows you to set the maximum frame size the Firebox wireless device can send and not fragment the frame. This is called the fragmentation threshold. This setting is rarely changed. The default setting is the maximum frame size of 2346, which means that it will never fragment any frames that it sends to wireless clients. This is best for most environments.

When to Change the Default Fragmentation Threshold

A collision happens when two devices that use the same medium transmit packets at exactly the same time. The two packets can corrupt each other, and the result is a group of unreadable pieces of data. If a packet results in a collision, the packet is discarded and it must be transmitted again. This adds to the overhead on the network and can reduce the throughput or speed of the network.

Larger frames are more likely to collide with each other than smaller frames. To make the wireless packets smaller, you lower the fragmentation threshold on the Firebox wireless device. If you lower the maximum frame size, it can reduce the number of repeat transmissions caused by collisions, and lower the overhead caused by repeat transmissions.

Smaller frames introduce more overhead on the network. This is especially true on a wireless network, because every fragmented frame sent from one wireless device to another requires the receiving device to acknowledge the frame. When packet error rates are high (more than five or ten percent collisions or errors), you can help improve the performance of the wireless network if you lower the fragmentation threshold. The time that is saved when you reduce repeat transmissions can be enough to offset the extra overhead added with smaller packets. This can result in higher throughput.

If the rate of packet error is low and you lower the fragmentation threshold, wireless network performance decreases. This occurs because when you lower the threshold, protocol overhead is added and protocol efficiency is reduced.

If you want to experiment, start with the default maximum 2346, and lower the threshold a small amount at a time. To get the most benefit, you must monitor the network for packet errors at different times of the day. Compare the effect that a lower threshold has on network performance when errors are very high with the effect on performance when errors are moderately high.

In general, we recommend that you leave this setting at its default of 2346.