The speed at which a piece of radio transmission/receiver equipment can communicate with another depends on how much signal is available. In order to maintain communications as the available signal fades, the radios need to transmit data at a slower rate. Normally, the radios attempt to work out the available signal on their own and automatically select the fastest possible speed.
In fringe areas with a barely adequate signal, packets may be needlessly lost while the radios continually renegotiate the link speed. If you can't add more antenna gain, or reposition your equipment to achieve a better enough signal, consider forcing your card to sync at a lower rate. This will mean fewer retries, and can be substantially faster than using a continually flip-flopping link. Each driver has its own method for setting the link speed. In Linux, set the link speed with iwconfig:
iwconfig eth0 rate 2M
This forces the radio to always sync at 2Mbps, even if other speeds are
available. You can also set a particular speed as a ceiling, and allow
the card to automatically scale to any slower speed, but go no faster.
For example, you might use this on the example link above:
iwconfig eth0 rate 5.5M auto
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