Wireless speed management

• Difficulty: Intermediate
• Application: iwconfig

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

Using the auto directive this way tells the driver to allow speeds up to 5.5Mbps, and to run slower if necessary, but will never try to sync at anything faster. To restore the card to full auto scaling, just specify auto by itself:

iwconfig eth0 rate auto

Cards can generally reach much further at 1Mbps than they can at 11Mbps. There is a difference of 12dB between the 1Mbps and 11Mbps ratings of the Orinoco card - that's four times the potential distance just by dropping the data rate!