802.11d (Score 2, Informative)

The country codes are related to the 802.11d addendum. 802.11d is for International Regulatory Domains. It provides a mechanism to allow subscriber stations to automatically detect the country that they are operating in and limit their maximum transmit power accordingly. Each country's regulating body (like the FCC) sets limits on transmit power for different operating bands. If you look around, you'll discover very little consistency from country to country. It's a regulatory nightmare, really. An AP that supports 802.11d will include the country code in its Beacon and Probe Response messages. A subscriber station that has 802.11d enabled shall not transmit at all until it discovers the country code from at least one AP beacon. When it discovers the country code, it looks up in its own regulatory table (eeprom or flash) the maximum transmit power for that country. Once it determines max power, it may begin its normal transmit operations. So, the intention of the feature is to allow subscriber stations to be certified for operation in multiple countries. For those countries, it would know the maximum transmit power in advance and be able to transmit. However, the spec is not all that clear on its operation, and specifies no regulatory requirements itself, so it's really just a feature that enables operation, but isn't necessarily widely used and implemented. Most mobile station solutions support 802.11d by now, but most have it disabled by default. When 802.11d is disabled, it just uses a home country profile that is programmed in eeprom or flash. Also, there is no requirement to not transmit until discovering the country, so the mobile station can transmit probe requests right away.