What Frequencies Do Wi-Fi Networks Transmit?

802.11b uses the popular 2.4 GHz frequency band. Frequencies within this band (with minor variations in certain countries) are available for wireless networking worldwide. One drawback, however, is that other devices such as cordless phones and microwaves can cause interference and congestion in this band.

802.11g is rapidly growing in popularity. It is backward-compatible with 802.11b, uses the same 2.4 GHz range, but provides for transmission rates up to 54 Mbps.

Because 802.11g networks operate in the popular 2.4 GHz frequency band, they are  subject to interference by devices such as microwave ovens, baby monitors, and cordless phones. However, such interference generally only causes a decreased data rate, not a loss of connection or loss of data. That disadvantage is offset in normal use by the more robust signals associated with the 2.4 GHz band, which readily pass through solid objects such as interior walls of homes. Since AirPort Extreme operates in the 2.4 GHz frequency band, it is approved for use in the same countries as 802.11b

More info from Apple.com …
AirPort Developer Note
Apple – 802.11