802.11g is backward compatible with 802.11b. On the 1200 Network interfaces configuration page, select 802.11 radio interface -> Settings page, just make sure 11, 5.5, 2 and 1 are set to "require". If these are set to disable, b clients will be denied association.
Sankar Nair
UC Solutions Architect
Pacific Northwest | CDW
CCIE Collaboration #17135 Emeritus