Anti-lock Brakes and Drum brakes?

I have always wondered about how the abs system works on cars with a front disc/rear drum layout. Does abs only work on wheels that are disc or will it work on wheels that are drum as well? So on a car like this would only the front (disc brakes) have the abs feature? Thanks|||There is a gear-like disc attached to the inside of the wheel, %26amp; a sensor that almost touches it. When the wheel rotates, the sensor counts the number of gear teeth that pass by it, %26amp; thru that the computer can tell exactly how fast that wheel is turning. This gear disc is separate from the brakes, so it can be used with disc or drum brakes.|||The gear is in the rear end around the ring and it picks up its signal using a magnet that mounts on the housing...