When the cable company installs cable TV, how do they limit channels to only what you ordered?

I ordered "basic digital cable" from my local cable company (Comcast). I was just wondering when they come out to install it, how will they limit my channels to just the ones in the basic package? Do they handle this at the head-end, or is it done at the junction box outside my apartment? Do they install some kind of filter on the line? And... if so... do you suppose the installer comes out... with... other... filters in his truck? Just curious, of course....