A cable television system provides conditional access to services. The cable television system includes a headend from which service "instances", or programs, are broadcast and a plurality of set top units for receiving the instances and selectively decrypting the instances for display to system subscribers....http://www.google.com/patents/US6744892?utm_source=gb-gplus-sharePatent US6744892 - Method and apparatus for geographically limiting service in a conditional access system
