Common Interface - (CI) - Describes hardware and software architecture for Conditional Access Systems where the generic "host" set-top box is not dedicated to one particular encryption system. Consumers use detachable hardware which allows them to receive encrypted services they pay subscription fee(s) for. The interface between the host set-top box (or equivalent) and the CI hardware is standardized (PCMCIA-Interface) so that the same set-top box (or equivalent) can be used to handle a variety of encryption systems. Also see
http://en.wikipedia.org/wiki/Common_Interface.