The Common Interface (CI) is the interface for set-top boxes and satellite receivers. This interface was specified by Digital Video Broadcasting (DVB) and is used to connect an access module.
The CI module is inserted into a slot in the set-top box or DVB receiver. It is similar to a PCMCIA card and contains all functions for descrambling, processing of Entitlement Management Messages (EMM), Entitlement Control Messages (ECM) and Conditional Access (CA).
A smart card is inserted into the CI module, on which the access authorisations for the various TV programmes are stored.CAM module for decodingpay TV can also be plugged into the Common Interface, as can modems, memory expansions and game consoles.
Since cable network operators and programme providers offer proprietary CI interfaces, there is an initiative by the Consumer Electronics Association in the ZVEI, the Gesellschaft für Unterhaltungs- und Kommunikationselektronik (gfu) and Kabel Deutschland GmbH to standardise the CI interface under the name CI-Plus (CI+). The CI-Plus standard supports copy protection and offers network operators the possibility to use their individual encryption techniques with their own decryption modules.