The present document of DVB-Simulcrypt addresses the requirements for interoperability between two or more conditional access systems at a head-end. It specifies the system architecture@ timing relationships@ messaging structures@ extended interoperability and control. The components within the system architecture represent functional units. The boundaries between physical units are not required to match the boundaries between functional units. It is possible that the SCS could be in the MUX or the SCS and MUX could be built independently. Neither architecture is mandated.