"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. Common scrambling algorithm The DVB-Simulcrypt group has looked at issues relating to the concepts of the common scrambling algorithm@ within the DVB-Simulcrypt environment. The DVB-Simulcrypt system is based on the concept of a shared scrambling and descrambling method. The group has looked at the possible constraints@ which the DVB-Simulcrypt architecture might impose on the use of such a shared scrambling and descrambling method. No problems were noted. Language The word ""shall"" is used in a normative statement that can be verified and is mandatory. The word ""should"" is used in the context of a recommendation or a statement that cannot be verified or is not mandatory (it may be optional)."