The Session Layer is concerned with the logical processes of selecting and accessing the various broadcast information streams available within the multiplex. This layer acts as the gatekeeper, providing the receiver with the necessary information to find, tune to, and, if required, descramble the user’s chosen service.

5.1 Programme Selection

To enable a receiver to access any of the individual services within the multiplex, essential information is carried in the Multiplex Configuration Information (MCI). This information is non-time-interleaved and transmitted in the Fast Information Channel (FIC) to ensure it is immediately available to the receiver with minimal delay. The MCI contains a detailed map of the multiplex, including information about the services offered, their precise timing, and their location within the overall multiplex frame. This allows the user to select programmes based on textual information, such as the programme service name, programme type, or language.

A key feature managed at this layer is the ability for a receiver to seamlessly switch to an alternative source if the primary digital service reception becomes unacceptable. Using “cross-reference” information, the receiver can identify and switch to an equivalent FM service, for example. The system allows the receiver to switch back to the original digital service as soon as reception quality improves.

5.2 Conditional Access

The system architecture includes provisions for both the synchronization and control of conditional access. This functionality can be applied independently to specific service components carried in either the Main Service Channel (MSC) or the Fast Information Channel (FIC), or it can be applied to the entire multiplex. This enables broadcasters to offer subscription-based or pay-per-view services.

Once a service has been selected and access has been granted, the Transport Layer is responsible for identifying and multiplexing its constituent data elements into a cohesive stream.