The strategic function of the Transport Layer is to identify groups of data as complete programme services and to manage the multiplexing and association of the various audio and data components that constitute the complete broadcast stream. This layer assembles the individual elements defined in the higher layers into a structured multiplex ready for transmission.

6.1 Multiplex Structure

A programme service is defined at this layer as comprising at least one audio service component and, optionally, one or more additional audio and/or data service components. The entire capacity of the multiplex can be devoted to a single service provider or divided among several providers.

The broadcast stream is organized into two primary channels:

• Main Service Channel (MSC): This channel carries the bulk of the service data, including all audio streams and most data services. The MSC is a time-interleaved channel that has a gross capacity of approximately 2.3 Mbit/s within a 1.5 MHz bandwidth. Depending on the level of error protection (code rate) applied, the available net bit rate for services varies from approximately 0.6 to 1.7 Mbit/s.
• Fast Information Channel (FIC): This channel is external to the MSC and is not subject to time-interleaving. This ensures its data can be accessed by the receiver with very low latency. The FIC’s primary role is to carry essential data required for service selection, most notably the Multiplex Configuration Information (MCI).

The Network Layer takes these organized data groups from the Transport Layer and further structures them into logical frames for transmission.