2.0 System Architecture: The OSI Layered Model
The strategic decision to model Digital System A on the OSI reference framework provides significant benefits for system interpretation, design, and implementation. By partitioning the complex processes of digital broadcasting into distinct, self-contained layers, the model creates a clear and logical structure. This layered abstraction is crucial, as it allows the complexities of the physical transmission—such as the Orthogonal Frequency Division Multiplex (OFDM) modulation and guard interval design detailed in the Physical Layer—to be developed and optimized independently from the audio source encoding standards handled at the Presentation Layer.
The interpretation of the OSI layered model as it applies to Digital System A is summarized in the table below.
| Layer Name | Description | Features Specific to the System |
| Application layer | Practical use of the system | System facilities<br>Audio quality<br>Transmission modes |
| Presentation layer | Conversion for presentation | Audio encoding and decoding<br>Audio presentation<br>Service information |
| Session layer | Data selection | Programme selection<br>Conditional access |
| Transport layer | Grouping of data | Programme services<br>Main service multiplex<br>Ancillary data<br>Association of data |
| Network layer | Logical channel | ISO audio frames<br>Programme associated data |
| Data link layer | Format of the transmitted signal | Transmission frames<br>Synchronization |
| Physical layer | Physical (radio) transmission | Energy dispersal<br>Convolutional encoding<br>Time interleaving<br>Frequency interleaving<br>Modulation by 4-DPSK OFDM<br>Radio transmission |
The fundamental purpose of the system is to deliver sound programmes and associated data services to the end user. To provide a comprehensive analysis of how this is achieved, the following sections will analyze the system’s architecture by proceeding downwards through these layers. The examination will begin with the application layer—the broadcast information as experienced by the user—and conclude with the physical layer, which governs the means of radio transmission.
This detailed examination begins with the highest layer in the model: the Application Layer.