The Network Layer in Digital System A has a limited but essential role: it is concerned with the identification of groups of data as programmes through the specific use of ISO audio frames. This layer imposes a logical, time-based structure on the audio bit stream generated by the Presentation Layer.

The processes in the audio source encoder are carried out during discrete ISO audio frames, each with a duration of 24 ms. The resulting bit stream from the quantizer and coding unit is assembled into a complete frame containing several distinct parts: header information, Cyclic Redundancy Check (CRC) words for error detection, the audio data itself, and intimately linked Programme Associated Data (PAD). PAD is information such as dynamic label text or music/speech indicators that must travel with the audio to avoid timing discrepancies. Each audio channel contains a PAD channel with a variable capacity, generally at least 2 kbit/s.

This complete structure forms an ISO frame that is fully compliant with the ISO 11172-3 Layer II format, allowing a standard ISO/MPEG-Audio Layer II decoder in the receiver to correctly interpret the bit stream.

These logical frames are next passed to the Data Link Layer, where they are formatted into a transmission frame for the physical signal.