8.0 Data Link Layer: Transmission Frame Synchronization
The Data Link Layer performs the critical function of providing the means for receiver synchronization. It achieves this by formatting the transmitted signal into a precisely defined transmission frame. This regular, repeating structure allows the receiver to lock onto the signal, identify its constituent parts, and correctly decode the broadcast information.
The transmission frame is composed of a fixed sequence of symbols. It begins with a null symbol, which provides a period with no RF signal transmission, allowing the receiver to perform a coarse synchronization. This is followed by a fixed reference symbol, which is used for fine synchronization, automatic gain control (AGC), automatic frequency control (AFC), and phase reference functions. The remainder of the frame is divided into two distinct parts: the Fast Information Channel (FIC) and the Main Service Channel (MSC).
The key timing and carrier parameters of the transmission frame vary depending on the transmission mode being used, as detailed in the table below.
| Mode I | Mode II | Mode III | |
| Total frame duration, TF | 96 ms | 24 ms | 24 ms |
| Null symbol duration, TNULL | 1.297 ms | 324 µs | 168 µs |
| Overall symbol duration, TS | 1.246 ms | 312 µs | 156 µs |
| Useful symbol duration, tu | 1 ms | 250 µs | 125 µs |
| Guard interval duration, Δ | 246 µs | 62 µs | 31 µs |
| Number of radiated carriers, N | 1536 | 384 | 192 |
The final stage of signal preparation and transmission, including modulation and error correction, is handled by the Physical Layer.