🔍 Key Concepts

• SITOR (NBDP) modes of operation: ARQ vs FEC • Use of paired frequencies in maritime HF/MF data services • Difference between SITOR/NBDP and DSC functions and frequencies

💭 Think About

• Ask yourself: Why would a system need paired frequencies instead of a single working frequency? What does that imply about how it sends and receives data? • Think about ARQ (Automatic Repeat reQuest) versus FEC (Forward Error Correction): which one requires a return path from the receiving station? • Consider what type of digital service normally uses dedicated DSC frequencies—are these the same as SITOR/NBDP paired working channels?

✅ Before You Answer

• Verify which SITOR mode (ARQ or FEC) needs a two‑way exchange during the message transmission itself. • Confirm whether Distress on HF/MF normally uses special DSC channels or regular SITOR paired working frequencies. • Make sure you distinguish DSC calling frequencies from NBDP working channels in the GMDSS HF/MF band.