🔍 Key Concepts

• SITOR (Simplex Teletype Over Radio) uses two main modes: ARQ (Automatic Repeat reQuest) and FEC (Forward Error Correction) • In ARQ mode, there is a two‑way exchange: one station sends, the other checks and then sends a short control signal back. • In FEC mode, there is one‑way broadcasting with no return signal, and characters are usually sent twice for error correction.

💭 Think About

• Ask yourself: In a true ARQ exchange, what does the receiving station send back after it checks the block for errors? A repeat of the message, or a short control signal? • Which mode (ARQ or FEC) is associated with each character being sent twice and not needing a reply from the receiving station? • Which mode is appropriate for broadcasts like Maritime Safety Information, where the sender usually doesn’t know who is listening or get acknowledgments back?

✅ Before You Answer

• Verify which mode (ARQ vs FEC) sends each character twice for error correction. • Verify in ARQ whether the reply from the receiving station is a short control character (ACK/NAK) after checking parity of a block. • Check which mode is used for one‑way broadcasts (e.g., MSI, traffic lists) where there is no interactive exchange between two identified stations.