🔍 Key Concepts

• SITOR (NBDP) uses two main operating modes: ARQ (Automatic Repeat reQuest) and FEC (Forward Error Correction) • In ARQ mode, messages are sent in short blocks and the receiving station must acknowledge (accept or reject) each block • The structure of a SITOR data block (information characters plus check/accept characters) is standardized, not arbitrary

💭 Think About

• Which choice describes something that is specific to ARQ mode, not just radio communication in general? • How does ARQ handle errors compared with FEC? Does ARQ rely on repeating blocks or on adding enough redundancy to correct errors without repeats? • In ARQ, are characters sent as one long uninterrupted stream, or in discrete blocks that can be accepted or rejected?

✅ Before You Answer

• Verify which option refers to error handling by repeating data blocks on request, which is characteristic of ARQ. • Check which options mention continuous transmission versus block-based transmission—ARQ depends on blocks that can be acknowledged. • Check which options mention a fixed number of characters in each ARQ block or acceptance code and whether that matches what you’ve studied for SITOR/NBDP ARQ.