What best defines the SITOR (NBDP) acronym "ARQ"?

The acronym "ARQ" stands for Automatic Repeat reQuest and is a protocol used in the SITOR (Simplex Telex Over Radio) system, which is a type of Narrow Band Direct Print (NBDP) communication. ARQ is designed to ensure reliable data transmission between two stations engaged in direct TELEX communications.

When a message is sent, the receiving station can request the sender to repeat a certain part of the message if it has not been received correctly. This means that both stations are actively involved in the communication process, continuously checking for errors and confirming the successful receipt of data. Because of the interactive nature of ARQ, it is particularly effective in environments where data integrity is critical and where potential for error exists due to factors like distance or interference.

In contrast, other options that imply one-way communications do not incorporate the same level of interaction or error correction that ARQ is designed for. Therefore, the correct answer reflects the definition and purpose of ARQ within the context of direct communication between two stations in the SITOR protocol.