Which acronym refers to a communications system that does not use ARQ?

The acronym that refers to a communications system that does not use Automatic Repeat reQuest (ARQ) is SITOR (NBDP). SITOR stands for Simplex Telex Over Radio, which is utilized for various types of maritime communication, including distress messages.

SITOR communications can operate in a broadcast mode, sending messages without waiting for an acknowledgment from the recipient, thus making it distinct from systems that include ARQ where the sender waits for an acknowledgment of receipt before proceeding.

NBDP, or Narrow Band Direct Printing, is a mode within SITOR that enables the transmission of text-based data without the requirement of confirming message receipt. This characteristic aligns with the definition provided in the question, as SITOR effectively transmits messages even in cases where there may be no successful acknowledgment from the receiving end.

To summarize, SITOR (NBDP) is specifically designed for efficient communication without ARQ, facilitating critical operations in maritime settings without the delays associated with acknowledgment requirements.