Starting in the early 1900s, "Q" codes were developed, a collection of standard questions routinely used in maritime and aeronautical situations. They were given three letter codes, all beginning with the letter Q. You can find information on Q codes in this Wikipedia article.

Two of the Q codes deal with the quality of the communications link:

QRK: What is the intelligibility of the signal?

QSA: What is the strength of the signal?

Each has five possible responses:

5-1

5-2

5-3

5-4

5-5