Reliable transmission of alarm, supervisory, and trouble signals is critical in a security system to prevent degradation of the signal in transit, which in turn would result in failure of the signal to be displayed and recorded at the monitoring station or an incorrect corrupted signal displayed and recorded at the monitoring station.

Below is a sample of NFPA 731. For the complete section, see the actual NFPA 731 text at NFPA.ORG. Once there, click on the free access link to the latest edition of NFPA 731.

9.7.11 Signal Error Detection and Correction.

9.7.11.1 Transmission of alarm, supervisory, and trouble signals shall be in a highly reliable manner to prevent degradation of the signal in transit, which in turn would result in either of the following:

(1) Failure of the signal to be displayed and recorded at the monitoring station

(2) An incorrect corrupted signal displayed and recorded at the monitoring station

9.7.11.2 Reliability of the signal shall be achieved by any of the following:

(1) Signal repetition — multiple transmissions repeating the same signal

(2) Parity check — a mathematical check sum algorithm of a digital message that verifies correlation between transmitted and received messages

(3) An equivalent means to 9.7.11.2(1) or 9.7.11.2(2) that provides a certainty of 99.99 percent that the received message is identical to the transmitted message