This part of IEC 60839 specifies the requirements for standard interfaces between the Public Switched Telephone Network (PSTN) and alarm system transceivers. It specifies a 1 200 Bd interface using ITU-T Recommendation V.23. This standard applies equally to the interface between an alarm system and the PSTN where the alarm system transceiver functions are integrated into the control and indicating equipment (CIE). It also applies to the interface to the PSTN from the terminal receiver at the alarm receiving centre. This is required in order to ensure compatibility between equipment from different suppliers. The standard applies equally to the transmission of alarms and other messages to/from intrusion, fire, access control and social alarm systems, and to the transmission of information to/from other similar systems