This part of IEC 60839 specifies the requirements for standard interfaces employing ITU-T V.24/V.28 signalling for communications between the control and indicating equipment of an alarm system and transmission equipment used for remote communications, where the transmission equipment is general purpose equipment, not exclusively designed for the alarms industry (and not therefore complying with the requirements of other parts of this standard). It is intended to be used as interfacing to standard modems, packet switching networks, X.25 PADs, etc. 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