This part of IEC 60839 specifies the data link layer message structure, formats and transmission procedures which should be used at standard serial data interfaces in alarm transmission systems where the transmission network employed does not offer a standard protocol. This is necessary in order to ensure compatibility of 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. The protocol is based on a simple poll response algorithm, with a single MASTER and one or more SLAVEs. The protocol defined allows for point-point and point-multipoint operation. Multipoint to multipoint operation is not supported, however the structure does support the transmission of messages from one SLAVE to another, using the MASTER as a simple router. The structure follows the OSI recommendations for a layered protocol to allow flexibility in the use of the physical layer.