This part of IEC 60839 specifies a common application layer protocol (message structure, formats and transmission procedures) to be used at standard interfaces in alarm transmission systems. This should be used at all such interfaces where equipment from one supplier is intended to inter-work with equipment from other suppliers, where the underlying system architecture does not impose its own application layer on the interface (for example as in some bus systems). The structure follows the OSI recommendations for a layered protocol to allow flexibility in the choice and use of lower level transmission media and protocols. The common application layer protocol defined has a minimum subset that should be provided by all equipment which supports this standard, and has defined extensions which may be offered. The protocol is also designed such that it can be extended beyond the messages defined herein in order to provide further facilities and manufacturer specific extensions. This standard applies equally to the transmission of alarms and other messages to/from intrusion, fire and social alarm systems, and to the transmission of information to/from other similar systems