Alarms are messages send by a managed element to the network manager to indicate an abnormal condition such as a fault or an exception. Any other autonomous messages reported by managed element, for instance threshold alert or change in an attribute value are known as Events (events can also be reported by the network manger itself for instance an event about excessive cpu usage).
When the underlying management protocol is SNMP (Simple Network Management Protocol), Trap is used to refer to an alarm or an event. In other words, Trap is just a terminology used for alarms/events reported by SNMP devices to the network manager.
Similarly in TL1, alarms and events are denoted by Autonomous Messages. Thus, there is no significant difference between an alarm and a trap or an autonomous message, it is only about the management protocol being used.
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