Asynchronous Event Notification (AEN)
Asynchronous Event Notification (AEN) - Note: this is now referred to as Asynchronous Event Reporting (AER).
Asynchronous Event Notification (AEN) is a process or procedure that may be used by SCSI targets to notify a SCSI initiator of “events” that occur in the target, when the target is NOT executing an Active Process (a command). An example of an “event” could be a Unit Attention caused by removing a CD or DVD from a player or recorder (medium is removed). These events normally result in a Contingent Allegiance Condition (CAC). Without AEN, the initiator will not know of any events until it tries to send another command, or another command is processed from the queue.
The hard part of AEN involves the target becoming a “temporary initiator” and being able to use the SEND command to tell an initiator about the event. This initiator would also have to become a “temporary target” to be able to receive this report. Since devices rarely change rolls, especially initiators, this is a seldom used feature of the SCSI world.
But the good news is, with the advent of Serial Attached SCSI (SAS), parts of the infrastructure, called SAS Expanders, have the capability to send and receive “broadcasts” – which can be used to notify “management processes” about different types of events. This can be loss of synchronization on a link, a link that comes up or goes down, and the ability to send SCSI event notifications. Expanders usually also contain another function called a SCSI Enclosure Services (SES) device. This can also enhance the system by monitoring things like air flow, temperature, and backplane voltages.
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