?– Critical event notification, confirming the occurrence of an unspecified critical
event, affecting the link quality and transmission capabilities of the particular link
station. Such a notification flag is transmitted immediately after the onset of the
event and in a continuous manner (all link peers are notified about this event).
170 Chapter 7
Definitions of the dying gasp and critical events are left open for system vendor-specific
implementation, thereby providing increased system design flexibility. Examples of
the unrecoverable condition for dying include power failure, electrical power instabilities,
and so on, causing the given piece of equipment to restart continuously and thus
providing poor link quality or no service at all.
The link fault event, on the other hand, is applicable only to any particular situation
when the physical sublayer is capable of independent transmission and reception, thus
providing signaling even when the receiver is impaired or damaged. Providing the
receiver fails to detect any data transmission from its peer at the PHY layer, due to, for
example, laser malfunction in the peer station, the local entity can set this flag to let
the peer know that its transmission interface is inoperable.
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