Cambium Networks PTP 820S, Техническое описание, Страница: 131 / 208

Chapter 4: Feature Description Synchronization
SSM Support and Loop Prevention
In order to provide topological resiliency for synchronization transfer, PTP 820S implements the
passing of SSM messages over the radio interfaces. SSM timing in PTP 820S complies with ITU-T
G.781.
Note
SSM support is planned for future release.
In addition, the SSM mechanism provides reference source resiliency, since a network may have
more than one source clock.
The following are the principles of operation:
•
At all times, each source interface has a “quality status” which is determined as follows:
o
If quality is configured as fixed, then the quality status becomes “failure” upon interface
failure (such as LOS, LOC, LOF, etc.).
o
If quality is automatic, then the quality is determined by the received SSMs or becomes
“failure” upon interface failure (such as LOS, LOC, LOF, etc.).
• Each unit holds a parameter which indicates the quality of its reference clock. This is the
quality of the current synchronization source interface.
•
The reference source quality is transmitted through SSM messages to all relevant radio
interfaces.
• Each unit determines the current active clock reference source interface:
o
The interface with the highest available quality is selected.
o
From among interfaces with identical quality, the interface with the highest priority is
selected.
•
In order to prevent loops, an SSM with quality “Do Not Use” is sent towards the active source
interface
At any given moment, the system enables users to display:
•
The current source interface quality.
• The current received SSM status for every source interface.
•
The current node reference source quality.
As a reference, the following are the possible quality values (from highest to lowest):
•
AUTOMATIC (available only in interfaces for which SSM support is implemented)
•
G.811
•
SSU-A
• SSU-B
•
G.813/8262 - default
• DO NOT USE
•
Failure (cannot be configured by user)
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