Ssms And Quality Level - Symmetricom TimeProvider 1000 User Manual

Chapter 1 Overview of the TimeProvider
Reference Input Signals
Revertive Switching
Telcordia GR-378 and GR-1244 define two reference selection modes: Revertive
and Non-revertive. In the revertive mode, when an input used as the system
reference is disqualified (for any reason), if that input returns, it reverts to the
system reference when the disqualifying reason is removed. The system reference
switches two times: once when the disqualifying event occurs, and again when the
input is no longer disqualified. In the non-revertive mode, the system reference does
not revert to the initial input when the reason for disqualification is removed. The
system reference switches only once when the disqualifying event occurs.
Quality Level and Priority Level
When the REFMODE parameter is provisioned to AUTO, the system reference
switches when the input signal is disqualified. When the input signal is re-qualified,
the TimeProvider can either keep the current reference or switch back to the
re-qualified signal. When the REFMODE parameter is set to FORCED, then the
reference does not switch and remains locked to that reference and the local
oscillator enters Holdover mode if the input is not re-qualified. You may also
provision the REFMODE parameter to FORCED to select an input as the system
reference regardless of the priority levels of other outputs. If this input is lost or is
disqualified, then the REFMODE automatically changes to AUTO and the next
available reference based on priority and QLEVEL becomes the system reference. If
no other input references are available, the TimeProvider enter the Holdover mode.
The QLEVEL (Setting the Input Quality Level, on page 104) and the PRIORITY
(Setting the Input Priority Level, on page 105) parameters work together with SSMs
to determine the switching strategy for the inputs when the unit is in the SSU mode.
If the active/primary input becomes unavailable, the switching strategy determines
which input to use: the TimeProvider uses the input with the next highest QLEVEL.
If all inputs have the same QLEVEL, then the TimeProvider uses the input with the
highest PRIORITY.
To provision the TimeProvider as non-revertive, you must provision the user-assigned
Priority Level on all inputs to the same value. To provision the TimeProvider as
revertive, you provision any one of the inputs to a different Priority Level.
If the original primary input becomes available again, the REFMODE, QLEVEL and
PRIORITY parameters determine whether the TimeProvider switches back to that
input. If REFMODE is set to AUTO, then the reference switches according to the
QLEVEL and PRIORITY settings. If the QLEVEL and/or PRIORITY parameter of
the original input is higher than the current input, the TimeProvider reverts back to
the original input. If the QLEVEL parameter is the same for all inputs, then the
TimeProvider does not revert back to the original input.

SSMs and Quality Level

Synchronization Status Messages (SSMs) can be included in the bit stream of the
incoming signal to indicate its quality level. You must provision the input to read the
SSM, and you must provision the bit position of the incoming SSM.
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