Symmetricom SSU-2000e User Manual page 26

Chapter 1 Product Overview
System Architecture
If the incoming signal does not include SSMs, or if you disable them, then you can
provision the Quality Level to an appropriate value. The PQL value is used in the
same manner as the incoming SSM to determine which input is used when the
active input is disqualified. SSMs are described in more detail in
Status Messages
Setting the Reference Switching Mode
There are two reference selection modes available: Revertive and Non-revertive. In
the Revertive mode, if an input used as the system reference is disqualified (for any
reason), the reference switches to the next-highest input and reverts to the initial
input when that input is re-qualified. In this mode, the reference switches two times:
once when the disqualifying event occurs, and again when the initial input is
re-qualified. In the Non-revertive mode, the system reference switches to the
next-highest input but does not revert to the initial input when the initial input is
re-qualified. The system reference switches only once when the disqualifying event
occurs.
The ICS command REF and the TL1 command SET-INPUT-REF allows you to set
the reference switching mode to AutoReturn, AutoSwitch, or OFF. The AutoReturn
mode is Revertive, and enables automatic switching with automatic return to the
input with the highest Priority or PQL level. The AutoSwitch mode is Non-revertive;
the reference automatically switches to the next-highest input on failure of the
selected input. When set to OFF, only manual switching is allowed. If the selected
reference input is disqualified for any reason, the clock enters the Holdover mode
and you must correct the fault and re-provision the reference input.
Setting the Input Selection Mode
The ICS command REF and the TL1 command SET-INPUT-REF determine what
type of reference input selection and reference switching is used. You can set the
input selection to either Priority or PQL. The reference switching mode is either
Autoreturn, Autoswitch, or No switching.
After you select the reference switching mode, the Input module reads and
processes the Priority level and PQL value to determine the traceability of inputs.
This traceability information is then used by the Clock modules in selecting a
reference signal and for embedding the SSM into the outputs.
If you select Priority mode, the SSU-2000e selects the input reference with the
highest user-defined Priority setting; if all inputs have the same Priority level, then
the input with the highest PQL (quality level, SSM) is used.
If you select PQL mode, the SSU-2000e selects the input reference with the highest
assigned PQL level or associated SSM; if all inputs have the same PQL/SSM value,
then the input with the highest Priority levels is used. In either case, error conditions
such as Loss of Signal (LOS), Out of Frame (OOF), MTIE, TDEV, and Phase
performance data collected at the inputs are used to disqualify inputs (prevent input
selection) based on user defined thresholds.
26 SSU-2000e User's Guide
(SSMs), on page 47.
Synchronization
12713140-002-2 Revision E – October 2006