Symmetricom SSU-2000 User Manual page 30

Chapter 1 Product Overview
System Architecture
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-2000 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-2000 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.
In either mode, when the Priority and PQL values are equal for each input, the
physical location of the Input module determines the priority for selecting the input
reference. Port 1 on the Input module in the leftmost slot has the highest priority; the
priority decreases to port 1 on the Input module in the rightmost slot. Port 2 on the
module in the leftmost slot has the next lower priority, decreasing to Port 2 on the
Input module in the rightmost slot. Port 3 on the module in the leftmost slot has the
next lower priority, and Port 3 on the module in the rightmost slot has the lowest
priority.
If the Input module is configured for provisioned mode, it reports the user-assigned
SSM. If it is configured for automatic mode, the Input module uses the most recent
valid SSM. If a valid SSM is not received, the module uses the user-assigned SSM.
Setting an Input Alarm Strategy
You can provision the SSU-2000 to elevate input alarms to the next higher level
after a specified amount of time. You can also specify that an alarm clears
automatically after a specified period of time without the alarm condition. For more
information on alarms, see
Bypass Clock (Clock C)
In the event that both Clock modules fail, an input connected to a D1, E1, or
Composite Clock Input module can serve as the system reference. The input must
be a valid reference, and it must have a Priority setting. When the Bypass clock is
the active reference, then the output signal is not filtered and any wander or jitter on
the input signal appears on the outputs. The Bypass clock is intended only for
short-term use; you should resolve the failures on the Clock modules as quickly as
possible.
30 SSU-2000 User's Guide
Alarms and Events, on page 32.
12713020-002-2 Revision F.01 – January 2007