Symmetricom SSU-2000 User Manual page 29

Chapter 1 Product Overview
System Architecture
You can provision the SSU-2000 to automatically select the highest-quality input
signal, or you can manually select the input signal you want to use. Two factors
determine the quality of the input: the user-defined Priority level, and the PQL/ SSM.
These factors are used to determine which input is used, and what happens if that
input becomes disqualified.
Priority Level
You can assign a Priority level to each input; this value helps determine which input
is used as the system reference. The range is 0 to 10; the lower the Priority level,
the higher the priority. Use the TL1 command SET-PRMTR- INPUT or the ICS
command INPUT to provision this parameter. See Setting the Input Selection Mode,
on page 30, for details on using the Priority level.
SSMs and Quality Level
Synchronization Status Messages (SSMs) may be included in the bit stream of the
incoming signal to indicate its quality level. To read the incoming SSM, you must
provision the input to read the SSM; for E1 signals, you must also provision the bit
position of the incoming SSM. Use the TL1 command SET-PRMTR-INPUT or the
ICS command INPUT to provision these parameters.
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 Synchronization
Status Messages (SSMs), on page 51.
Setting the Reference Switching Mode
Telcordia GR-378 and GR-1244 define two reference selection modes: 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.
12713020-002-2 Revision F.01 – January 2007 SSU-2000 User's Guide 29