Symmetricom SSU-2000 User Manual page 274

Chapter 9 Hardware Configuration Guide
Configuring a Subtending SSU
The CC Input modules make phase measurements between the input signal and
the clock oscillator(s). From these phase measurements on the selected input, the
Clock modules compute frequency offset correction and holdover value. The
selected Input module also generates a 4 kHz "C" clock that is used to align the
phase of the CC output signals.
Operating Mode
The operating mode of the SSU is set with the ICS command OPERMODE
[NORMAL | JAPAN | SUB]. In Normal mode, the unit operates as specified in
Chapter 1, Product
inputs as reference sources. The Japan mode uses only JCC inputs as reference
sources; the JCC Output module's signals are byte and polarity phase aligned with
the 4 kHz clock signal from the JCC Input module. The bipolar violation (BPV)
occurrences in the selected input are used to generate a signal output for phase
alignment of the CC/JCC output signals. In addition, phase measurements on the
selected input are used to control the average frequency of the outputs. If there is a
significant difference in the BPV alignment between the selected CC/JCC input and
any secondary CC/JCC inputs, the unit issues a warning that the reference signals
are not phase aligned. It is the operator's (or installer's) responsibility to ensure that
the CC/JCC inputs are from the same (master) BITS and that they are wired
correctly to maintain the same polarity. A valid CC/JCC input is required when
starting the Subtending mode in order to align the phase of the CC/JCC outputs.
Note: When provisioning the SSU-2000 to phase align Composite
Clock Outputs (CC, JCC, or JCC4) to Composite Clock Inputs (CC,
JCC, or JCC4) you can instruct the SSU to transition or change
modes of operation (Normal, Subtending, Japanese Subtending).
Changing modes of operation is a system "turn-up procedure" and
should only be performed when the SSU Composite Clock Outputs
are not "in-service". Switching modes of operation is not defined by
any Telecom Standard or Requirement; if you instruct the system to
change the current mode of operation, you should expect the CC,
JCC, and/or JCC4 outputs to generate a momentary phase transient.
Selecting Input References and Switching References
In the Subtending and Japan modes, the unit selects the highest priority valid
CC/JCC input available. If the selected input is lost, the unit switches to any
alternate CC/JCC input as specified by priority assignments; the phase and
frequency of the CC outputs follow the new reference signal. If all CC/JCC inputs
are invalid, the output phase continues at the last valid alignment before the input
was lost; the frequency is established by the learned holdover values in the Clock
modules. When a valid CC/JCC input is re-established, the CC/JCC output is
immediately aligned to the selected input signal. A CC/JCC input is invalid when it is
in LOS or it has invalid BPV occurrences. LOS is declared when the Input module is
unable to track the expected 64 kHz input; an invalid BPV event occurs when the
expected 8000 BPVs per second varies by more than ±10%.
274 SSU-2000 User's Guide
Overview. In the Subtending mode, the unit uses only the CC
12713020-002-2 Revision F.01 – January 2007