363-206-285
Circuit Pack Descriptions
Issue 3 June 2001
7-17
General Description of Operation 7
The TG circuit pack provides timing signals to the DDM-2000 OC-3 circuit packs.
The TG circuit pack is microprocessor controlled and has the capability to
synchronize to external DS1 references or to loop references from an incoming
optical signal. The on-board oscillator has sufficient accuracy to provide timing
signals without synchronization references for point-to-point systems (free-
running) and a DS1 timing output for network timing distribution.
The TG circuit pack supports three timing modes to serve a wide range of
DDM-2000 OC-3 synchronization needs: external timing, line (formerly loop)
timing, and free-running. In addition, external timing or line (formerly loop) timing
with DS1 output provisioned for network synchronization is provided.
In external timing mode, each TG circuit pack accepts one DS1 reference from an
external stratum 3 or better clock and one cross-coupled from its companion TG
circuit pack. A high-stability digital phase-locked loop (DPLL) removes any
transient impairments on the DS1 reference for improved jitter performance.
In line (formerly loop) timing mode, the TG circuit pack derives local shelf timing
from the received OC-N signal. In addition, the timing function can also be derived
from an OLIU equipped in function unit C. This option is only available through
software provisioning
In free-running mode, the TG circuit pack derives timing from a high stability
temperature-compensated, voltage-controlled crystal oscillator.
In case of unprotected synchronization reference failure, the TG circuit pack will
switch to "holdover mode" and continue to provide system timing, using the
internal oscillator to maintain the last known good reference frequency.
DS1 Timing Output 7
The DS1 Output provided by the TG circuit pack can be provisioned to one of two
modes of operation:
â– MULT
— This mode is normally used to distribute a DS1 timing reference to
other shelves in the bay. In this mode, the distributed DS1 Output is
buffered from the external DS1 reference that is input to the shelf.
â– SYNC OUT
— This mode is used to distribute a derived DS1 timing reference to a
BITS. In this mode, the distributed DS1 Output is based on the
timing signal the TG circuit pack has derived from the received OC-3
signal from either the main OLIU or an OLIU equipped in function
unit C.
— If the DS1 output is provisioned for SYNC OUT mode, DS1 AIS will
be inserted on detection of unprotected OC-N line failure.
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