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Transmission and Synchronization Interfaces
5-28 Issue 3 June 2001
Synchronization Interfaces 5
Timing Modes 5
Each DDM-2000 OC-3 Multiplexer can be provisioned to one of three timing
modes:
■ Free-running from an internal oscillator
■ Line-timed from an incoming optical interface. This timing reference is used
to generate all outgoing signals.
■ External timing from the digital synchronization network via DS1 references
(DS1 External).
NOTE:
The synchronization and timing examples used throughout this chapter
reference the timing generators as “TG.” This could represent either the
BBF2B TGS or the stratum 3 BBF4 TG3.
Free-Running 5
For free-running operation, the TG derives timing from a temperature-
compensated, voltage-controlled crystal oscillator (TCVCXO) and a digital phase-
locked loop (DPLL) with a full temperature range end-of-life accuracy of ±15 parts-
per-million (ppm) for the BBF2B TGS, and ±4.6 ppm for the BBF4 TG3.
Line Timing 5
The reference signal feeding the PLL is selected from the internal oscillator or a
line timing clock derived from the incoming optical line. In line timing mode, the
OC-N line being selected for transmission is also selected as the timing reference.
In addition, the OC-3 line in function unit C selected for transmission can
optionally be selected for the timing reference. This selection is under the control
of the on-board microcontroller and the SYSCTL circuit pack and is dependent on
the timing mode selected by the user via on-board selection switches and the
status of the references. Loss of both line timing references causes the TG circuit
pack to go into holdover mode to maintain system timing.
The terms loop timing and line timing have been changed to clarify timing
terminology. When the OLIU derives local shelf timing from the incoming
optical signal and the shelf is an intermediate shelf (choice of OC-N lines for
timing) in a linear network for example, it is called line timing. Loop timing is
a subset of line timing used to describe the timing mode of the terminating
node (single source of timing) of a linear network, for example.
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