12713020-002-2 Revision D – April 2004 SSU-2000 User’s Guide 227
Hardware Configuration Guide
Configuring a Subtending SSU
The Subtending SSU configuration provides Composite Clock (CC) and DS1
outputs that are frequency-locked and phase-aligned to the CC reference signal. If
the CC input signal fails, the Clock module provides holdover.
This section contains an introduction to the hardware and software that is specific to
a Subtending SSU configuration.
Functional Description
The SSU-2000 configured as a Subtending SSU broadens the Symmetricom
SSU-2000 family of products by providing subtending clock functionality when
referenced to a master TSG/BITS equipped with a Stratum 3E or better clock. The
following combination of hardware and software configuration meets Subtending
SSU criteria described in Section 7 of Telcordia Specification GR-378-CORE.
Hardware – Use the Subtending SSU when there is a need for more timing outputs
or longer distribution paths than can be supplied by a single TSG. The Subtending
SSU receives redundant Composite Clock (CC) reference signals directly from the
Master BITS Clock. The CC signals are used for primary and secondary inputs and
the SSU phase-locks to the selected reference to ensure proper DS0 phase
alignment throughout the office. If both CC reference inputs fail, the shelf provides
Stratum 3E holdover stability. If both 3E Clock modules fail, the Subtending SSU
uses the CC reference input for clock bypass operation.
Input Reference Signals – The Subtending SSU configuration requires at least
one CC input signal for phase alignment of the CC output signals. The selected CC
input provides measured phase information to each Clock module, where it is used
to control the clock’s frequency and establish the holdover value for the Clock
modules. The operator can manually or automatically select the reference.
Switching references can occur repeatedly without accumulating phase or
frequency error. Restoring the primary reference source can return the unit to
tracking the primary signal.
Software – The Subtending SSU software contains a command that allows the user
to select either the normal SSU mode or the subtending mode of operation.
A Communications module with Subtending SSU software installed identifies the
Input, Output, and Clock modules that are provisioned in the system.
Input Section
The Input section consists of one or more CC Input modules connected to CC
signals from a Master BITS shelf. Other input modules (DS1, etc.) may be installed,
but in the Subtending mode these inputs are used for monitoring only, and are never
selected as reference inputs for phase and frequency control of the outputs.
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