Network synchronization
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Introduction
For telephone exchanges or switches, accurate timing is critical. Therefore switches are
synchronized with each other. One way of synchronizing is through the network. In
this case all the SDH-network elements in the network are synchronized with one
master clock. All the switches in the network can synchronize with the network, and
consequently to the master clock.
Timing equipment
A network can use different types of equipment as a timing reference, each having its
own, well defined, accuracy.
• Primary Reference Clock (PRC)
• Synchronization Supply Unit-Transit (SSU-T)
• Synchronization Supply Unit-Local (SSU-L)
• SDH Equipment Timing source (SEC)
PRC
The master clock should be a primary reference clock (PRC), that provides a very
accurate network clock (a clock satisfying the ITU-T recommendation G.811). The
other network elements in the ring are slaved to this clock. A second clock in the
network can be used to act as the backup of the master clock.
The PRC can either be an autonomous clock or it is able to accept synchronization
from a radio or satellite signal.
SSU
Noise, called jitter, may accumulate on the signal after passing through many nodes. It
therefore becomes necessary at some point to recover and reshape the signal. In this
case a synchronization supply unit (SSU) will be used. In addition to recovering and
reshaping, the SSU will also be able to run on its own when the reference signal (for
instance coming from the PRC) is lost. In this case, the SSU will be able to maintain
the quality of the reference signal it produces by means of a holdover mode: the
frequency and phase of the reference signal are stored in a memory and used to
generate a reference signal locally.
Two kinds of SSUs exist:
• SSU-T: SSU-Transit
• SSU-L: SSU-Local
The functionality of both SSUs is equal, the accuracy of the SSU-T is higher than the
accuracy of the SSU-L.
Timing provisioning
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365-312-807R7.2
Issue 4, May 2007
Alcatel-Lucent - Proprietary
See notice on first page
7-7
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