Timing interface features
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Synchronization Status Message (SSM) signal
A timing marker or synchronization status message signal can be used to transfer the
signal-quality level throughout a network. This will guarantee that all network elements
are always synchronized to the highest-quality clock that is available.
On Metropolis
®
AMU systems, the SSM algorithm or the timing marker is supported
according to ITU-T recommendation G.781 and ETSI recommendation ETS
300-417-6-1. The SSM is supported on all STM-N interfaces.
External timing interfaces
Metropolis
®
AMU provides one external timing input and output per main card for
ITU-T compliant 2048 kHz / 2048 kbit/s timing signals. The timing output is realized
as RJ45 connector suitable for symmetrical twisted pair cables with an impedance of
120
Ω
or coaxial cables with an impedance of 75
Ω
.
2 Mbit/s tributary retiming
The user can choose whether individual 2 Mbit/s tributary outputs operate in
“self-timed” or “re-synchronized” mode. In the (standard) self-timed mode, the phase
of the outgoing signal is a moving average of the phase of the 2 Mbit/s signal because
the signal is embedded in the VC-12 that is disassembled. In the re-synchronized mode
the 2 Mbit/s signal is timed by the SDH Equipment Clock (SEC) of the network
element; frequency differences between the local clock and the 2 Mbit/s signal
embedded in the VC-12 to be disassembled are accommodated by a slip buffer.
There is also the following option: whenever the traceability of the local clock drops
below a certain threshold, the re-timing 2 Mbit/s interfaces automatically switch to
self-timing. When this fail condition disappears, these interfaces return to re-timing.
These changes do not involve any hits in the traffic.
Important! Re-timing should only be applied when the network element which
performs the re-timing and the network element which generated the 2 Mbit/s
signal have traced back their SECs to the same synchronization source. Otherwise a
continuous stream of 2 Mbit/s frame slips or skips will occur at the re-timing point
which is indicated by a FCS threshold crossing alarm.
The user has the option of operating individual 2 Mbit/s outputs in the
“re-synchronized” mode. In this mode the 2 Mbit/s output signal is timed by the
system clock of the network element.
Features
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3-30
Lucent Technologies - Proprietary
See notice on first page
365-312-847R4.0
Issue 4, November 2006
