Control, Transmission, and Synchronization Interfaces
Issue 8.0 July 2002
5-23
Externally Timed/Through Timed Configuration 5
The externally timed/through timed configuration allows the FT-2000 OC-48
Add/Drop-Rings Terminal to synchronize to the digital synchronization network as
required for synchronous operation by the SONET standard. The network is
synchronized to a local central office clock via DS1 external synchronization
references. The local office clock must be stratum 3 or better, with timing
traceable to a primary reference source (stratum 1 clock). If synchronous (EC-1,
OC-3, IS-3 and/or OC-12) low speed interfaces are present, external sources
must be traceable to a stratum 1 clock.
The externally timed/through timed configuration applies to all OC-48
point-to-point applications (with or without the FT-2000 OC-48 Repeater Bay).
The FT-2000 OC-48 Add/Drop-Rings Terminal in the through timed mode may be
equipped with two DS1 external synchronization references. If the OC-48 line
reference signals fail, the TG3 (DS1) circuit pack holds the on-board oscillator
frequency at the last good reference sample (holdover mode).
The FT-2000 OC-48 Add/Drop-Rings Terminal in the through timed mode can also
provide two DS1 synchronization outputs to the office BITS clock or other SONET
network elements. An FT-2000 OC-48 Add/Drop-Rings Terminal equipped with
the LAA18 TG3 (DS1) circuit pack derives the DS1 synchronization outputs
directly from the OC-48 line reference signals (Figure 5-12). For more information
about platform timing modes, refer to "Platform Synchronization" in this section.
Figure 5-12. Externally Timed/Through Timed Configuration
FT-2000
Stratum 3 or better
OC-48
BITS
Clock
Central Office
Equipment
Other
FT-2000
CO CO
Externally Timed
DS3/EC-1/OC-3/IS-3/OC-12
Through Timed
DS3/EC-1/OC-3/IS-3/OC-12
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