363-206-305
Transmission and Synchronization
Issue 3 June 2000
5-7
Synchronization
5
Each DDM-2000 FiberReach wideband shelf is provisioned to be line-timed from
an incoming high-speed interface. In line timing mode, the OLIU circuit pack
derives local shelf timing from the incoming OC-1/OC-3/OC-12 high-speed signal
in the Main slot. The OLIU digital phase-locked loop (DPLL) also serves to remove
any timing transients for improved network jitter performance. If one of the OC-N
references is corrupted or unavailable, the OLIU will make a nonrevertive
protection switch to the other reference without causing timing degradations. If
both OC-N signals are lost (for example, due to a cable cut), the OLIU circuit pack
will switch to holdover mode. The OLIU will normally switch back to the line timing
mode when a reference is no longer corrupted, but it can be provisioned to require
a manual switch.
Manual Timing Pack Switch
5
This capability adds the ability to manually switch which Main OLIU is supplying
timing to the shelf. In this way, the switch can be effected remotely. In addition,
timing may be switched off of a Main OLIU prior to removing it for maintenance.
Using this software switch creates a slightly smaller transmission hit. This feature
also causes an “active-fn” OLIU pack equipment switch which will be reflected in
the
rtrv-state-eqpt
report.
Subnetwork Configurations
5
Free Running/Line Timing
5
For initial SONET deployment with a DS1 low-speed interface, minimum first cost
may be a primary concern. The free-running/line timing network can operate
without an external clock source, so the expense of connecting to one is
eliminated. This configuration may be useful for initial loop feeder and customer
location applications, and it also meets the needs of an end-office trunk facility.
This configuration should not be used to provide OC-N timing distribution or where
SONET interconnections to other SONET subnetworks are needed. The local
DDM-2000 host times its transmitted signals at the low- and high-speed interfaces
from the internal
±
15 ppm oscillator. The DDM-2000 FiberReach Multiplexer
connected to the host recovers timing from the incoming OC-N signal and uses
this timing for its transmitted signals.
Because the free-running/line timed DDM-2000 network is asynchronous to the
DS1 services carried over it, additional jitter will be experienced on the
demultiplexed DS1s. Certain interconnected equipment may be sensitive to such
jitter, and this configuration should not be used in cases where it would cause a
problem for that equipment. In particular, downstream equipment containing
Stratum 3 or better clocks could be sensitive to this jitter.
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