363-206-285
Transmission and Synchronization Interfaces
5-58 Issue 3 June 2001
A previous drawback to using OC-N timing distribution was that network timing
failures could not be communicated to downstream clocks via DS1 AIS, since the
DS1 signal does not pass over the OC-N interface. With synchronization
messaging, clock stratum levels can be passed from NE to NE, allowing
downstream clocks to switch timing references without creating timing loops, if a
network synchronization failure occurs. If a quality timing reference is no longer
available, the DDM-2000 OC-3 sends AIS over the DS1 interface. If the local
OC-N lines fail, DDM-2000 OC-3 outputs AIS on the DS1 output or an upstream
DDM-2000 OC-3 system enters holdover.
Access Network Timing Distribution 5
OC-N timing distribution can also be used in access networks or to small COs. In
this configuration, a DS1 reference from the CO BITS clock still times the OC-N
transmitted to the remote site. The line-timing capability of the DDM-2000 OC-3
Multiplexers provides the ability to recover OC-N timing. The DS1 timing output
feature can be used to also extend timing to customer networks or remote sites. In
this case, the DS1 timing output can be used to time switch remotes, DDM-2000
OC-3 and OC-12 shelves, or other local equipment directly. Ideally, the equipment
can provide an external timing reference. Otherwise, the signal must be input to a
traffic DS1 port on the external equipment which will tie up some of this
equipment's bandwidth. In this configuration, it is important that the DS1 reference
to the DDM-2000 OC-3 in the CO be traceable to the same clock used to source
the DS1s being carried to the customer site or small CO. If it is not, slips may
occur.
Although an ideal source of timing, OC-N timing distribution, via a DS1 timing
output, cannot be used to provide timing in all applications. In cases where the
local equipment is not provided with an external timing reference input, or in some
private networks where the timing is to be distributed from another private network
location, timing may be distributed via traffic-carrying DS1s. In these applications,
a stable DS1 timing source can be achieved by ensuring that all elements in the
SONET network are directly traceable to a single master clock via line timing. In
this environment, the high-performance desynchronizer design of the DDM-2000
OC-3 Multiplexer allows a DS1 timing reference to be carried as a multiplexed
DS1 payload
*
.
It is recommended that, where possible, the DS1 sources (switch, PBX, or other
equipment) be traceable to the same timing source used to time the DDM-2000
* Synchronous operation via line timing eliminates the generation of VT pointer adjustments,
thus maintaining the phase stability needed for a high-quality DS1 timing reference. Cross-
connecting at the STS-1 level also eliminates the VT pointer adjustments. While the design
of the DDM-2000 OC-3 Multiplexer maintains jitter/wander within standard DS1 interface
requirements, even in the presence of VT pointer adjustments, and while the DS1 is likely
to be stable enough for most equipment to use as a timing reference, some equipment may
have more stringent stability requirements for its timing references.
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