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Timing Options
Overview
The flexibility and complexity of the SC 553 CSU/DSU's timing options require explanations that
are more detailed than those normally provided. This appendix describes the details and
applications of the SC 553 CSU/DSU's timing options:
•
Slave Timing
•
Internal Timing
•
Channel Timing (External/DTE Timing)
•
Cascade Timing
•
Station Timing
Each description is accompanied by an illustration showing how the timing clock is distributed
throughout the network, as well as typical applications. Following the timing option descriptions
are some representative network applications that show how to apply the timing options in a variety
of network configurations.
Timing Option Descriptions
In synchronous networks, all devices' transmitters and receivers are usually referenced to a single
master timing source. The timing source, or clock, which is frequently provided by the network and
must be highly accurate and stable. The network's clock is the preferred timing source, but the SC
553 CSU/DSU allows other timing options for use in applications where network timing is either
not available or not applicable.
The SC 553 CSU/DSU recovers the imbedded clock from the data stream and uses it to synchronize
its own internal timing reference to the master clock. That allows it to extract the data reliably and
to distribute timing to other devices connected to it. On the network and cascade ports timing is
imbedded in the data stream, but it is provided on separate channel interface leads for the channels.
Note
Several techniques exist for providing timing to the customer equipment: smooth clock and vari-
ations of gapped clock. With a smooth or continuous clock, the type employed by the SC 553
CSU/DSU, every clock pulse is the same length and occurs at the same interval. With a gapped
clock, however, pulses are intentionally omitted. Customer equipment that is expecting a smooth
clock may not function properly with a gapped clock if it interprets the missing pulses as loss of
timing. On the other hand, equipment that can tolerate a gapped clock will probably work well
with a smooth clock, so the smooth clock technique is more widely acceptable.
The default timing option for the SC 553 CSU/DSU is Slave Timing, used when the network
provides the timing source. When the network does not provide the timing source, one SC 553 CSU/
DSU must use Internal Timing (or Channel Timing when customer equipment connected to it
provides timing) and the others must use Slave Timing. Finally, with the optional T1 Cascade Card
installed, Cascade Timing allows the equipment on the cascade port to recover the clock from the
076R155-000
Issue 7
Appendix D:
SpectraComm 553 Fractional T1 DSU
Installation and Operation Manual
Timing Options
Overview
D-1
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