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Timing Options
timing source provided by the network.
One thing is common to all of the SC 553 CSU/DSU's timing options: the CSU/DSU recovers the
clock from the network receive data, then uses it to clock data into the receive buffer and to provide
the channel receive clock signals for the channels. What differs is the source of the master clock
reference and the source of the channel transmit clock signals, as shown in
•
With Slave Timing, the master clock comes from the network and the channel's transmit
clock signals are derived from the clock recovered from the network receive data.
•
With Internal Timing, the CSU/DSU provides the master clock, but the channels' transmit
clock signals are still derived from the recovered clock.
•
With DTE Timing, the customer equipment on the selected channel provides its own transmit
clock signal, from which the CSU/DSU derives a master clock.
•
Cascade Timing extends the network receive T1 data stream to the cascade port, and uses
timing imbedded in the network and cascade receive data.
Note
All timing options that include split timing require each timing source to be traceable to a
Stratum 1 clock.
On the network and cascade ports, the data rate is 1.544 Mbit/s. On the channel ports, however, the
data rate may be from 56 kbit/s to 1.536 Mbit/s. The SC 553 CSU/DSU compensates for this
difference in data rates by translating the clock frequency when passing it from one port to another.
Table D-1
Timing Reference
Timing Option
Slave Timing
Internal Timing
Channel Timing
(External/DTE)
Cascade Timing
(network source)
Cascade Timing
(cascade source)
D-2
Master Clock Source
CSU/
Net.
Cas.
DSU
X
X
X
SpectraComm 553 Fractional T1 DSU
Installation and Operation Manual
Timing Option Descriptions
Table
Channel
Trans.
Clock
Source
Ch.
Net.
Ch.
X
X
X
X
X
X
X
D-1:
076R155-000
Issue 7
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