Timing Options
Typical Network Applications
D-12
SpectraComm 553 Fractional T1 DSU
076R155-000
Installation and Operation Manual
Issue 7
PBX Application
In a typical private network PBX application, as illustrated in
, two SC 553 CSU/DSUs
are connected back-to-back. One PBX is configured as the master (the one at Site A in this example)
and provides the timing source, and the other is configured as a slave and loops timing to its CSU/
DSU. Cascade Timing is automatic for both SC 553 CSU/DSUs, but you must also select the
alternate timing source for each: Select Internal Timing for the CSU/DSU at Site A so that it will
provide timing if the master PBX connected to it fails.
If there is a loss of cascade data between the PBX and the SC 553 CSU/DSU, the SC 553 CSU/DSU
will switch to Internal Timing.
Select Slave Timing for the SC 553 CSU/DSU at Site B so that it will recover and loop timing if the
slave PBX connected to it fails. If both PBXs fail, the CSU/DSU at Site A will switch to Internal
Timing and provide timing, and the CSU/DSU at Site B will switch to Slave Timing and recover
and loop timing, so that the terminals on the channels can continue unaffected.
Figure D-8
Typical PBX Application
Note
DS0s used by the channel must not be used by the PBX. Configure the PBX to busy out all DS0s
used by the channel.
SC 553
CSU/DSU
SC 553
CSU/DSU
Site A
Site B
Channel
Terminal
Terminal
Timing Source
Timing Option:
Master Timing
Timing Option:
Slave Timing
Timing Option:
Cascade Timing
(Alt. Internal Timing)
Timing Option:
Cascade Timing
(Alt. Slave Timing)
Master
PBX
Slave
PBX
Cascade
Channel
Cascade
