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COMSPHERE 3500 Series Data Service Units
3-14 January 1998 3510-A2-GN32-50
Channel Loopback
The Model 3510 and 3511 DSUs perform a Channel
Loopback test whenever they detect reversal of the sealing
current in the local loop. In addition, these DSUs
connected to subrate or 56 kbps DDS local loops respond
to network loopback bipolar violation sequences by
performing a channel loopback, returning incoming data
to its source via the digital network (shown in Figure 3-7).
DSUs optioned for 64 kbps CC operation will respond to
DDS network latching loopback sequences.
While a channel loopback is underway, the yellow
REM, TEST, and DL indicators light, and the red OS/NS
and the green OK indicators are on, showing that the
digital network is carrying a Network Loopback/OOS
(Out-of-Service) message.
Troubleshooting
Additional diagnostic testing can be performed to
troubleshoot and correct out-of-service, no signal, and
streaming terminal conditions that occur on the digital
network. Figure 3-8 is a flowchart for troubleshooting an
OOS condition. Figure 3-9 is a flowchart for
troubleshooting a No Signal condition. Figure 3-10 is a
flowchart for troubleshooting a streaming condition.
Figure 3-7. Channel Loopback Diagram