Functional Description
700-712-100-01, Revision 01
16
May 10, 1999
UTU-712 and ETU-762 List 1
L
OOPBACKS
An HDSL system maintains several diagnostic loopback configurations, both toward and away from the local unit,
which can be used to verify proper transmission of test data through the local unit, the HDSL facility, and the
remote unit.
on
shows all possible loopback paths.
lists available loopbacks.
During loopbacks the system generates an Alarm Indication Signal (AIS) past the loopback point toward the far
unit. Loopbacks can be activated from the LOC (local) and REM (remote) front-panel pushbuttons, the Local
Loopback (LL)/Remote Loopback (RL) signals on the Nx64K port, the console Test menus, and the management
unit interface (
lists loopback equivalents for different activation methods). Only one loopback
option can be enabled at a time. During a loopback, the Loopback LEDs on the front panel indicate the type of
loopback currently present in the system. See
Table 4, “Line and Desktop Unit Front Panel LED Indications,” on
.
The system reverts to normal transmission of payload data after the specified timeout period has elapsed. If a
timeout period is not specified, the loopback must be manually disengaged before normal transmission of data can
resume.
Table 8.
HDSL Transmission and Application Interface Alarms
Alarm
Description
HDSL Alarms
HDSL alarms (loop-specific) include:
Loss of Sync Word
(LOSW)
The unit cannot receive data over the given HDSL loop.
Margin (MAR)
The HDSL noise margin of the loop has fallen below the set threshold.
Errored HDSL Second (ES)
Threshold Exceeded
The number of HDSL ES has exceeded the threshold set to give advance warning that HDSL
performance is deteriorating. This threshold can be set from 0 to 255 ES over a 24-hour period, or
disable the alarm completely.
Nx64k Interface Alarm
The Nx64k Interface alarm is Loss of Clock (LOC). The DTE transmit clock (TT) was lost for the
previous second. This alarm is reset when the clock is active again.
External Clock Alarm
The External Clock alarm is LOC. The external clock was lost for the previous second. Applies to loss
of external clock when EXT timing is used. This alarm is reset when the clock is active again.
Alarm Indicators
The HDSL ALM and I/F ALM indicators on the front panel of the unit indicate local unit alarm
conditions.
Alarm History
The units also compile an alarm history which can be viewed from a maintenance terminal, as
described in
“LTU and NTU Interface Alarm History Screens”
on
. For each alarm, the report
provides: the date and time of the first alarm occurrence (since the alarm history was cleared), the
last occurrence, the current status, and the number of times the alarm has occurred. This report is
very useful when evaluating system performance.
Test loopbacks disrupt normal end-to-end transmission of customer data and are the
equivalent of taking the circuit out of service. The circuit does not revert to normal operation
until loopbacks are disengaged manually or until after the specified loopback timeout period
has expired.
