Verilink Access Manager 2000 (896-502037-001) Product Manual - Manual (Page 311)

Activating/Deactivating CSU loopbacks
Access Manager 2000 User Manual 8-41
PLB - Payload Loopback
When a PLB is in effect, the signal from the DS1 line is looped back to
the line. Since the looped signal passes through the CSU (including the
repeater), the PLB is useful in testing the CSU from the far end of the
circuit. Most Verilink units can be configured to either pass the network
signal intact or send an Alarm Indication Signal (AIS, which is unframed
ALL-ONEs) to the EQPT during a PLB.
The following LEDs on the Verilink CSUs (NCC 2020, CCC 1020, and
TAC 1010) indicate a PLB in progress (SPAN is equivalent to network
[NET]):
LLB - Line Loopback
When a line loopback (LLB) is activated, the signal from the DS1
network is looped back to the network .
Since the looped signal does not pass through the CSU (except for the
repeater and critical protection circuitry), the LLB is useful for testing the
DS1 line. All BPVs received from the network are returned on the
transmit pair.
Most CSUs may be configured to pass the network signal intact to the
EQPT or send unframed ALL-ONEs (i.e., AIS) to the EQPT during
loopback.
A far-end LLB can be activated and deactivated via the 4 kbps ESF Data
Link or with inband codes sent from the near-end CSU. These inband
codes can be generated by all Verilink ESF CSUs except for the 551VST
Equipment Message
551 VST List 1
551 VST List 1/A SPAN MM LOOPED
4016 List 1 SPAN MM LOOPED
551 VST List 1/B
551 VST List 2 SPAN NEAR END LOOPED
4016 List 2 SPAN LOOPED
4016-R SPAN LOOPED
NCC or TAC NET LED flashes amber