Verilink AS2000: The Basics (880-502981-001) Product Manual - Manual (Page 75)

Monitoring and Troubleshooting
Verilink Access System 2000: The Basics 5-9
Repeater
Loopback (RLB)
When a repeater loopback (RLB) is activated, the customer Level 1
equipment and/or DTE is looped on itself through the application
module circuits. The RLB can be used to check the circuit path
from the application equipment through the application module
and back. The application module also passes the equipment
signal to the network. All other loopbacks in progress are
deactivated during an RLB (called local loop in ACP-based cards).
Line Loopback
(LLB)
When a line loopback (LLB) is activated, the data received from the
network is transmitted back toward the network. The looped signal
passes through the application module repeater so that the LLB can
be used to test the circuit from the far-end application module or
telephone company (telco). The application module sends the
network signal or AIS to the equipment, depending on how the
application module is configured option setting.
The LLB can also be activated by sending the industry-standard
inband loop-up code (00001) for at least 5 seconds to an
application module. It is then deactivated by sending the
corresponding loop-down code (001) for the same time period. In
T1 ESF and E1 circuits, the LLB is also activated and deactivated by
sending bit-oriented messages through the overhead channel.
Payload
Loopback (PLB)
When a payload loopback (PLB) is activated, the received
information bits (192 bits per frame) are transmitted to the
network. The framing bits (synchronization, CRC-6, and T1 ESF or
E1 data link) originate at the PLB point.
The PLB includes the repeater and signal processing circuits, surge
protection circuits, and Line Build Out (LBO). It provides end-to-
end testing of the circuit from the far-end application module
through the network line.
The PLB maintains bit sequence integrity for the information bits,
8-bit time slots, frames, and super-frames. During a PLB, the
application module sends either the network signal (intact) or an
AIS to the equipment, depending on the current application module
configuration setting.
The PLB can be activated and deactivated locally or at the far end
by sending bit-oriented messages over the T1 ESF or E1 data link.
Equipment
Loopback (ELB)
When an equipment loopback (ELB) is activated, the level 1
equipment (PBX, etc.) is looped on itself without going through any
application module circuits. The application module sends either
the equipment signal or an all-ones alarm indication signal (AIS) to