User`s guide

Performing Loopback Modes and Pattern Tests
6-5
7800-A2-GB26-80
May 1999
Payload Loopback
The Payload Loopback (PLB) mode loops the information received on the
Network T1 interface back to the network after it has passed through the receive
and transmit framing section (i.e., the CSU) of the DSU/CSU. This loopback is
useful for determining whether the problem is with the T1 facility or in the circuitry
of the remote device. Building on the previous example used in
Line Loopback , if
Device A does not detect an error as a result of a LLB test, but does detect an
error as a result of a PLB test, the likelihood is that the error is in Device B.
Compare
Line Loopback (LLB) with Payload Loopback (PLB) .
Network T1
Interface
DTE T1
Interface
CSU
DSU
DCE
Port 1
DCE
Port 2
496-15098
LLB
Line Loopback (LLB)
Network T1
Interface
DTE T1
Interface
CSU
DSU
DCE
Port 1
DCE
Port 2
496-15099
PLB
Payload Loopback (PLB)