User`s guide

Performing Loopback Modes and Pattern Tests
6-7
7800-A2-GB26-80
May 1999
DTE Loopback
The DTE Loopback (DLB) performs an equivalent function on the DTE T1
interface that the LLB does on the Network T1 interface (see the figure below). It
is useful for isolating problems on the DTE T1 line. For the DLB, a DTE device or
test equipment must generate data to be looped back.
Network T1
Interface
DTE T1
Interface
CSU
DSU
DCE
Port 1
DCE
Port 2
496-15100
DLB
DTE Loopback (DLB)
Repeater Loopback
The Repeater Loopback (RLB) performs an equivalent function to the PLB on the
Network T1 interface (see the figure below). The exception is that the RLB loops
the entire T1 data stream, which includes the data on the DTE T1 interface as
well as the synchronous data ports. The RLB is useful for ensuring that all of the
customer’s information is correct up to the point it is sent out the Network T1
interface (which helps indicate that the DSU/CSU is operational). For the RLB, a
DTE device or test equipment must generate data to be looped back.
Network T1
Interface
DTE T1
Interface
CSU
DSU
DCE
Port 1
DCE
Port 2
496-15101
RLB
Repeater Loopback (RLB)