16
User Guide
Metrobility T3/E3 17
Loopback Modes
The Metrobility T3/E3 media converter features two loopback modes to help verify
correct installation and to diagnose system problems.
Normal
During normal operation, without loopback, data from a local device (CSU, PBX,
etc.) enters the local T3/E3 unitʼs coaxial receiver, passes through the fiber line be-
tween the two converters, then exits the remote unitʼs coaxial transmitter to enter the
remote equipment, and vice versa.
Loopback
Loopback helps to isolate and identify traffic problems within a specific segment.
During loopback, the yellow LBK LED is lit. Loopback is enabled/disabled by set-
ting DIP switch 2 or 3. Once loopback is enabled, the unit will remain in this mode
until the DIP switch is turned off.
Copper and fiber loopback cannot be enabled at the same time. If both DIP switches
are enabled, only the first one set will be in effect until it is disabled.
Copper Loopback
During copper loopback, timing and data received on the coaxial line are looped
back to the sending device. The loop occurs in the FPGA. The data is also
transmitted to the remote T3/E3 converter, but the data from the remote unit is
ignored by the converter in loopback mode.
In the example shown below, the local T3/E3 unit has copper loopback enabled.
Data received at the BNC port is simultaneously returned to the local device and
sent to the remote T3/E3 unit and on to the remote device. Data from the remote
device is transmitted by the remote unit, however, it is ignored by the local unit
at its fiber optic interface.
Local T3/E3
Unit
Remote T3/E3
Unit
Coax
Coax
Fiber
Local
Device
Remote
Device
Local T3/E3 Unit
with Copper Loopback
Remote T3/E3
Unit
Coax
Coax
Fiber
Local
Device
Remote
Device
X
