16
User Guide
Loopback Modes
The Radiance T3/E3 line card 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 line card’s coaxial receiver, passes through the
fiber line between the two cards, then exits the remote card’s coaxial transmitter
to enter the remote equipment, and vice versa.
Local T3/E3
Line Card
Remote T3/E3
Line Card
Coax
Coax
Fiber
Local
Device
Remote
Device
Loopback
Loopback helps to isolate and identify traffic problems within a specific
segment. During loopback, the yellow LBK LED is lit. Loopback can be
enabled/disabled in two ways: (1) by setting a DIP switch, or (2) through
software by enabling loopback on Port 1 (copper) or Port 2 (fiber). Once
loopback is enabled, the card will remain in this mode until either the DIP
switch is reset or the software setting is changed.
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 card, but the data from the remote unit is
ignored by the card in loopback mode.
In the example shown below, the local T3/E3 line card has copper loopback
enabled. Data received at the BNC port is simultaneously returned to the
local device and sent to the remote T3/E3 card and on to the remote device.
Data from the remote device is transmitted by the remote card, however, it
is ignored by the local card at its fiber optic interface.
