When troubleshooting a T1 connection, the support representative from the provider instructs a technician to place a special device into the CSU/DSU. Using this device the provider is able to verify that communications are reaching the CSU/DSU.
Which of the following was MOST likely used by the technician?
A.
Cable analyzer
B.
Toner probe
C.
OTDR
D.
Loopback plug