Which device is tested using a loopback connector to diagnose WAN issues?

• A. CSU/DSU
• B. Demarc
• C. IDF
• D. Wireless LAN Controller

Answer: (A)

Loopback testing is used to verify the integrity of a WAN interface by feeding the device’s transmitted signal back to its receiver, confirming both transmit and receive paths work. The device that terminates the WAN circuit at the customer premises, typically a CSU/DSU, is designed for this kind of diagnostics and often provides a loopback plug or mode specifically to test the WAN port. By using the loopback connector on the CSU/DSU, you can determine whether faults are in the local device/cabling or further along the line, isolating the problem more effectively. The demarc is simply the boundary point between provider and customer and isn’t a device you test with a loopback for WAN issues; the IDF is an internal building distribution point, and a Wireless LAN Controller relates to Wi‑Fi, not WAN termination.