Leased-line modem troubleshooting: On a leased line installation, a known-good external modem shows no Carrier Detect (CD) light. Where is the problem most likely located?

Introduction / Context:
Carrier Detect (often labeled CD) on an external modem indicates that the modem is receiving a valid carrier from the far end over the leased circuit. When the CD light stays off on a known-good modem, the fault is usually not in the local terminal or the modem itself, but in the telecommunications path. This question checks practical leased-line troubleshooting skills and understanding of modem status indicators.

Given Data / Assumptions:

• A leased line (not a dial-up line) is in use between two sites.
• A known-good external modem is correctly powered and cabled to the computer/terminal.
• The Carrier Detect (CD) indicator is off continuously.
• No other alarms on the modem suggest local hardware failure.

Concept / Approach:
On a leased line, CD depends on the presence of a valid carrier from the remote end through the telco circuit. If CD is dark, the local modem is not receiving that carrier. The most probable cause is a fault in the line (open, miswired pair, excessive attenuation, CSU/DSU issues, or far-end modem down). Local DTE cabling or computer issues do not affect the CD light, since CD is a DCE-to-DTE signal derived from the received carrier.

Step-by-Step Solution:

Verification / Alternative check:
Loopback tests (local analog/digital loop) and contacting the carrier to run metallic/path tests typically confirm a line issue. If the far-end modem is off or disconnected, CD will remain off at the near end as well.

Why Other Options Are Wrong:

• In the modem: Already proven good; CD depends on receive path, not just the unit.
• In the computer: DTE problems do not extinguish CD; CD originates in the modem from the line signal.
• In the DTC/terminal equipment: Same reasoning as the computer; CD is independent of the terminal's state.
• None of the above: Incorrect because the phone/leased line is the prime suspect.

Common Pitfalls:
Confusing DTR/RTS control signals (from the computer) with CD (from the line), and replacing serial cables or PCs unnecessarily when the telco path is down.

Final Answer:
In the phone line