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?
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AIn the modem
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BIn the phone line
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CIn the computer
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DIn the DTC/terminal equipment
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ENone of the above
Answer
Correct Answer: In the phone line
Explanation
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:
Confirm the modem powers up and self-tests pass (local indicators normal).Observe CD: it is off, meaning no carrier is received from the line/far end.Swap the modem with a known-good unit (already done) to rule out local modem fault.Conclude the likely fault lies in the telco leased circuit or the remote modem path.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