Leased line 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:
On dedicated (leased) data circuits, the Carrier Detect (CD) indicator reflects the presence of a valid carrier from the remote end or network equipment. Absence of CD on a known-good modem points away from the local DTE (computer) and toward the line or network side (DCE path).

Given Data / Assumptions:

• The external modem is known good (recently tested elsewhere or loopback verified).
• Cabling between the modem and computer is correct.
• Only the CD indicator is off; power and other LEDs are nominal.

Concept / Approach:
CD depends on receiving carrier from the far end via the telco leased circuit or CSU/DSU path. If the modem is sound and cabling to the computer is fine, the likely fault is in the telco loop (wiring, provisioning, or remote equipment). While specific enterprise setups may route through DTC/CSU devices, the umbrella description 'phone line' here represents the leased circuit path under carrier control, which is the most common failure domain for no-carrier conditions.

Step-by-Step Solution:

Verification / Alternative check:
Perform a local analog/digital loopback; if CD appears in loopback but not on the live circuit, the telco path is implicated.

Why Other Options Are Wrong:
Modem failure is unlikely given 'known good'. The computer does not affect CD (it is a DTE signal). 'DTC equipment' is too specific and not generally implicated without evidence.

Common Pitfalls:
Swapping PC serial cables or COM settings when the issue is on the telco side; overlooking mis-provisioned circuits.

Final Answer:
In the phone line