2 Cabling and Physical Connectivity

Recognize physical-layer faults

Physical-layer faults interrupt the signal between network devices. Common causes include damaged or incorrectly terminated cables, loose or misplaced connections, faulty ports, and cable or transceiver types that are unsuitable for the link.

Start with simple checks and change one component at a time. This helps identify whether the fault follows a particular cable or remains associated with a port or device.

Trace and isolate the connection

Trace the complete connection from the device to the wall jack, patch panel, and switch. Confirm that both ends are in the intended ports and that intermediate connections are patched correctly.

Inspect the cable and connections for cuts, kinks, crushing, tight bends, damaged plugs, bent or dirty contacts, and loose connections. Unplug and firmly reseat each end, and replace cables that are visibly damaged.

Check the link indicators at both ends. Then substitute a suspect patch cable with one known to work and of the correct type. If the fault remains, try a known-good port. A fault that follows the cable suggests a cable problem; one that stays with the port points to the port or device for further investigation. Also check that ports are enabled and devices are powered.

If the cable runs through a wall jack or patch panel, test that run rather than testing only the short patch cords. Record the test result and the location tested.

Interpret link symptoms

A link indicator usually means the devices have detected a physical connection, but its meaning depends on the device model. Consult the device label or documentation. No light can indicate a disconnected or incorrect cable, a disabled or failed port, an unpowered device, or a mismatch between the cable and transceiver. A light on only one end can still occur with a broken conductor or a disabled port.

For no link, check power, seating, port selection and status, cable type, and both ends of the path. Include couplers, patch panels, media converters, and transceivers in the check.

An intermittent link or poor performance can result from damaged or stressed cable, loose terminations, or a faulty port. Check interface counters for errors and whether the link repeatedly drops and returns. A link light can remain on even when a marginal cable causes errors.

For an unexpected connection, verify that the cable is patched to the intended wall jack and switch port. A cable connected at both ends may still lead to the wrong destination.

For a fiber link fault, confirm that the fiber type and transceivers match the equipment and intended link. Check that connectors are seated and handled according to the equipment maker’s instructions.

Test cabling and understand limits

A simple cable tester with a remote unit can check wiremap: whether conductors reach the expected pins at the far end. Depending on the tester, it can identify several wiring faults:

• Open: a conductor is broken or not terminated.

• Short: conductors are unintentionally connected.

• Reversed or crossed pair: conductors or pairs land on the wrong pins.

• Split pair: the pins may appear continuous, but conductors from different twisted pairs are combined, which can impair transmission.

A wiremap pass checks basic continuity and pin connections. It does not necessarily show that a cable meets the performance requirements for its intended Ethernet speed. Use an appropriate qualification or certification tester when performance needs to be verified. Follow the tester’s instructions; ordinary cable testers are not substitutes for testing live network service.

Apply the checks to a no-link fault

A workstation has no network connection, and its switch-port link light is off. Reseat both ends, then replace the patch cord with a known-good one. If the light comes on, the original cord was likely faulty.

If the light stays off, try a known-good switch port and check that the port is enabled. If neither substitution restores link, inspect and test the in-wall run and its terminations.