Cabling

  • Physical layer issues are often the root cause of network problems - always check cables first
  • Cable problems manifest as intermittent connectivity, slow speeds, or complete link failure
  • Use systematic approach: visual inspection → cable tester → protocol analysis

Cable Testing Methods

  • Visual inspection - Check for obvious damage, kinks, or improper terminations
  • Cable continuity testers - Verify all wire pairs have end-to-end connectivity
  • Cable certification testers - Measure electrical characteristics (attenuation, crosstalk, impedance)
  • Time Domain Reflectometer (TDR) - Locates exact distance to cable faults or breaks

Common Cable Problems

Problem Symptoms Cause Solution
Open circuit No link light, no connectivity Broken wire or poor termination Re-terminate or replace cable
Short circuit No link light, possible equipment damage Wires touching each other Re-terminate with proper separation
Miswiring Link up but no data flow Incorrect pin assignments Follow T568A or T568B standard
Excessive crosstalk Intermittent errors, slow performance Untwisted pairs, poor shielding Use higher category cable, proper termination
Impedance mismatch Signal reflections, errors Mixed cable types or poor terminations Use consistent cable category throughout

Copper Cable Standards

  • Cat5e - 100MHz bandwidth, supports Gigabit Ethernet up to 100 meters
  • Cat6 - 250MHz bandwidth, reduced crosstalk, supports 10GbE up to 55 meters
  • Cat6a - 500MHz bandwidth, supports 10GbE up to 100 meters (full distance)

Fiber Optic Troubleshooting

  • Light loss - Measured in decibels (dB), caused by dirty connectors, bends, or splices
  • Optical Time Domain Reflectometer (OTDR) - Locates breaks and measures loss in fiber links
  • Clean fiber connectors with appropriate cleaning supplies - dirt is the #1 cause of fiber problems
  • Check for minimum bend radius violations (typically 10x cable diameter for multimode)
  • Fiber Type/Transceiver Mismatch

Cable Length Limitations

Cable Type Maximum Distance Speed Limitation
Cat5e/Cat6 100m (328ft) Gigabit Ethernet
Cat6a 100m (328ft) 10 Gigabit Ethernet
Multimode Fiber 300m-2km Depends on fiber type and wavelength
Single-mode Fiber 10km-40km+ Limited by optics, not cable

Vocabulary

  • Attenuation - Signal loss over distance, measured in dB
  • Crosstalk - Interference between wire pairs in the same cable
  • NEXT (Near End Crosstalk) - Crosstalk measured at the same end where signal is transmitted
  • Return Loss - Amount of signal reflected back due to impedance mismatches
  • Wire Map - Test showing which pins connect to which pins on far end

Troubleshooting Process

  • Step 1: Visual inspection for obvious damage or improper connections
  • Step 2: Verify cable type matches application requirements (Cat5e for Gigabit, etc.)
  • Step 3: Test with known good cable to isolate problem
  • Step 4: Use cable tester to verify wire map and continuity
  • Step 5: Check interface statistics for CRC errors or collisions

Interface Diagnostics

  • CRC errors - Often indicate cable problems or EMI (electromagnetic interference)
  • Late collisions - Suggest cable length violations in half-duplex networks
  • Input errors - General category including CRC, frame, and other physical layer errors
  • Use show interfaces command to view error counters and reset statistics for baseline

Notes

  • Always test cables before permanent installation - fixing problems later is exponentially more expensive
  • Keep cable runs away from fluorescent lights and electrical equipment to minimize EMI
  • Document cable runs and test results for future troubleshooting reference
  • For intermittent problems, temperature and humidity changes can affect marginal cables
  • When in doubt, replace the cable - it’s usually the cheapest component in the network path
  • Patch cables fail more frequently than permanent cabling due to repeated flexing and handling