Wired Network Troubleshooting | Network+ N10-007 | 5.3

In this video you will learn about wired network troubleshooting as it relates to bad cables or connectors, bad ports, opens & shorts, splitting pairs in a cable, dB loss, transposed Tx/Rx leads, cable placement, exceeded distance limitations, crosstalk, speed & duplex mismatches, and transceiver mismatches.

Bad Cables or Connectors

Faulty cables (with electrical characteristics preventing a successful transmission) or faulty connectors (which do not properly make a connection) can prevent successful data transmission at Layer 1.  A bad cable could simply be an incorrect category of cable being used for a specific purpose.  Ben pins in a connector or incorrect pinouts could also cause issues.

Bad Port

A port may be bad on your network device.  Oftentimes, you can easily confirm this using the network connection LED and status indicators.

Opens & Shorts

An open is a broken stand of copper, preventing current from flowing through a circuit.  However, a short occurs when two copper connectors touch each other, resulting in current flowing through that short rather than the attached electrical circuit because the short has lower resistance.

Splitting Pairs in a Cable

An unshielded twisted pair (UTP) cable consists of eight separate copper leads.  However, only four of those eight leads are used for data (two transmit leads and two receive leads).  This results in four unused leads.  Some installers use those four extra leads to support a second Ethernet connection on a single UTP cable.  Although such an approach can function, nonstandard wires are being used for connecting the second Ethernet connection.  Therefore, you should be aware of any nonstandard pinouts used in the network that you are troubleshooting.

dB Loss

The signal power of a data transmission might be degraded to the point where the transmission is not correctly interpreted by a receiving device.  This loss of signal power, called a decibel loss could result from exceeding the distance limitation of a copper or fiber cable.

Transposed Tx/Rx Leads

Some Ethernet switches support medium dependent interface crossover (MDIX), which allows a switch port to properly configure its leads as transmit or receive leads.  You can interconnect such switches with a straight-through cable (as opposed to a crossover cable).  However, if a network device does not support MDIX, it needs an appropriate cable (that is, a crossover cable) to allow its Tx leads to connect to the Rx leads on a connected device, and vice versa.  Therefore, care must be taken when selecting cable types interconnecting network components.

Cable Placement

Because copper cables are subject to electromagnetic interference (EMI), you should arrange cables to minimize interference.  Ideally, Ethernet cables should not be placed in close proximity with high-voltage cables, generators, motors, or radio transmitters.  For example, when running cables between buildings via underground conduit, you should ideally place network cabling in a conduit separate from the electrical cables.

Distance Limitations Exceeded

If Ethernet devices are interconnected using a cable that exceeds the Ethernet distance limitations for the cable type, a digital transmission between those devices can be degraded to the point where the receiving equipment is unable to correctly interpret the transmission.  Attenuation is the technical category for this problem.  Therefore, network designs should consider distances between devices.

Crosstalk

Crosstalk can occur when an analog connection creates an electromagnetic field around its conductors, inducing its waveforms on a nearby analog connection.  This phenomenon is most commonly experienced in an analog phone call.  Crosstalk can be minimized by using a higher category of cabling because higher categories of cables better limit the radiation of electromagnetic waves.

Speed/Duplex Mismatch

Speed and duplex mismatches can be tricky to troubleshoot in the network, especially considering that connectivity is often maintained, just at unacceptable levels.

Transceiver Mismatch

Transceivers must be selected carefully to ensure compatibility with the cable type and wavelengths in use.

References

1. Sequeira, A. CompTIA Network+ Cert Guide.