Troubleshoot Common Cabling Problems by Anthony Sequeira

A lot can go wrong when it comes to the physical media (cables) that make up a network. But the great news is that there are more tools and techniques than ever before to proactively head off physical layer issues.

speed: The fastest speed of a cabled network is around 10 Gbps or even higher. While the fastest speed of WiFi max is theoretically 6.9 Gbps, but actual speeds are much slower, usually less than even 1 Gbps.

security: Hackers use packet sniffers to capture network data to see what information, such as passwords and authentication tokens they can access. To use a packet sniffer on a cabled network, a hacker needs to get physical access to the cable, but wireless network data radiates outside the building, and so hackers called "wardrivers" search for wireless networks from a moving vehicle, using a laptop or smartphone and software that is freely available on the internet.

Interference: Radio devices such as baby monitors, walkie talkies, microwave ovens and drones flying near your building can interfere with a wireless network.

For these reasons many corporations use cables for most or all of their network connectivity.

To be an effective troubleshooter, it helps to understand the types of things that commonly go wrong in networks so that you can become proficient in recognizing and solving issues. Here are just some of the things you need to be on the lookout for with cabling in a network:


Distance is the enemy of cabling. The longer the signal travels through the media, the more it weakens. It is very important to know the maximum distances for the various media in your enterprise network. Attenuation is measured in decibels (dB), which is covered shortly.


While they are not as sensitive to interference as wireless media, cables can suffer from EMI (Electromagnetic Interference) and other types of interference. Just as with wireless networks, you need to do proper testing for interference issues before deploying physical media. In addition to reduce interference, you should follow the recommendations of the cable manufacturer.

Decibel (dB) loss:

If physical cable has lots of different splits or splices in it, there might be a great deal of dB loss. This loss is also prone to happen as the signal needs to go farther and farther in the media.

A deciBel (dB) is unit for expressing the ratio between two quantities, usually between two levels of sound energy electric power. One Bel is when the output signal is 10x that of the input, and one deciBel is 1/10th of a Bel. If the difference is 4 dB, or 0.4 Bel, The antilog base 10 of 0.4 is 2.5., so the louder sound is 2.5 times the sound of the quieter sound.

Bad cables, incorrect pinouts, or bent pins:

Faulty cables (with electrical characteristics preventing successful transmission) or faulty connectors (which do not properly make connections) can prevent data transmission at Layer 1. A bad cable could simply be an incorrect category of cable being used for a specific purpose. For example, using Cat 5 cable (instead of a Cat 6 or higher cable) to connect two 1000BASE-TX devices would result in data corruption. Bent pins in a connector or pinouts could also cause data to become corrupted.

Bad ports:

A physical port on a network device might be bad. You can often easily confirm whether a port is bad by looking at the network connection LED and status indicators.


An open is a broken strand of copper that prevents current from flowing through a circuit. A short occurs when two copper connections touch each other, resulting in cureent flowing through that short rather than through the attached electrical circuit because the short has lower resistance.

Light-emitting diode (LED) status indicators:

Many network devices are very specialized and require special types of indicators. Oftentimes, indicators are implemented in the form of special LED status indicators. Thanks to such indicators, with a single glance, a network administrator can ascertain whether there is a problem with the network device. For example, a solid green LED for a network card normally indicates that the card is connected or receiving a signal. If there are no lights present or if the lights are orange or red, the network may not be connected properly or may be receiving a signal from the network.

Incorrect transceivers:

A transceiver is a device that is able to both send and receive analog or digital signals. You must be sure to select transceivers carefully. The transceiver you select for a network device must match the cable type used and the wavelengths that are in use in the network.

Speed and duplex issues:

Speed and duplex mismatches can be tricky to troubleshoot in a network, especially considering that connectivity is often maintained (although at unacceptable levels).

Transmit and receive reversed:

Some Ethernet switches support media-dependent interface crossover (MDIX), which allows a switch port to properly configure its leads as transmit (Tx) or receive (Rx) leads. You can interconnect such switches by using a straight-though cable (as opposed to a crossover cable). However, if a network device does not support MDIX, it needs a crossover cable in order for its Tx leads to connect to the Rx leads on a connected device and vice versa. Therefore, you must take care when selecting cable types for interconnecting network components.

Dirty optical cables:

An often-overlooked aspect pf fiber-optic media care and maintenance is keeping the fiber-optic connector end faces clean. A dirty fiber connection can either slow down or completely inhibit network traffic. One method to clean optical cables is to use a combination of wet and dry cleaning approaches. For example, you might use a small amount of solvent on a wiping material and immediately dry the surface.

About The Author

Anthony Sequeira, CCIE No. 15626, is a Cisco Certified Systems Instructor (CCSI) and author regarding all levels and tracks of Cisco Certification. Anthony formally began his career in the information technology industry in 1994 with IBM in Tampa, Florida. He quickly formed his own computer consultancy, Computer Solutions, and then discovered his true passion-teaching and writing about Microsoft and Cisco technologies. Anthony joined Mastering Computers in 1996 and lectured to massive audiences around the world about the latest in computer technologies. Mastering Computers became the revolutionary online training company, KnowledgeNet, and Anthony trained there for many years. Anthony is currently pursuing his second CCIE in the area of Security and is a full-time instructor for the next-generation of KnowledgeNet, Anthony is also a VMware Certified Professional.

CompTIA Network+ N10-008 Cert Guide contains proven study features that allow you to succeed on the exam the first time. Expert instructor Anthony Sequeira shares preparation hints and test-taking tips, helping you identify areas of weakness and improve both your conceptual knowledge and hands-on skills, essential for successful completion of the performance-based testing items on the exam. This complete, CompTIA-approved study package includes the following:

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Network topologies and media types
IP addressing
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Routing, Ethernet switching, and wireless networking
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Network security
Remote access
Network troubleshooting

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