5.0 Network Troubleshooting

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5.2 Given a scenario, troubleshoot common cable connectivity issues and select the appropriate tools

Last edited 426 days ago by Makiel [Muh-Keel]

Specifications and Limitations

When working on the Physical layer, you must understand the specifications of the equipment you are working with to ensure that you are not exceeding cable and interface capabilities.
Throughput
Network throughput capabilities are always increasing, so with the higher speeds come more stringent cable requirements. It is critical that we check the interface specifications that detail the supported cable types.
Speed + Distance
Generally, the higher the speed of an interface the more sensitive it is to distance. Remember this when troubleshooting why data connectivity in a cable deteriorates.

Cable Considerations

Understanding cable types and use cases is important to meet local regulatory requirements that are usually related to fire safety and to keep costs down by selecting the correct cable type for your needs.
Shielded and Unshielded
Unshielded UTP, the wires are twisted together in pairs to reduce noise and crosstalk, and multiple pairs are enclosed in a standard non-metal, non-shielded outer covering.
UTP is the most common cable type used in networks.
Shielded STP, twists wires into pairs, but for added protection from electromagnetic interference, the cable is wrapped with a foil or mesh shielded outer wrapper.
Plenum and Riser-Rated
Plenum cables are highly fire resistant and have a low emittance of fumes. Plenum cables can be installed in air ducts (ceilings).
Riser cables are less costly and are used for building runs between floors that do not traverse air ducts.
It’s important to know your cable types so you’ll choose the best one for the job.

Cable Application

Rollover Cable/Console Cable It is used for local connections into network equipment and servers for configuration and troubleshooting.
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Crossover Cable Used when connecting a PC directly to another PC or connecting two Ethernet switches together.
When connected two PCs or two switches, one side has to be a transmitter and the other side has to be a receiver.
Keep this mind when connecting either of these two devices together.
Power Over Ethernet (PoE) Many IP phones could be powered from a central access Ethernet port to save energy finding power outlets at every desk. There’s a single ethernet cable used to provide electrical power + stream data.


Common Cable Connectivity Issues

Remember that the OSI model is additive meaning that if the layer below has a problem, all of the other layers above it probably won't work! Therefore, it is important to understand all of the issues with cabling and how to identify and resolve them to get the network back up and running.
Consider Attenuation
As a signal moves through any medium, the medium itself will degrade the signal. Often referred to as End-to-end loss—a phenomenon known as Attenuation that's common in all kinds of networks.
Keep this in mind when choosing the distances your data will have to travel; It’s important to choose the correct cable that will adequately carry your signals without degradation.
Copper cabling has a max distance of 100 meters before needing to be amplified by a hub or repeater, while Fiber Optics can run signals for literally miles.
Check area for nearby Interference
EMI and radio frequency interference (RFI) occur when signals interfere with the normal operation of electronic circuits.
Computers are very sensitive when it comes to interference; TVs, radio transmitters, microwaves, and two-way radios tend to be the culprit when it comes to interference.
Use shielded network cables like shielded twisted-pair (STP) and coaxial cable (rare today) or run expensive EMI/RFI-immune fiber-optic cable to avoid Interference.
Decibel (dB) loss
The measurement for loss is in power decibels (dB). Cables will have signal loss regardless of whether they are copper or optical.
Too much decibel loss and the receiver will not be able to even turn on the link light.
Use a cable tester to test the DB loss.
Verify the cable does not have Incorrect Pinouts
For a straight-through cable, the correct pinouts are 1-1, 2-2, 3-3, and 6-6.
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For a crossover cable we use 1-3, 2-6, 3-1, and 6-2.
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R-J45 jacks are clear plastic and can be held up to a light to confirm the pinouts of the cable.
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Bad Ports
The actual port on the networking equipment can be broken sometimes.
You can move the cable over to an unused port and see if the problem remains on the original port.
Try the cable on a different port and see if the issue persists; If it still doesn’t work, then the cable is bad. If the cable does work on a different port, then you know that the port itself is bad.
Open/Short
Shorts happen when the current flows through a different path within a circuit than it's supposed to; In networks, they're usually caused by some type of physical fault in the cable.
Best thing to do is swap the cable out with a new one
Light-Emitting Diode (LED) Indicators
LED Indicators provide a quick visual reference — Green is good, Amber is Okay, and No Lights is just bad!
Check for Incorrect Transceivers
You would need a 1G copper transceiver on both ends of the cable for the link to communicate if you are connecting two 1G copper interfaces together that have copper interfaces.
Since transceivers can all look alike, this can be an easy mistake to make
Make sure Duplexes are Identical
A problem arises when one end thinks it's half-duplex while the other is convinced it's on a full-duplex connection. They will communicate, just not very well!
Collisions are common when Duplex settings do not match on both ends.
The way around this is to either set both ends to negotiate their speed and duplex or hard-code in the interface configuration on both ends the speed and duplex setting you require.
Transmit and Receive (TX/RX) reversed
When connecting from a PC-type device into a switch, the PC must use pins 1 and 2 to transmit and 3 and 6 to receive a digital signal. This means that the pins must be reversed on the switch, using pins 1 and 2 for receiving and 3 and 6 for transmitting the digital signal.
Make sure these two (TX + RX) are flipped so they can communicate efficiently.
Dirty Optical Cables
When the ends of a fiber-optic cable are dirty or have oily substances on them, there can be a substantial loss of signal or attenuation.
Fiber Cable testers can determine whether or not this is the case.
Check for Split Pairs
A split pair is a wiring error where two connections that are supposed to be connected using the two wires of a twisted pair are instead connected using two wires from different pairs
If you buy your cables precut, you won't have this problem.

Common Hardware Tools

Loopback Adapter (Plug)
A Loopback Adapter can be used to loop an Ethernet interface's transmit back to its receiver. This does not loop the data back on an Ethernet interface. Loopback adapters are only useful to verify that the port's transmit and receive electronics are working
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Wire-Map Tester A wire-map tester is a device that transmits signals through each wire in a copper twisted-pair cable to determine if it's connected to the correct pin at the other end. Wire mapping is the most basic test for twisted-pair cables because the eight separate wire connections involved in each cable run are a common source of installation errors.

A Cable Tester a device used to test the strength and connectivity of a particular type of cable or other wired assemblies.

A Time-Domain Reflectometer (TDR) is a tool that finds and describes faults in metallic cables like twisted wire pairs and coaxial cables. The equivalent device for optical fiber is an optical time-domain reflectometer (OTDR).
Works similar to a radar because it transmits an electric pulse along the conductor. If the pulse doesn’t reflect back, this means the cable did it’s job at terminating the signal completely.
If there are any pulse reflections sent back to the TDR source, something is not right.

An Optical Time-Domain Reflectometer (OTDR) is an optoelectronic instrument used to test the physical status of a fiber-optic cable. It works by putting out a series of optical pulses into the specific fiber you want to test.
It can also estimate fiber cable length, measure signal loss, and create accurate wire maps.
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A Multimeter includes features like the ability to measure voltage, current, and resistance. They can be used to troubleshoot electrical problems in a wide array of electrical devices like batteries, motor controls, appliances, power supplies, and wiring systems.
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A Cable Crimpers are primarily used for attaching ends onto different types of network cables via a process known as crimping. Crimping involves using your hands to apply a certain amount of force to press some kind of metal teeth into the inner conductors of a cable
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A Punchdown Tool is used for inserting wire into insulation-displacement connectors on punch down blocks, patch panels, keystone modules, and surface mount boxes.
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A Tone Generator is a simple copper cable tester that is simple to use and can be used to trace a wire in a wall. It is a two-piece unit that's basically a tone generator and probe, sometimes called a “fox and hound” wire tracer.
You will use a toner probe to locate a specific connection in a punch-down block.
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Taps can be used to monitor for intrusions and for packet captures during troubleshooting. Taps are only used for monitoring the data flow in a cable. A tap is a network hardware device that lets you passively insert test equipment in the traffic flow between two devices.
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Fiber Fusion Splicer When two fiber cables need to be spliced, a fusion splicer fuses the two cables together and minimizes the light scattering or being reflected back in the cable. They also ensure that the splice is strong and resistant to separation.
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Spectrum Analyzers are used to analyze wireless or radio frequency signals. Spectrum analyzers are primarily used to identify and measure the strength of radio signals that are present in the area.
They can visually display these signals by frequency on the device. These devices are used to locate sources of interference that may impact the operation of a wireless network.
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Fiber Light Meter A fiber light meter can tell you the receive levels and wavelengths to test and verify your fiber cable runs.
Cable Strippers portable handheld tool used by workers, especially electricians, for removing the protective coating of an electric wire in order to replace or repair the wire.

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