Network Troubleshooting Study Guide for the CompTIA Network+

Connectivity Issues

Connectivity issues are common in the network environment and can be caused by logical or physical problems within the network. Given a scenario, you should be able to troubleshoot the problem and employ the use of the most appropriate tool for the situation.

Specifications and Limitations

The networking environment contains numerous physical components, such as cables, connectors, and devices. Each component has unique specifications for use and limitations.

Throughput

Throughput is the amount of data that is able to move through a network successfully at a given time. Throughput is dependent on multiple factors, both physical and logical. At the physical level, throughput is limited by the medium, such as the cabling type and networking devices, that is used for data transmission.

Speed

Network speed, also known as data transfer rate, refers to how quickly data is transferred between devices on a network. It is typically measured in megabits per second (Mbps). Network speed is dependent on the type of medium used as well as the distance between the sending and receiving devices.

Distance

Distance refers to the physical difference in location between a transmitting and receiving device. As distance increases, signal strength is reduced. The rate of reduction is dependent upon the medium used for the data transmissions. In general, fiber-optic cabling is able to transmit data to distances significantly farther than copper cabling and with less signal attenuation or degradation. Also, in general, the higher the speed, the greater it is affected by distance.

Cable Considerations

In a network environment, it is important to be aware of the specifications, proper usage, and limitations of different types of cables. When choosing which type of cabling to use in a specific situation, the location where the cable will be placed, the cost of the cabling, and regulatory compliance should be considered.

Shielded and Unshielded Twisted-Pair

Copper Ethernet cables can be either shielded twisted-pair (STP) cables or unshielded twisted-pair (UTP) cables. STP cables are twisted-pair cables that are contained within a foil or mesh outer layer to protect against electromagnetic interference. UTP cables are twisted-pair cables that are contained in a non-metal outer jacket.

Plenum and Riser-Rated

Plenum cable is a type of cable that is rated to be used in plenum spaces, the spaces that facilitate circulation for heating and air conditioning systems of a building or structure. Plenum cabling is sheathed in a highly fire-resistant material that is commonly made from low-smoke PVC or a fluorinated ethylene propylene (FEP) compound.

A riser-rated cable is a cable that is designed to traverse the space between floors or a building or structure. Riser-rated cables are sheathed in a flame-retardant material, commonly plastic, and are less expensive than plenum cables.

Cable Application

In a networking environment, the type of cable used is dictated in part by the application the cable will be used for. There are multiple common types or configurations of cables that can be used in different situations.

Rollover Cable/Console Cable

A rollover cable, also known as a console cable, is a common cable that is used for local connection of an interface to a networking device (e.g., a laptop to a switch). It is also used for troubleshooting and configuration. A rollover cable commonly has a serial port or Universal Serial Bus (USB) port connector on one side and an RJ-45 connector on the opposite. The wires of rollover cables are in opposite locations (they are “crossed over”) in the connectors.

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Crossover Cable

A crossover cable is a cable that switches the placement of the transmit and receive wires between the two end connectors. Crossover cables are used when two devices with the same transmit-and-receive configuration are connected to one another, such as two PCs or two switches.

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Power Over Ethernet (PoE)

A PoE cable is an Ethernet cable that has the ability to provide DC power as well as data to a connected device, such as an IP phone, security cameras, or Wi-Fi access point. PoE, as specified in the IEEE 802.3af standard, provides up to 15 watts of DC power, while power over Ethernet plus (PoE+), as specified in the IEEE 802.3at standard, provides up to 30 watts of DC power.

Common Issues

The backbone of a network is the cables that are used to connect devices at Layer 1, or the physical layer. When evaluating cable issues, it is important to know the specifications of the particular cable or interface being used in order to identify potential causes of common issues that may affect the network’s cabling and interfaces.

Attenuation

Attenuation is the degradation of a signal as it traverses a cable or wire. The rate of attenuation is dependent on the type of cable or wire being used, with copper attenuating more quickly than fiber-optic cabling.

Interference

Electromagnetic interference (EMI), also known as radio frequency interference (RFI) occurs when an electrical path or circuit is affected in a negative way by the electromagnetic field of another source. EMI and RFI only affect copper cables, as fiber-optic cables are immune to this type of interference.

Decibel Loss

The power of a signal sent through wires or cables is measured in decibels (dB). Attenuation, or dB loss, occurs naturally as a signal travels farther from the source of the signal on the wire or cable.

Incorrect Pinout

An incorrect pinout occurs when wires connected at one end do not correspond correctly to the connection point, or pin, at the other end. For example, for a straight-through cable connected to an RJ-45 connector, wires should be connected at pin 1 on one side and pin 1 on the other side, pin 2 on one side and pin 2 on the other side, and so forth.

Bad Ports

A bad port is when the port a cable is connected to is faulty, causing a failure in connectivity. A bad port may be identified through the device console or by testing the cable on a known working port.

Open/Short

An open circuit occurs when a signal cannot reach its terminating point to complete the circuit, either unintentionally through a break or intentionally. A short circuit occurs when an unwanted path is created in the circuit, possibly due to unintentional contact between wires.

Light-Emitting Diode (LED) Status Indicators

LED status indicators are visual signals on networking devices that indicate the status of connections on the device. The standard color coding for LED status indicators is green for good (with flashing indicating that traffic is being actively sent or received), amber for potential problems, and no light for no connection.

Incorrect Transceivers

A transceiver is located within a device and converts electrical signals to the correct Ethernet specification for communications with the other end. Incorrect transceivers occur when the transceivers on either end do not match. For example, a device using a 1 G copper interface needs a 1 G copper interface on the terminating end.

Duplexing Issues

Duplexing refers to how data is transmitted between devices. A half-duplex system can only transmit or receive data at one time. A full-duplex system can both transmit and receive at the same time. Most Ethernet interfaces currently in use are capable of supporting half- and full-duplex and determine which duplex setting to use with autonegotiation.

Duplexing issues occur when the duplex settings on the sending and receiving sides do not match, causing a reduction in communication efficacy. Duplexing issues are most commonly caused by one device being set to autonegotiation while the other is static.

Transmit and Receive (TX/RX) Reverse

To create a connection between two devices, the correct type of cable is required, either straight-through or crossover. This depends on the devices being connected, which dictates what pins are used for transmitting and receiving data. If the incorrect cable is used, the transmission and reception may be switched, a problem known as TX/RX reverse.

For example, connecting a switch and a PC requires a crossover cable, where pins 1 and 2 are used by the PC to transmit and pins 3 and 6 are used to receive, while pins 1 and 2 are used by the switch to receive and pins 3 and 6 are used to transmit. The TX/RX would be reversed if a straight-through cable was used, resulting in both the PC and switch using pins 1 and 2 to transmit and pins 3 and 6 to receive.

Dirty Optical Cables

Fiber-optic cables transmit signals by sending light down a clear glass or plastic fiber. If there is dirt or oil present on the end of a fiber-optic cable, it may result in signal attenuation and dB loss.

Common Tools

In the networking environment, there are numerous hardware tools that are used to install and troubleshoot networking devices and cabling. A network technician should be able to identify and use common tools in the appropriate situations.

Cable Crimper

A cable crimper, also known as a wire crimper, is a hand tool used to attach connectors onto network cables by exerting pressure, or crimping, the metal teeth contained in the connector onto the network cable.

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Punchdown Tool

A punchdown tool is a hand tool used to push, or punch, a wire down into an insulation displacement connector (IDC) to create contact between the wire and the IDC.

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Tone Generator

A tone generator, also known as a toner generator and probe or simply a wire tracer, is a device that is used to identify the path of a copper wire. . The tone generator portion of the tool is attached to a cable, either through a jack or directly to the wire with an alligator clip, and sends a signal through the cable. The probe is then touched to another portion of the cable, either directly onto the wire end or through the sheath. If it emits a tone, that indicates the signal is running through the touched cable.

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Loopback Adapter

A loopback adapter, also known as a loopback plug, is a small device that can be used to verify the transmit and receive signals from a port by sending the transmit signal back to the interface being tested.

Optical Time-Domain Reflectometer (OTDR)

An OTDR is an optoelectronic device that transmits light pulses through a fiber. These pulses are collected and measured to identify how the light is refracted, which can indicate fiber length, attenuation, and faults in the fiber.

Multimeter

A multimeter, also known as a multitester or volt/ohm meter (VOM), is a device that can measure multiple facets of electronic signals, including voltage, current, and resistance. A multimeter can be either analog or digital.

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Cable Tester

A cable tester is a hardware device that provides information on how a connected wire or cable is performing. Cable testers are available for both copper and fiber-optic wires and, depending on the specific device, can provide information such as grounding status, configuration (straight-through or crossover), connectivity status, and performance.

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Wire Map Tester

A wire map tester is a hardware device that provides basic information on copper twisted-pair wires by sending a signal through each connected wire to verify the output pin at the opposite end. A wire map tester can identify issues such as open (broken or unconnected wires), transposed or crossed (wires connected to the wrong pin), shorts (wires or pins improperly touching), and reversed (wire pairs with mismatched polarity).

Tap

A tap is typically a dumb hardware device designed to test networks by inserting testing equipment, such as a network traffic analyzer, into the flow of traffic between two devices. Taps are only used for monitoring by receiving data and cannot insert frames into the data stream.

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Fusion Splicers

A fusion splicer is a hardware device that is used to connect, or splice, two fiber-optic cables together with minimum light scattering or reflection.

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Spectrum Analyzers

A spectrum analyzer is a hardware tool that identifies and measures signals in a tested area. A spectrum analyzer can be used for audio or radio frequency signals as well as to visually display the signals in the area, potentially identifying signal interference locations and affected channels.

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Snips/Cutters

Cable snips or cutters are hardware tools used to cut wires and cables. Snips are often included in a combination tool that also has a crimper and stripper on it.

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Cable Stripper

A cable or wire stripper is a hardware tool used to remove the protective covering or sheath off of a wire. Strippers have multiple stripping locations that can be used for different wire thicknesses.

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Fiber Light Meter

A fiber light meter, also known as an optical power meter, is a hardware device that identifies receiving levels and wavelengths on a fiber-optic cable.

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