3.4 Troubleshooting Network Media

Troubleshoot Copper Wiring Issues

Interference

EMI

Presence of a foreign electrical signal on a wire. Typically EMI, electromagnetic interference. Absorbed electromagnetic field results in the interference. This can lead to seeing CRC errors on an interface.

EMI can be caused by nearby devices that use electricity. Say near generators, or motors or even fluorescent lights

How to eliminate interference:

• use fiber optic cables
• use STP shielded twisted pair cables. typically uses foil sheathing.
• Use a drain wire, a wire that doesn't have plastic coating. EMI will be absorbed by the drain wire.
• Cat 6 and Cat 7 are typically used since they have STP

To protect against EMI/RFI:

• Use fiber optic instead of copper cables. Fiber optic cables are immune to EMI/RFI.
• Use shielded twisted pair cables. Shielded cables have a metal foil that encloses all the wires. Some cables might also include a drain wire that is a bare wire outside of the foil, but within the cable jacket. The drain wire can be grounded to help absorb EMI/RFI.
• Avoid installing cables near EMI/RFI sources.

Crosstalk

Interference that can result from signal sent on one pair inside the Twisted Pair getting absorbed by another.

Types of Crosstalk:

• Near End Cross Talk (NEXT), signal generated on one wire is measured on the other wires at the same end. It measures crosstalk that occurs at the same connector on different wires
• Far End Cross Talk (FEXT), we measure crosstalk that happens on the opposite end of where the transmission occurs.
• Alien Crostwalk: When a single wire bundle running parallel with another wire bundle picks up signal from the other bundle. This can be amplified if bundling cables together with a tie.

Sources of possible crosstalk:

• Plastic Sheath wearing Down
• Within connectors where twists are removed to add the connector

Preventing Crosstalk:

• Make sure all connectors are properly connected
• Use pre-made certified cables
• Keep wires twisted
• The tighter the twist, the better
• Cat 6 is twisted more tightly than Cat3

Power over Ethernet (PoE)

Power over Ethernet is a networking feature defined by the IEEE 802.3af and 802.3at standards. It describes any of several standard or ad-hoc systems that pass electric power along with data on twisted pair Ethernet cabling. Keep in mind the following:

• PoE technology is used on twisted-pair Ethernet cabling (CAT 5 or higher).
• Power is usually supplied by a PoE-enabled Ethernet switch.
• PoE is commonly used to power network devices that are located where physical access to a power outlet may not be available. For example, a PoE-enabled surveillance camera mounted on a tall pole can be powered via its Ethernet cabling.
• You can use a PoE injector to add PoE capability to regular non-PoE network links.
• PoE injectors can be used to upgrade existing LAN installations to PoE and provide a solution where fewer PoE ports are required.
• To upgrade a network connection to PoE, patch it through the PoE injector. Power injection is controlled and automatic.

Attenuation

Loss of signal strength over distance

Preventing Attenuation:

• Don't exceed the max distance allowed by network architecture
• Ex., Ethernet using copper, usually about 100 meters - this includes the full distance traveled
• As temps rise, it can get worse
• Use Repeaters to receive, regen, and amplify the electrical signal

Impedance Mismatch

Impedance is the measure of resistance within the transmission medium, represented by ohms. Problems can occur when the electricity hits the spot where the impedance changes. Where this occurs, it can reflect it back resulting in an Echo.

• Impedance is measured in ohms (Ω).
• All cables must have the same impedance rating. The impedance rating for the cable must match the impedance of the transmitting device.
• Impedance is mostly a factor in coaxial cables used for networking. Be sure to choose cables with the correct rating (50 or 75 ohm) based on the network type. Never mix cables that have different ratings.
• When signals move from a cable with one impedance rating to a cable with another rating, some of the signal is reflected back to the transmitter, distorting the signal (known as an echo). With video (cable TV) impedance mismatch is manifested as ghosting of the image.
• Cable distance does not affect the impedance of the cable.

Shorts/Open Circuit

When an electrical signal takes a path other than the one intended. Where a signal can hop to another wire. Remember, path of least resistance for electricity.

Open circuits where the wire has a break in it can lead to electrical signals not reaching to where it needs to go.

Miswiring

When wires don't match up correctly within a connector.

Reversal:

• Straight through instead of a crossover cable between two computers.
• Basically one end's transmit is the other's receive.
• This is known as a Reversal, when the connection won't work

Wiremapping:

• Determines whether wires connected to each pin are correct or not.
• When a wire isn't properly placed into the correct or matching pin.

Split Pair:

• Occurs when the pins between two pairs are crossed. When wires are crossed between pin pairs on both ends.
• Problem is when the signals aren't traveling down the correct wires which can lead to cross talk.

Troubleshooting Fiber Optic Wiring

Common Issues:

• Connector Issues
• Cable Issues
• Media Adapter Issues
• Attenuation

Connector Issues

Fiber inside needs to line up with the connector. Keep things clean, using lint-free cloth or specialized cleaning tools

Insertion Loss can occur whenever connector is used. Better the polish, the better light can pass through. We want to reduce Optical Return Loss (ORL).

Polishing Types:

• Physical Contact (PC), ends are polished with a slight curvature so that only the cores touch
• Super Physical Contact (SPC)/Ultra Physical Contact (UPC), used to reduce ORL reflections
• Angled Physical Contact (APC), 8 degree cut that prevents reflected light from traveling back into the fiber. These are colored green to prevent mixing with non-APC connectors.

Several issues can occur when you are working with fiber optic cabling.

Fiber optic cabling is much less forgiving of physical abuse than copper wiring. The fiber core is fragile and can be easily damaged by rough handling. For example, bending a fiber cable at too tight of a radius will break the core. Wavelength mismatch causes serious issues with fiber optic cables. You cannot mix and match types of cable.

• For example, if you connect single-mode fiber to multi-mode fiber, you will introduce a catastrophic signal loss of up to 99%.
• Even connecting cables of the same type that have different core diameters can cause a loss of up to 50% of the signal strength.

Signal Loss

This can be contributed to things like cable length, connectors, and splices. Measured in decibels (dB).

Several physical cable attributes can contribute to signal loss:

Cable length - While higher quality cables carry light signals further, the longer the cable, the more signal absorption and the greater the signal loss. Connectors - Every connector causes some level of signal loss, mostly due to reflection. While patch cables at each end of a run are to be expected, minimize any other connections. Splices - There are tools that you can use to splice a cut fiber optic cable. However, the signal loss from a splice is comparable to the signal loss from a connector. Bends - Micro bends in the cable due to things such as temperature change or manufacturing anomalies can cause signal loss. While you have little control over micro bends, even macro bends that can't be detected by the human eye can contribute to signal loss. The straighter the fiber optic cable, the less the signal loss.

Adding average losses of all losses within the cable to give an estimate of the attenuation that can occur is called a loss budget.

Connectors and Splices: 0.3 dB Multi-mode cabling:

• 3 dB loss per 1000 meters when using an 850 nm light source.
• 1 dB loss per 1000 meters when using a 1300 nm light source. Single-mode cabling:
• 0.5 dB loss per 1000 meters when using a 1310 nm light source.
• 0.4 dB loss per 1000 meters when using a 1550 nm light source.

Should be nore more than 3 dB less than the total power at the transmission source. Link loss margin less than 3 dB ensures the cable run will function.

Troubleshooting Tools

Spare Parts

One of the first things is to have spare parts.

• Change the drop cable that connects a computer to the network.
• Replace a NIC with a verified working NIC.
• Move a device from one switch port to another.

Loopback Plugs

Helps test ports to determine network or NIC issues.

If data sent is the same as the one received confirms that the device works.

• There are loopback plugs for both copper and fiber connections.
• A failure in the loopback test indicates a faulty network card.
• A successful loopback test means the problem is in the network cabling or another connectivity device.

You can purchase pre-made loopback plugs, or you can make an inexpensive one by cutting the end of a cable and manually connecting the transmit wires to the receive wires. To do this, connect the wire from pin 1 to the wire at pin 3, and the wire at pin 2 to the wire at pin 6.

Smartjack

A special jack on a WAN link near the demarc point. This tells us if the issue is tied into the service provider or on the site.

• Technicians at the central office can send diagnostic commands to the smartjack to test connectivity and performance between the central office and the demarc.
• When you contact a WAN service provider for assistance, the provider might execute a test using the smartjack.
• A successful test indicates that the problem is within the customer premises equipment (CPE).

Cable Tester

Used to test if cables function. Specifically to verify if the cable can carry a signal from one end to the other and all wires are in their correct positions. Sometimes includes a Timed Domain Reflector (TDR) - which helps measure the signal travel time on a wire run to measure the distance to the fault. An OTDR is used for testing fiber cables.

• High-end cable testers can check for various miswire conditions such as wire mapping, reversals, split pairs, shorts, or open circuits.
• You can use a cable tester to quickly identify a crossover and a straight-through cable.
• Most testers have a single unit that tests both ends of the cable at once.
• Many testers come with a second unit that can be plugged into one end of a long cable run to test the entire cable.

Cable Certifiers

Multi-function tool that validates a cable's specification. Certifier checks the physical compnents. Includes TDR and cable tester.

• A certifier is very important for Cat 6 cable used with bandwidths at or above 1000 Mbps. Slight errors in connectors or wires can cause the network to function at 100 Mbps instead of the desired 1000 Mbps (10 Gbps).
• Certifiers can also validate the bandwidth capabilities of network interface cards and switches. Many can detect the duplex settings of network devices.
• Most certifiers include features of a toner probe, TDR, and cable tester.
• Certifiers are very expensive and are typically used by organizations that specialize in wiring installations.

TDR/OTDR

A time-domain reflectometer is a special device that sends electrical pulses on a wire to discover information about the cable. The TDR measures impedance discontinuities (the echo received on wire in response to a signal on the same wire). The results of this test can be used to identify several variables:

• Estimated wire length.
• Cable impedance.
• The location of splices and connectors on the wire.
• The location of shorts and open circuits.

An optical time-domain reflector performs the same function as a TDR, but is used for fiber optic cables. An OTDR sends light pulses into the fiber cable and measures the light that is scattered or reflected back to the device. The information is then used to identify specifics about the cable:

• The location of a break.
• Estimated cable length.
• Signal attenuation (loss) over the length of the cable.

Tone Generator and Probe

Emits a tone along the wire where it gets picked up by the probe.

Multimeter

Helps measure electrical point difference across two points in an electronic circuit. A multimeter can measure several parameters:

• AC and DC voltage
• Current (amps)
• Resistance (ohms)
• Capacitance
• Frequency

Wiring Tools

• Wire Strippers

  • Wire strippers are rated to specific gauge (cable width) ranges.
  • Most wire strippers are combination tools. They can strip, cut, and crimp cables.
  • Almost all wire strippers have multiple holes or can be adjusted for specific cable sizes.

• Wire Cutters

• Wire Crimpers

Speed Test Sites

Transfers ICMP packets between a computer on the network and a speed test server to measure bandwidth. Helps Measure:

• Connection latency (ping)
• Download speed
• Upload speed

Tool	Description
Loopback plug	A loopback plug, or loopback adapter, reflects a signal from the transmit port on a device to the receive port on the same device. Use the loopback plug to verify that a device can both send and receive signals.

• There are loopback plugs for both copper and fiber connections.
• A failure in the loopback test indicates a faulty network card.
• A successful loopback test means the problem is in the network cabling or another connectivity device.

You can purchase pre-made loopback plugs, or you can make an inexpensive one by cutting the end of a cable and manually connecting the transmit wires to the receive wires. To do this, connect the wire from pin 1 to the wire at pin 3, and the wire at pin 2 to the wire at pin 6. | | Smartjack | A smartjack is an intelligent loopback device installed at the demarcation point for a WAN service. Key points are:

• Technicians at the central office can send diagnostic commands to the smartjack to test connectivity and performance between the central office and the demarc.

• When you contact a WAN service provider for assistance, the provider might execute a test using the smartjack.

• A successful test indicates that the problem is within the customer premises equipment (CPE). | | Known good spares | One valuable troubleshooting method is to keep a set of components that you know are in proper functioning order. If you suspect a problem in a component, swap it with the known good component. If the problem is not resolved, troubleshoot other components. Examples of using this strategy include the following:

• Change the drop cable that connects a computer to the network.

• Replace a NIC with a verified working NIC.

• Move a device from one switch port to another. | | Cable tester | A cable tester (line tester) verifies that the cable can carry a signal from one end to the other and that all wires are in the correct positions.

• High-end cable testers can check for various miswire conditions such as wire mapping, reversals, split pairs, shorts, or open circuits.

• You can use a cable tester to quickly identify a crossover and a straight-through cable.

• Most testers have a single unit that tests both ends of the cable at once.

• Many testers come with a second unit that can be plugged into one end of a long cable run to test the entire cable. | | Time-domain reflectometer (TDR) | A time-domain reflectometer is a special device that sends electrical pulses on a wire to discover information about the cable. The TDR measures impedance discontinuities (the echo received on wire in response to a signal on the same wire). The results of this test can be used to identify several variables:

• Estimated wire length.

• Cable impedance.

• The location of splices and connectors on the wire.

• The location of shorts and open circuits. | | Optical time-domain reflectometer (OTDR) | An optical time-domain reflector performs the same function as a TDR, but is used for fiber optic cables. An OTDR sends light pulses into the fiber cable and measures the light that is scattered or reflected back to the device. The information is then used to identify specifics about the cable:

• The location of a break.

• Estimated cable length.

• Signal attenuation (loss) over the length of the cable. | | Cable certifier | A cable certifier is a multi-function tool that verifies that a cable or an installation meets the requirements for a specific architecture implementation. For example, you would use a certifier to verify that a specific drop cable meets the specifications for 1000BaseT networking.

• A certifier is very important for Cat 6 cable used with bandwidths at or above 1000 Mbps. Slight errors in connectors or wires can cause the network to function at 100 Mbps instead of the desired 1000 Mbps (10 Gbps).

• Certifiers can also validate the bandwidth capabilities of network interface cards and switches. Many can detect the duplex settings of network devices.

• Most certifiers include features of a toner probe, TDR, and cable tester.

• Certifiers are very expensive and are typically used by organizations that specialize in wiring installations.

The following image shows some outputs you might see when using a cable certifier:

| | Toner probe | A toner probe is composed of two devices used together to trace the end of a wire from a known endpoint to the termination point in the wiring closet. To use a toner probe:

• Connect the tone generator to one end of the wire. It will send a signal on the wire.

• In the wiring closet, touch the probe to wires or place the probe close to wires. A sound at the probe indicates that the generated tone has been detected and the wire that you are touching is the termination point for the wire you are tracing. | | Multimeter | A multimeter is a device used to test various electrical properties. A multimeter can measure several parameters:

• AC and DC voltage

• Current (amps)

• Resistance (ohms)

• Capacitance

• Frequency | | Voltage event recorder | A voltage event recorder tracks voltage conditions on a power line. Basic recorders track only undervoltage or overvoltage conditions. More advanced devices track conditions over time and create a graph, saving data from a program running on a computer.

Some UPS systems include a simple voltage event recorder. Use a voltage event recorder to identify periods of low or high voltage that can adversely affect network components. | | Environmental monitor | An environmental monitor does what its name implies, it monitors the environmental conditions of a specific area or device.

• Monitors are often used to track the conditions within server rooms, such as temperature, humidity, water, smoke, motion, and air flow.

• Typically, computers (especially servers) have an internal monitor that measures fan speed and CPU temperature.

• Many monitors sound an alarm if a specified temperature or other environmental condition is reached. | | Wire stripper, snips, and crimpers | Wire strippers remove the protective sheath of a cable in order to expose the conductive wire.

• Wire strippers are rated to specific gauge (cable width) ranges.

• Most wire strippers are combination tools. They can strip, cut, and crimp cables.

• Almost all wire strippers have multiple holes or can be adjusted for specific cable sizes.

A crimping tool is used to attach connectors to wires. Some crimpers are designed for power connections. A special crimper is used to attach RJ45 connectors to twisted pair cables.

Snips are cutting tools used to cut cables or wires to a specific length or to remove damaged sections. A diagonal cutter is an example of a snip tool.

Whenever possible, use a wire stripper instead of snips to strip a cable. Wire strippers are specifically designed to cut only the protective sheath without cutting the internal wire. | | Speed test website | A speed test website is an online tool that is used to test the bandwidth of the internet connection. There are countless speed test websites available, all of them provide essentially the same information:

• Connection latency (ping)
• Download speed
• Upload speed | | Fusion splicer | A fusion splicer is a tool that uses heat to join two optical fibers together. It fuses them together end-to-end. This is done in situations when the cable is broken or too short for the purpose it's being used for.

The fusion splicer ensures that the two cables are fused together in an accurate and precise way to eliminate (as much as possible) any light being scattered or reflected back by the splice. The source of heat used by the fusion splicer is usually an electric arc, but could also be a laser, a gas flame, or a tungsten filament through which current is passed. | | Tap splicer | A tap splicer is a vinyl plastic wire termination device with a sharp metal insert that cuts through the plastic insulating jackets of two wires and crimps them together. You use it to make quick splices or connections using two or more pieces of wire within the recommended range of the tap splice. |