Troubleshoot Network Media- copper wiring Flashcards
Electromagnetic interference
(EMI)
An external signal that interferes with normal network communications. Common sources of EMI include nearby generators, motors (such as elevator motors), radio transmitters, welders, transformers, and fluorescent lighting. When working with the radio frequency spectrum, this is known as radio frequency interference (RFI).
Crosstalk
Interference caused by signals within twisted pairs of wires. For example, current flow on one twisted pair causing a current flow on an adjacent pair.
Attenuation
The loss of signal strength from one end of a cable to the other. This is also known as dB loss.
Electrical short
A situation in which an electrical signal takes a path other than the intended path. In the case of twisted pair wiring, a short means that a signal sent on one wire arrives on a different wire.
Open circuit
A condition that results from a cut in the wire preventing the original signal from reaching the end of the wire. An open circuit is different from a short in that the signal stops with an open circuit. Electricity cannot flow because the path is disconnected.
Electromagnetic interference
(EMI)
and
Radio frequency interference
(RFI)
Electromagnetic interference and radio frequency interference are external signals that interfere with normal network communications. Common sources of EMI/RFI include nearby generators, motors (such as elevator motors), radio transmitters, welders, transformers, and fluorescent lighting.
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
Crosstalk is interference that is caused by signals within the twisted pairs of wires. For example, current flow on one twisted pair causing a current flow on an adjacent pair.
You can reduce crosstalk between pairs by twisting wires into pairs. Each pair of wires is twisted at a different rate.
Crosstalk is often introduced within connectors where the twists are removed to add the connector.
Crosstalk can also occur where wires are crushed or where the plastic coating is worn.
There are several forms of crosstalk:
Near-end crosstalk (NEXT) is measured on the same end as the transmitter. For example, when a signal is sent on one wire pair, near-end crosstalk measures the interference on an adjacent wire pair at the same connector end.
Far-end crosstalk (FEXT) is measured on the end without the transmitter. For example, when a signal is sent on one wire pair, far-end crosstalk measures the interference on an adjacent wire pair at the opposite connector end.
Alien crosstalk is introduced from adjacent, parallel cables. For example, a signal sent on one wire pair causes interference on a wire pair that is within a separate twisted pair cable bundle.
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
Attenuation is the loss of signal strength from one end of a cable to the other. This is also known as dB loss .
The longer the cable, the more attenuation. For this reason, it is important to never exceed the maximum cable length defined by the networking architecture.
Cables at a higher temperature experience more attenuation than cables at a lower temperature.
A repeater regenerates the signal and removes the effects of attenuation.
Impedance
Impedance is the measure of resistance within the transmission medium.
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.
Throughput
Throughput is the rate that signals are processed and delivered over a communication channel. Some facts about throughput include the following:
Each circuit module has a specified link bandwidth and estimated throughput. Larger number designations usually mean more bandwidth.
Most modern wireless access points can be configured to run in the 5-gigahertz mode, 2.4-gigahertz mode, or mixed mode. The access point supports both standards, meaning that one radio runs 5-gigahertz, and the other runs 2.4-gigahertz. Be aware that:
Support for both modes is convenient because it supports a wide range of devices.
It also presents a problem because you can’t use a 2.4-gigahertz radio for your 5-gigahertz communications.
You can turn the 2.4-gigahertz off and use that extra radio and antenna for your 5-gigahertz clients and increase throughput using MIMO.
Another factor is the antenna used to transmit the signal. Just as there are differences between access points, there are also differences between antennas.
If you buy an access point with a high-quality antenna and you have a high-quality wireless network card, you’re going to get better range and better throughput.
If, you purchase inexpensive equipment with a low-quality antenna, then throughput is impacted and your range is reduced.
Short
An electrical short occurs when electrical signals take a path other than the intended path.
In the case of twisted pair wiring, a short means that a signal sent on one wire arrives on a different wire.
Shorts occur when two wires touch. This can be caused by worn wire jackets, crushed wires, or a metal object piercing two or more wires.
Open circuit
An open circuit is when a cut in the wire prevents the original signal from reaching the end of the wire. An open circuit is different from a short in that the signal stops. Electricity cannot flow because the path is disconnected.
Miswired cable
A miswired cable is caused by incorrect wire positions on both connectors. Several wiring problems might exist:
A reverse connection is a cable wired using one standard on one end and another standard on the other end.
This creates a crossover cable.
While this condition might be intentional, it can cause problems when a crossover cable is used instead of a straight-through cable.
This will reverse the transmit and receive match up; the transmit pins on one end maps to the transmit pins on the other end when they should map to the receive pins on the other end.
Wiremapping is matching a wire with a pin on one end with the same pin on the other end. For example, an error in the wiremapping results when the wire at pin 1 connects to pin 4.
A split pair condition is a single wire in two different pairs reversed at both ends.
For example, if instead of the solid green wire, the solid brown wire is matched with the green/white wire in pins 1 and 2.
With a split pair configuration, the cable might still work (especially if it is short), but it could introduce crosstalk.
When you follow the T568A/B standards for making drop cables, one pair is split to meet the standards. In this case, a common split pair error is placing all wire pairs in order in the connector instead of splitting the pair according to the standard.
When you connect cables using a punch down block, pairs are not split.
Incorrect termination
An incorrect termination occurs when an incompatible or incorrect connector is used. This can result in reduced performance or complete connection loss.
