N10-007 Given a scenario, troubleshoot and resolve common copper cable issues

Shorts

In addition to mis-wiring, other problems that can occur with cables (and that can be checked with a multifunction cable tester) include open/short faults (referenced by CompTIA simply as “open short”). An open fault means that the cables are not making a full circuit; this can be due to a cut in the cable (across all or some of the wires). A short fault means that the data attempts to travel on wires other than those for which it is intended; this can be caused by mis-wiring or a twist in the cabling at a cut allowing the bare wires to touch.

Opens

Any network segment may consist of a single continuous section of cable or be constructed from multiple cable sections attached through switches and other hardware. If multiple cable sections are used, it can result in impedance mismatches caused by slight differences in the impedance of each cable section.

Impedance refers to the total opposition a circuit or device offers to the flow of a signal, measured in ohms. All media such as twisted-pair cable has characteristic impedance. Impedance characteristics for twisted-pair cable include 100, 120, and 150 ohms. UTP typically has an impedance of 100 ohms, whereas STP has an impedance of 150 ohms. Mixing these two wires in the same cable link can result in an impedance mismatch, which can cause the link to fail. To help prevent impedance mismatch, use cable rated with the same impedance rating.

Incorrect termination (mismatched standards)

Any network segment may consist of a single continuous section of cable or be constructed from multiple cable sections attached through switches and other hardware. If multiple cable sections are used, it can result in impedance mismatches caused by slight differences in the impedance of each cable section.

Impedance refers to the total opposition a circuit or device offers to the flow of a signal, measured in ohms. All media such as twisted-pair cable has characteristic impedance. Impedance characteristics for twisted-pair cable include 100, 120, and 150 ohms. UTP typically has an impedance of 100 ohms, whereas STP has an impedance of 150 ohms. Mixing these two wires in the same cable link can result in an impedance mismatch, which can cause the link to fail. To help prevent impedance mismatch, use cable rated with the same impedance rating.

Cross-talk

Whether it’s coaxial cable or UTP, copper-based cabling is susceptible to crosstalk. Crosstalk happens when the signal in one cable gets mixed up with the signal in another cable. This can happen when cables are run too closely together. Cables use shielding to help reduce the impact of crosstalk. If shielded cable is not used, cables should be separated from each other. Crosstalk can also occur when one wire pair within the twisted-pair cable interferes with the signals on other wires. Crosstalk can be a result of insufficient cable shielding, disparity between signal levels in adjacent circuits, and twisted terminations at connection points. There are two types of crosstalk interference: Near End (NEXT) and Far End Cross Talk (FEXT).

EMI/RFI

EMI can reduce or corrupt signal strength. This can happen when cables are run too close to everyday office fixtures such as computer monitors and fluorescent lighting fixtures, elevators, microwaves, and anything else that creates an electromagnetic field. Again, the solution is to carefully run cables away from such devices. If they have to be run through EMI areas, shielded cabling or fiber cabling needs to be used.

Attenuation/Db loss

All media have recommended lengths at which the cable can be run. This is because data signals weaken as they travel farther from the point of origin. If the signal travels far enough, it can weaken so much that it becomes unusable.

The weakening of data signals as they traverse the medium is called attenuation. All copper-based cabling is particularity susceptible to attenuation.

When cable lengths have to be run farther than the recommended lengths, signal repeaters can be used to boost the signal as it travels. If you work on a network with intermittent problems and you notice that cable lengths are run too far, attenuation may be the problem.

Bad connector

One of the most obvious signs that connectors may be bad and causing a problem is when the network’s problems can be isolated to one location. Depending upon how bad the connectors are, you may have no connection, or a spotty connection that comes and goes at odd intervals.

Examine the connectors to look for loose wiring, broken tabs, and other physical signs of a problem. If the connectors appear questionable, change them. If the connectors do not appear questionable but the problem is indeed isolated to one run, consider bad wiring (discussed next) as a possible culprit.

Bad wiring

The bad wiring could be a patch cable (easy to replace) or the in-wall wiring (more difficult to replace). If you suspect wiring to be the faulty component, you can diagnose rather quickly by taking the device that is having trouble connecting to another location and/or bringing a working machine to this environment.

You can use a multifunction cable tester D to troubleshoot most wiring problems. You must check for cable continuity, as well as shorts (discussed next).

Split pairs

Most splits in a cable are intentional—enabling you to run the wiring in multiple directions with the use of a splitter. Depending upon the type of cabling in question, it is not uncommon for each split to reduce the strength of the signal. It is also not uncommon for splitters to go bad. You want to split the cable as few times as possible and check the splitter if a problem in a run that was normally working suddenly occurs.

Tx/Rx reverse

A straight-through cable (as the name implies, all wires run straight through and are the same on both ends) and a crossover cable. In a crossover cable, two pairs of the wires are reversed; these are the TX and RX pairs (transmit and receive).

A crossover cable is intended to be used in specific applications only (such as to directly network two PCs without using a hub or switch) and will cause problems when used where a straight-through cable is called for (as a general rule, in all fixed wiring).

Cable placement

The field of cable placement encompasses using the right cable in the right location. Incorrect cable placement such as using cable outside of wiring standards (running it too close to electrical devices, for example) can make it susceptible to problems with interference, attenuation, and overall usability.

Bad SFP/GBIC – cable or transceiver

On routers, Small Form-Factor Pluggable modules (SFPs) and Gigabit Interface Converter modules (GBICs) are often used to link a gigabit Ethernet port with a fiber network (often 1000BASE-X). Both SFPs and GBICs exist for technologies other than fiber (Ethernet and Sonet/SDH are usual), but connecting to fiber has become the most common use.

With either an SFP or GBIC, there is a receiver port (RX) and transmitter port (TX). These devices are static-sensitive as well as dust-sensitive. Care should be taken to not remove them more often than absolutely necessary to keep from shortening their life. After a module goes bad, they can be swapped for a new one to resolve the problem.