CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 98 pdf

The Trimout PhaseIn the rough-in phase of cable installation, excess cable was left at both ends of the cable run.. Important aspects of the finish phase include these: ■ Cable testing ■

The Trimout Phase

In the rough-in phase of cable installation, excess cable was left at both ends of the

cable run These coils of cable, which are used to take up slack and facilitate later

changes, are known as service loops Service loops are discouraged by EIA/TIA

stan-dards It is not uncommon to have 1m (3 ft.) of ends hanging out of a wall jack at the

finish of the rough-in stage In the telecommunications room (TR), where hundreds of

cables are terminated, it is not uncommon to have 2m to 3m (6 ft to 10 ft.) of ends

Although this practice appears to be wasteful, experienced installers know that an

excess of cable provides more flexibility in cable routing and provides greater access

to cables when toning (testing) individual cables A common mistake of new installers

is to cut the cable short Remember, excess can always be cut off, but a short cable

can-not be extended If a cable is too short, the only alternative is to pull acan-nother cable,

and this is a costly alternative in terms of labor and time

If there is 1m (3 ft.) of cable coming out of the wall at the jack location, it is best to cut

this back to about 25 cm (9.8 in.) A new label should be applied to the cable about 15 cm

(5.9 in.) from the end The jacket then should be stripped back about 5 cm to 7 cm (2 in

to 2.8 in.), exposing the individual twisted pairs The completed jack termination should

have no more that 1.5 cm (.6 in.) of unjacketed conductor exposed and no more than

1.5 cm (.6 in.) of untwist in the cable pairs Excess conductor length should be cut off

at the final termination (see Figure A-40)

Figure A-40 Cutting Cable to Length

Lab Activity Terminating Category 5e to a 110-Block

In this lab, you learn how to terminate Category 5e cable to a 110-type termi-nation block, as well as how to properly use a 110 punchdown tool and a 110 multipunch tool

940 Appendix A: Structured Cabling

The jack is terminated with approximately 15 cm to 20 cm (6 in to 8 in.) of cable still protruding from the wall This excess cable is coiled carefully into the wall or wall box when the jack is installed This excess cable can be used to reterminate the jack at a later date or enable the removal of the faceplate and the addition of another jack to the outlet At workstation terminations, it is common for the wires in the jack to lose con-tact with the pins because the patch cord to the work area often is pulled, kicked, or stretched by the workstation users

Terminate or Punchdown

The termination of communications cables at a TR is referred to as punching down Cables also are punched down on termination panels mounted on wall fields and at the rear of cross-connect panels

Wires are inserted into the appropriate locations on termination panels, and then the punchdown tool is placed over the wires Depending on the type of termination hard-ware used, replaceable blades in the termination tool can changed out to accommodate the termination type (see Figure A-41)

Figure A-41 Removable Termination Blade

As pressure is exerted on the tool, spring tension increases to a point at which a firing-pin type mechanism releases the energy stored in the spring The wire instantly is forced between two insulation-displacement connections, and excess wire is cut off in the same operation The connection is referred to as insulation displacement because the insula-tion is pushed out of the way by the contacting points on the terminal

Insulation-displacement connections provide a secure, gas-tight connection, which means that the actual connection is not exposed to the atmosphere because the dis-placed insulation presses tightly against the block This is necessary to provide long-term corrosion-free connections Patch panels typically are used for data networks, as are 110-blocks, which also are used for voice applications

Wire Management

Some termination systems come with a wire-management scheme built in 110-blocks

use plastic troughs and spacers between blocks Troughs can be used both horizontally

and vertically Rack-mount installations incorporate a variety of wire-management

features (see Figure A-42) Some use a combination of D-rings and troughs

Figure A-42 Panduit Wire Management

When purchasing cable-management systems, consider the following:

■ The system should protect the cable from pinching and should maintain the

max-imum bend radius

■ The system is scalable, so when more cables are needed, it can handle them

■ The system is flexible, so cables can come into it from all directions

■ The system offers a smooth transition to horizontal pathways so that cable is not

damaged or exceeds maximum bend radius

■ The system is durable, so it will last as long as the cables and equipment mounted

on it

Careful Labeling

Labeling is another important part of a structured cabling system If cables are not

labeled clearly on both ends, there can be confusion TIA/EIA-606 specifies that each

hardware termination unit must have some kind of unique identifier This identifier

must be marked on each termination hardware unit or on its label When identifiers

are used at the work area, station terminations must have a label on the faceplate, the

housing, or the connector itself Most Requests For Proposals and specifications require

that labels be computer generated so that they are permanent, legible, and more

pro-fessional in appearance

942 Appendix A: Structured Cabling

Use labels that will remain understandable to someone who might work on the system many years in the future Many network administrators incorporate room numbers in the label information They assign letters to each cable that leads to a room Some label-ing systems, particularly those in very large networks, also incorporate color-codlabel-ing

To ensure that the labels do not rub off or get cut off (the end) later, mark the cable several times, approximately 60 cm (23.6 in.) apart, at the free end After the cable

is run, repeat the procedure at the box or spool end To keep all cables tied securely together, use electrical tape Bind the cable ends with the end of a pull string Ensure that the pull string does not come loose by tying some half-hitch knots around the cables with the pull string before taping the ends Do not skimp on the tape If the string or cables pull out later, it could cost time and money

After pulling the cable along the selected route, bring it into the TR Allow enough cable for the ends to reach all the way to each jack location, plus enough excess or slack to reach the floor and extend another 60 cm to 90 cm (23.6 in to 35.4 in.)

Go back to the spools of cable at the central point or TR Use the labels on each spool

as a reference, and then mark each cable with the appropriate room number and letter

Do not cut the cables unless they have a label If each of these steps is followed, the networking media used for the horizontal cabling run should be labeled at both ends

Finish Phase

The finish phase is the point at which installers test and, in some cases, certify their work Testing makes certain that all the wires route to their appointed destinations Certification is a statement of the quality of the wiring and connection

Important aspects of the finish phase include these:

■ Cable testing

■ Time domain reflectometer (TDR)

■ Cable certification and documentation

■ Cutting over Diagnostic tools are important in determining existing and potential problems or flaws

in a network cabling installation

Cable Testing

Cable testers are used to test cables for opens, shorts, split pairs, and other wiring prob-lems After the cable installer has terminated a cable, the cable should be plugged into the cable tester to verify that the termination was done correctly If a wire accidentally

was mapped to the wrong pin, the cable tester will indicate the wiring mistake Similarly,

it can test for problems with the cable, such as shorts or opens A cable tester should be a

part of every cable installer’s toolbox After the cable has been tested for continuity using

these cable testers, the cables can be certified by using certification meters

Testing is the most important step in the finish phase of cable installation Testing

veri-fies that all wires are working so that the customer does not find that there are problems

later It is better to catch a problem before it becomes a major issue

Tests relating to cable function are found in TIA/EIA-568-B.1 Common things to test

for include the following (see Figure A-43):

■ Opens—Wires in cables fail to make a continuous path from end to end This is

usually the result or improper termination or breakage Occasionally it is because of faulty cable

■ Shorts—Wires in cables touch each other, shorting the circuit.

■ Split pairs—Wires are mixed among pairs.

■ Wire-mapping errors—Wires in a multipair cable do not terminate at the

appro-priate contacts in the connector at the far end

Figure A-43 Wiring Faults Caused by Improper Termination

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

Split

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8 Wire-Mapping Errors

G/W G/W

O/W O/W

B/W

B/W

B/W = Brown/White

O/W = Orange/White

944 Appendix A: Structured Cabling

In most cases, simple functional testing for opens, shorts, split pairs, and wire-mapping errors are done from one end of the cable only

Testing for Shorts

A short is formed when the two wires in a pair touch each other, providing an undesired shortcut in the flow of signal (see Figure A-44) This shortcut is a completion of the circuit before the voltage reaches the intended target

Figure A-44 Wire Short

To determine whether there is a short, measure the continuity or resistance between the wires No continuity should be measured between them, and there should be an infinite amount of resistance between them Make these measurements with an ohm-meter using a low-resistance scale If a higher-resistance scale is used, the installer runs the risk of inadvertently measuring the installer’s own body resistance when the wires are held to the probes Some installers find it useful to create a small test fixture to avoid this problem Many test probes can be fitted with slip-on alligator clips They can hold one of the wires that so both leads are not touched at the same time

Testing for Reversals

A reversal occurs when the tip (or ring) side of a pair is terminated on the ring (or tip) position at the opposite end of the wire (see Figure A-45)

Figure A-45 Reversal

Short = Bare Cables Touch

1 2

1 2

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#

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!

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#

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&

To repair a reversed pair in a cable, the RJ-45 connector must be removed and the

cable end with the pair reversal must be terminated again

Testing for Split Pairs

Split pairs happen when wires are mixed among pairs (see Figure A-46) One way to

test for splits is with an ohmmeter First, test the pairs for shorts If none is found,

place a short across each pair When it is tested with an ohmmeter, finding a short is

the anticipated result If an open is found, something is wrong The pair is either split

or open A tone generator then can be used to determine which is the case Higher-end

testers detect split pairs by measuring crosstalk between pairs

Figure A-46 Split Pairs

A simple cable tester can be used to check for split pairs as well This type of tester

uses LEDs that immediately notify the installer if there is a problem with polarity or

continuity

To repair a split, one or both of the connectors must be removed and the cable end

must be terminated again

Time Domain Reflectometer

Atime domain reflectometer (TDR)works by sending a pulse down the wire and then

monitoring the electronic echoes that occur on the cable because of cable problems

TDRs determine whether there is a cable fault and, if so, whether it is an open or short;

they also determine the distance from the meter to the fault The signal is reflected

back when it reaches the end of the cable, as well as anytime it encounters a defect in

the cable along the way The speed at which the signal travels is known as the nominal

velocity of propagation This is a known quantity for different cable types When set,

1 2 3 6 5 4 7 8

1 2 3 6 5 4 7 8

Split

946 Appendix A: Structured Cabling

the tester knows how fast the signal travels and can measure the length of the cable by measuring the amount of time that it takes for the signal to be sent and reflected back

A TDR readout typically is calibrated in feet or meters This is an extremely efficient means of locating cable problems, although the instrument must be adjusted properly and used with skill

Cable Certification and Documentation

Testing is not the same as certification Testing is for functionality—that is, it determines whether the wire can carry the signal from end to end Certification, or performance testing, is a statement about cable performance It answers these questions: How well does the signal travel down the cable? Is the signal free from interference? Is the signal

of adequate strength at the other end of the cable?

Certification Tester

Certification goes beyond functionality testing Performance testing also must be done Structured cabling systems that adhere to installation standards are required to be certi-fied Certification meters perform all of the required performance tests to adhere to the ANSI/TIA/EIA-568-B standards (see Figure A-47) Meters have an autotest function,

so all required tests are performed with the touch of a single button These tests include near-end crosstalk (NEXT), wire map, impedance, length, DC loop resistance, propaga-tion delay, return loss, delay skew, attenuapropaga-tion, and attenuapropaga-tion-to-crosstalk ratio These meters hold multiple test results in memory Test results are downloaded to a computer

so that a test report can be generated and presented to the customer In addition to cer-tification, these meters include diagnostic features that not only identify problems, but also actually show how far these problems are from the end of the cable being tested

Figure A-47 Fluke Networks 4000 Cable Certification Meter

Performance testing usually takes place at a designated test frequency The frequency is

selected to exercise the cable at a speed that will be part of its intended operation For

example, Category 5e cable is tested at 100 MHz, and Category 6 cable is tested at

250 MHz Performance testing is described in various addenda to TIA/EIA-568-B

Modern testing hardware and software can provide both text and graphic output This

allows ready comparisons as well as analysis at a glance

The cable certification process forms a baseline measurement for the cabling system

When the contract is established, the certification standard to which the resulting job

must conform usually is included as part of the contract The installation must meet or

exceed the specifications for the wire grade that is being used Detailed documentation

showing that the cabling has reached these standards is submitted to the customer

The certification procedure is an important step in completing a cabling job It enables

the installer to say unequivocally that at a certain day and time, the cables performed

to certain specifications Any later change in cable performance must be attributable to

some cause, and it will be easier to figure out what that cause is if there is hard, fast

evidence of the cables’ condition at an earlier point Different grades of cable require

different minimum test results to be acceptable Generally, the higher the cable

cate-gory is, the tighter the manufacturing tolerances are, the higher the quality is, and the

better the performance is

Certification Tests

To pass certification, cables must meet the minimum test results for their grade Cables

must meet or exceed these specifications Actual test results that outperform the

mini-mum often are encountered The difference between the actual test results and the

minimum test results is known as headroom If the results show lots of headroom, less

cable maintenance should be needed in the future, and the network should be more

tolerant of poor-grade patch cords and equipment cables

The commonly used specifications include these:

■ Specified frequency range—Each cable is tested at a frequency range that it is

most likely to be used in daily service The higher the grade is, the higher this range is

■ Attenuation—The amount of signal that a cable will absorb is a measure of its

attenuation The lower the attenuation is, the more perfect the conductors are and the higher quality the cable is

■ Near-end crosstalk (NEXT)—Near-end crosstalk occurs when signals from one

pair interfere with another at the near end of the cable Crosstalk can affect the capability of the cable to carry data The amount of NEXT that a cable must be capable of tolerating is specified for each grade

948 Appendix A: Structured Cabling

■ Power Sum NEXT—In cables in which all the conductors are used (such as

Giga-bit Ethernet), the signals on one cable interfere with several pairs, not just one Calculating the effect of these disturbances requires that the interactions of all pairs in the cable be taken into account The power sum NEXT equation mea-surement does this

■ ACR—The attenuation-to-crosstalk ratio (ACR) is an indication of how much

stronger the received signal is when compared to the NEXT or noise on the same cable Sometimes this measurement is referred to as the signal-to-noise ratio (SNR) Be aware that SNR takes into account external interference as well

■ Power sum ACR—When all of the pairs in a cable are used, the interaction

between the pairs becomes more complicated This is because more wires are involved, meaning that there are more mutual interactions The power sum equations help take this greater mutual disturbance into account

■ Equal-level far-end crosstalk (ELFEXT)—Equal-level far-end crosstalk is a

calcu-lated measurement of the amount of crosstalk that occurs at the far end of the wire If this characteristic is high, it means that the cable is not carrying the sig-nals well and that the ACR (signal-to-noise) ratio is not well controlled

■ Power sum ELFEXT—As with the other power sum measurements, interaction

among multiple pairs in the same cable increase the complexity of equal-level far-end crosstalk characteristics The power-sum version of the measurements takes this into account

■ Return loss—Some of the signal traveling down a wire bounces off imperfections

such as impedance mismatches in the wire It can be reflected back toward the sender and can form a source of interference This is called return loss

■ Propagation delay—The electrical properties of the cable can affect the speed

at which signals travel through it The value of this delay must be known to perform certain measurements, such as time domain reflectometry Propagation delay for cable usually is specified as a maximum allowable amount of delay, in nanoseconds

■ Delay skew—Because each pair in a cable has a different number of twists, signals

that enter the cable at the same time are bound to be slightly out-of-sync when they get to the far end This lagging and leading of signals on adjacent pairs is called delay skew This problem can be heightened by sloppy termination, in which the cables are asymmetric with respect to the connector pins Finally, if there is a difference in propagation delay between the wires in a cable pair, it could affect the signal because of delay skew