Copper Cabling Troubleshooting Handbook - Fluke · PDF file3 NETWORKSUPERVISION Troubleshooting Basics Copper cabling failures are most commonly caused by one of the following: 1

Copper CablingTroubleshooting Handbook

Table of Contents

Introduction 2

Troubleshooting Basics 3

Link Models 4

Finding Cabling Faults 5

Troubleshooting Flowchart 8

Escalation Procedure 10

Using Advanced Troubleshooting Diagnostics 12

Examples 13

Conclusion 16

2

Introduction

Today’s cabling installation professionals must know how to

troubleshoot and diagnose high-performance cabling systems.

Structured cabling systems have changed dramatically since the

TIA-568A and IS11801 cabling standards were published in

1995. The revised and updated standards no longer recommend

the Cat 5 cabling system – and instead support Cat 5e or Cat 6

installations.

Why the need for advanced diagnostics?

These high-performance cabling systems must be tested and certified

in the field with new test parameters, new link definitions, more data

points, higher bandwidth, tighter margins, new connector types, as

well as patch cord requirements. The importance of component com-

pliance and excellent workmanship during installation has risen

accordingly.

Due to the increased complexity of these cabling systems, determin-

ing the cause of failure and quickly restoring suitable performance

has become a challenging task. This handbook has been written to

guide you through troubleshooting of advanced structured cabling

systems, so you can increase your productivity and deliver better

value to your organization.

3

N E T W O R K S U P E R V I S I O N

Troubleshooting Basics

Copper cabling failures are most commonly caused by one of the following:

1. Installation errors (especially wiremap and pair twisting; always keep the “original twist” in wire pairs as much as practical)

2. Bad connectors

3. Inappropriate tester set up

4. Bad cable

5. Bad patch cords*

6. Cabling effects not accounted for in the link model (e.g. reflected FEXT)

*Patch cords would be higher on the list, except that they are only tested in the channel

configuration, and the vast majority of tests are of the permanent link.

Before you begin testing, you should verify the basics:

• Has the appropriate autotest been selected?

• Has the correct link model been selected?

• Are you using the appropriate adapter with a plug that matchesthe jack in the TO or patch?

• Are you using the most current version of tester software?

• Is the NVP set correctly for the cable under test?

• Is the tester within its operating temperature range and in calibration?

User patch cords

OFFON

PASS

TEST

ING

FAIL

TALK

ING

LOW

BAT

TERY

TALK

Patch Panel TelecommunicationsOutlet

Beginning ofchannel

End ofchannel

ConsolidationPoint

SINGLETEST

AUTOTEST

OFF

MONITOR

SETUPPRINT

SPECIALFUNCTIONS

12

34

EXIT

FAULTINFO

TALK

TESTENTER

SAVE

4

Link models

For meaningful results, it is essential to choose the appropriate autotest

and link model. The Basic Link is now obsolete, and the majority of all field

tests should use the Permanent Link model. The impact of this change is to

move the reference point for the measurements from the tester interface to

the far end of the test cord (as shown). In a practical sense, this means

field testers must be much more sophisticated, because they must subtract

all of the test cord effects for each measurement. From an installer’s per-

spective, the change from Basic to Permanent Link also means a loss of

approximately 2 dB of NEXT margin at 250 MHz, which can lead to more

failures and marginal results on Category 6/Class E links.

When performing Permanent Link tests, take special care to ensure you are

using adapters that are appropriate for the cabling under test (especially

for Category 6/Class E links).

Channel measurements are typically done when restoring service, or to

verify cabling for application support. It is uncommon to perform channel

tests during normal installation, since the patch cords are rarely available

at that time. Correct channel measurements must cancel the effects of the

mated connection in the tester’s the channel adapters.

Permanent linkadapters

Patch Panel TelecommunicationsOutlet

Beginning ofpermanent link

End ofpermanent link

ConsolidationPoint

5


Finding cabling faults

For each of the required TIA and ISO structured cabling measurements, you

will find troubleshooting tips to help quickly pinpoint the cause of failures

when they occur. In some instances, you will find suggested reasons why

the measurement does not fail in cases you would expect to see a failure.

Wiremap

Length

Note: Standard practices dictate that length be defined by the length of the shortest pair.NVP varies per pair, meaning each pair could be reported with a different length. These twoconditions can result in a cable with three of four pairs over the length limit, yet the linkresult is a Pass (e.g. a channel with 101, 99, 103, 102 meters for the four pairs). A Pass inthis case is the correct interpretation.

Test Result Possible Cause of Result

Open • Wires broken by stress at connections• Cables routed to wrong connection• Damaged connector• Cuts or break in cable• Wires connected to wrong pins at connector or punch block• Application-specific cable (e.g. Ethernet using 12/36 only)

Short • Improper connector termination• Damaged connector• Conductive material stuck between pins at a connection• Damage to cable• Application-specific cable (e.g. factory automation)

Reversed pair • Wires connected to wrong pins at connector or punch block

Crossed pair • Wires connected to wrong pins at connector or punch block• Mix of 568A and 568B wiring standards (12 and 36 crossed)• Crossover cables used (12 and 36 crossed)

Split pair • Wires connected to wrong pins at connector or punch block


Length exceeds limits • Cable is too long check for coiled service loops and remove in this case• NVP is set incorrectly

Length reported is • Intermediate break in the cableshorter than known length

One or more pairs • Damage to cablesignificantly shorter • Bad connection

6

Delay/Skew

Insertion Loss (Attenuation)

NEXT and PSNEXT


Exceeds limits • Cable is too long Propagation delay• Cable uses different insulation materials on different pairs Delay skew


Exceeds limits • Excessive length • Non-twisted or poor quality patch cables• High impedence connections Use time domain techniques to troubleshoot• Inappropriate cable category e.g. Cat 3 in a Cat 5e application• Incorrect autotest selected for cabling under test


Fail *Fail, or *Pass • Poor twisting at connection points• Poorly matched plug and jack (Category 6/Class E applications)• Incorrect link adapter (Cat 5 adapter for Cat 6 links, or

incompatible Cat 6 adapter on Cat 6 links• Poor quality patch cords• Bad connectors• Bad cable• Split pairs• Inappropriate use of couplers• Excessive compression caused by plastic cable ties• Excessive noise source adjacent to measurement

Unexpected Pass • Knots or kinks do not always cause NEXT failures, especially on good cable

• Incorrect autotest selected (e.g. “Bad” Cat 6 link tested to Cat 5 limits)

• “Fails” at low frequency on NEXT graph but passes overall. When using the ISO/IEC standards, the so-called 4dB rule states all NEXT results measured while insertion loss <4dB cannot fail

Return Loss

ELFEXT & PSELFEXT

Resistance

7


Test Result Likely Cause of Result

Fail *Fail, or *Pass • General rule: troubleshoot NEXT problems first. This normally corrects any ELFEXT problems

• Service loops with many tightly coiled windings


Fail *Fail, or *Pass • Patch cord impedence not 100 ohms• Patch cord handling causing changes in impedence• Installation practices (untwists or kinks of cable – the original

twists should be maintained as much as possible for each wire pair)• Excessive amount of cable jammed into the Telecom Outlet box• Bad connector• Cable impedance not uniform• Cable not 100 ohm• Impedance mismatch at junction between patch cable and

horizontal cable• Poorly matched plug and jack (Category 6/Class E applications)• Use of 120 ohm cable• Service loops in telecommunications closet• Inappropriate autotest selected• Defective link adapter

Unexpected Pass • Knots or kinks do not always cause return loss failures, especially on good cable

• Incorrect autotest selected (easier to pass RL limits)• “Fails” at low frequency on RL graph but passes overall.

Due to the 3 dB rule, whereby all RL results measured while insertion loss < 3 dB cannot fail.

Test Result Likely Cause of Result

Fail *Fail, or *Pass • Excessive cable length • Poor connection due to oxidized contacts• Poor connection due to marginally attached conductors • Thinner gauge cable• Incorrect patch cord type

• Has the Cat 6 test been selected?

• Is the testers’ software up to date?

• Has the correct Link Adapter been used?

• Verify installation/termination quality

• Is it a channel test? Do you have the correct adapter?Must use Cat 6-rated channel adapter

• Is it a permanent link test?Draft 6+ limits will cause more NEXT failures than Draft 5 due to less margin available by switching to permanent link from basic link.

8

Overview troubleshooting flowchart

Category 6/Class E Failure

Note: High RL can causeNEXT failures via reflectedFEXT

Fail Length Check NVP/Cable setting

Fail Insertion Loss Check lengthon all pairs • If failure on only one pair:

re-punch that pair at both ends• Use time domain return loss

analysis or time domain reflectomerty (TDR) to look for events

Fail NEXT or PSNEXT Using OMNIScanner:• Perform S-Band diagnostics• Is connection within the bands?

If not, connection out of limits • If connection OK: - bad cord or

cable (see time domain plot)

Fail Return Loss Using OMNIScanner:• Perform S-Band diagnostics• Is connection in the bands?

If not, connection out of limits • If connection OK: bad cord or

cable (see time domain plot)

Still Fails? Go to Escalation Procedure

• Has a Cat 5e test been selected?

• Is the testers’ software up to date?

• Verify installation/termination quality

9


Category 5E/Class D Failure

Using DSP 4X00:• Perform HDTDX

– Locate fault and correlate NEXT event(s) with cabling to find source of failure

Using DSP 4X00:• Perform HDTDR

– Locate fault and correlate RL event(s) with cabling to find source(s) of failure

• Check for presence of service loops– If service loop present, remove tie wraps and spread out coils

10

Escalation procedure

1. Be able to answer “Yes” to the following questions:

• Do the cable and connectors used in the installation match the performance rating of the autotest selected?

• If a Category 6/Class E installation, are all components matched appropriately?

• Has the appropriate autotest been selected?

• Has the correct link type been selected?

• Are you using the appropriate adapter for this test?

• Are you using the most current version of tester software?

• Is the NVP set correctly for the cable under test?

• Is the tester at ambient temperature and in calibration?

• Have you carefully reviewed the installation quality of the terminations, re-punched where necessary, and ensured the cable is not snagged or kinked?

• Wiremap passes?

2. Does the link fail with no marginal results (those marked with a *)?

• If so, it is likely a bad component or cable, and any “tuning or tweaking” is not likely to get you to a Pass.

• Verify your results and methodology with your senior troubleshooting specialist. Use time domain analysis techniques to attempt to isolate the connection, cable, or patch cord as the source of failure.

• Save all test results with full swept frequency data and record the tester’s serial number and software version.

• Contact the appropriate supplier, share your test results with them, and work to resolve the problem.

11

3. Does the link fail with marginal results?

• You may be able to restore a PASS condition. Isolate the measurement(s) withthe * results. Find the worst-case result.

• If possible, physically inspect the location of the fault. Refer to “Finding CableFaults” above for tips on failure causes.

• The following suggestions may assist in improving the performance of a marginal link:

– Re-punch connections

– Replace connectors

– Replace patch cords (if a channel test)

– Remove or loosen any tightly binding cable ties

– Remove or loosen any service loops of cable in the wiring closet

12

Using advanced troubleshooting diagnostics

Fluke Networks’ certification testers include advanced troubleshooting diagnostics

tools. The DSP-4300 and OMNIScanner®2 provide a unique window into the cabling

link. If these tools are properly utilized, they can dramatically improve troubleshooting

productivity and help to restore service quickly.

While both are DSP-based digital testers, and both show both frequency and time

domain representations of the cabling, they approach troubleshooting from slightly

different perspectives. In order to get the most out of each tool, it is important to

understand these differences so that you can interpret the information displayed

correctly. While slightly different, each approach is equally viable.

Advantages of the DSP-4300 include:

• Fault Info button shows the failures by parameter and wire pair or wire pair combination as shown in example below

• Schematic diagnostic display provides easy to understand overview of problem

• Crosstalk vs. length means it is easier to isolate faults further down the cable

• Plots can be stored for later analysis

Advantages of the OMNIScanner2 include:

• S-Bands make it easier to see if connections meet connector standards

• NEXT vs. length means all anomalies are shown in proportion to the effect theyhave on the FAIL result

• NEXT phase information (useful for connector interoperability analysis) can beexported to PC-based applications

DSP-4300 OMNIScanner2

Schematic diagnostics display X

Shows crosstalk vs length X

Shows NEXT vs length X

Shows NEXT phase information X

Shows impedance vs length X

Shows return loss vs length X

Includes pass/fail S-Bands X

Time domain plots can be saved X

Magnitude and phase information can be exported X

DSP-4300 OMNIScanner2

13

To access advanced diagnostics on the DSP-4300, run an autotest and

press the “Fault Info” key. Alternatively, under “single tests” on the rotary knob,

select HDTDX for NEXT analysis or HDTDR for Return Loss analysis.

To access advanced diagnostics on the OMNIScanner2, run an autotest,

and then press the “doctor’s bag” icon (meaning “diagnostics”). Alternatively, from

the main menu, press the same icon (which is softkey #3) and select TDNXT for NEXT

analysis or TDRL for Return Loss analysis.

Examples

The following examples illustrate NEXT failures. Similar results can be seen using

Return Loss time domain diagnostic techniques. The first example is of the DSP-4300

display, followed by a comparison of the same NEXT cable fault using both the

DSP-4300 and the OMNIScanner2. The last example illustrates a faulty connection.

These examples should help to clarify the differences in appearance of faults and

diagnostics techniques of each product.

Example 1a: Category 6 link with good connection but bad cable

DSP-4300 schematic display suggestscable may be bad over distance shown.

14

Example 1b:

Both DSP-4300 and OMNIScanner2 time domain displays show NEXT at connection

is low, but cable has high NEXT throughout its length.

DSP-4300 OmniScanner2

On this magnified view,

note that both testers see

a high NEXT event nine

meters into the cable.

Note that the magnitude of NEXT events remains roughly constant on the DSP display

because it is showing crosstalk vs. length, while on the OMNI2 display the magnitude

is gradually attentuating because it is showing NEXT vs. length.

Example 2: Category 6 link with goodcable, but bad connection.

In this case, the DSP-4300 clearly shows the

largest event at 1.2 meters from the tester (which

you should know is the length of the adapter

cord). Note also a smaller event 3.7 meters down

the cable.

The OMNIScanner2 testing the same link with the

same personality module provides similar informa-

tion. You can see that the connector response

barely exceeds the allowable limit for Category 6

connectors (as indicated by the S-Bands), and

there is the same secondary event 3.7 meters

down the cable.

15

Maximum magnitude ofevent (~35% ov scale)

Cursor

Secondary event

Maximum magnitude of event (1.06times the Cat 5e connector limit)

Cursor

Secondary event at 3.7 meters

Positive NEXT indicatesinductive coupling

Negative NEXT indicatescapacitive coupling

S-Bands indicate connector limits. S-Band limits are angled in because

NEXT attenuates with distance

16

Conclusion

Despite good products and careful installation procedures, some failures are

inevitable when testing Category 5e and 6 cabling. By knowing the nature of

typical faults, and how they are represented in your testers’ display while

troubleshooting, you can significantly reduce network downtime and restore

service quickly. Taking the time to familiarize yourself with the capabilities of

your test tool will be a modest investment that pays for itself many times over.

For the latest information on cable testing standards, news, and issues, visit

www.cabletesting.com

17

Partner with Fluke Networks

Fluke Networks provides the most comprehensive line of premises network test

solutions for the inspection, verification, certification and documentation of

high-speed copper and fiber cabling systems.

Advanced certification for premium network testing

The DSP-4300 and OMNIScanner2 provide supe-

rior vision into high-performance cabling

systems. These advanced cable test solu-

tions certify and troubleshoot Cat 6/5e links

to 300 MHz using an extendible digital plat-

form that ensures unprecedented Level III accu-

racy, delivering repeatable accuracy no

matter how many times you retest a link. With

superior diagnostics, Cat 6 Permanent Link and

Channel support standard and comprehensive test results

management software, they are the preferred testers by

cable installers and network owners worldwide.

For more information about Fluke Networks’ copper and fiber cabling solutions

go to www.flukenetworks.com.


Fluke Networks, Inc.P.O. Box 777, Everett, WA USA 98206-0777(800) 283-5853 Fax (425) 446-5043

Western Europe00800 632 632 00, +44 1923 281 300Fax 00800 225 536 38, +44 1923 281 301Email: [email protected]

Canada (800) 363-5853 Fax (905) 890-6866 EEMEA +31 (0)40 267 5119Fax +31 (0)40 267 5180Other countries call (425) 446-4519 Fax (425) 446-5043

E-mail: [email protected] access: http://www.flukenetworks.com

©2002 Fluke Networks, Inc. All rights reserved. Printed in U.S.A. 10/2002 2029409 H-ENG-N A

Documents

Copper Cabling Troubleshooting Handbook - Fluke · PDF file3 NETWORKSUPERVISION Troubleshooting Basics Copper cabling failures are most commonly caused by one of the following: 1