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Field Testing ADSL - Sunrise Telecom, Inc.

March 2000 1

Field Testing ADSL Circuits

Reasons for Testing

Testing with Modem Emulation

Testing the Physical Layer

Line QualificationTroubleshooting Techniques

Objectives

1. List two reasons that service providers should test ADSLcircuits before attempting to install at the CPE.

2. List at least two single-ended physical layer tests that canbe performed by a fully optioned Sunrise SSxDSL tester.

3. Identify the most likely end of a subscriber loop to haveBridge taps (CO or CPE)

4. Identify the cursor and marker on the Sunrise SSxDSL

tester.

5. Identify the characteristic TDR signatures of a Bridge tap,short, open and loading coil.

6. Define the difference in physical connections used inBridge and term modes


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Why Test the Local Loop?

n For DSL Transmission, the local loop canbe very unpredictable

n Verify the system works before giving it tothe customer

n Troubleshoot performance problems

Central Office Customer

Premises

xTUxTU--CC

DD

SS

LL

AA

MM

x T Ux T U-- RR

Why Test?

ILEC must verify the pair is capable of running ADSL before

handing it over to the Customer or the CLEC.

Published Speeds by Modem Vendors reflect:Lab Results using line simulators

Theoretical limits

Line conditions vary

Cable Make-Up can include impairments including:

Bridge Taps, Load Coils, Noise, etc.

Cable records may not be accurate

Recording performance of a loop when ready for installationprovides a record of the cable pair for later reference.


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March 2000 3

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The SunSet xDSL

ADSL

ATU-R

TDR/LINE

/DMM

E S C A P E E N T E R

F1 F2 F 3 F 4

BA T T E R Y

FRA ME A L A R M

x D S L xTU- C xTU- RP O W E R

F 1 F2 F 3 F 4

R E S Y N CERR INJ

A U T OHIST O R Y

1 2 3

4 5

S H IF T

7 8 9

*

0

6

A B C D

P O W E R

#

D M M T D R LINE

T 1 / E 1 S I G L P 1 S Y N C L P 2 S Y N C

A I S

E R R O R S P A T S Y N CB P V / C O D E

H OL D R E S P O N D B IT E RR

G R A P H I C

Su n Se t xD SL

SunSet xDSL Applications

Single Set Testing

DMM

Loop ResistanceTDR

Load Coil Detector

PSD Background Noise Measurement

ATU-R Emulation

Dual Set Controller & Responder Testing

PSD Insertion Loss

Loop ResistanceFrequency Generator and Level Meter


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Verify Connectivity to DSLAM

ATU-C

ATU-C

ATU-C

ATU-C

SPLITTER

ESCAPE ENTER

S u n S e t I

F1 F2 F3 F4

BATTERY

P UL S ES E RR /A LM

SIGNAL ACT/RDY ERROR POWER

F1 F2 F3 F4

R ES YN C E RR IN J

AUTO HISTORY

1 2 3

4 5

SHIFT

7 8 9

*

0

6

A B C D

POWER

#

SIGNAL ACT/RDY ERROR


ST

UL

UT

PULSES E RR /A LMFRMSYNC

L1

L2

PATSYNC BITERROR RINGING

GRAPHIC

07:37:00> LOW LOW 5 M ; R/G > 5 M

Otherwise, ground exists in circuit

Tip to Ring > 5 M

Otherwise, short exists in circuit

Use DMM-OHM Meter to measure

Use a TDR to locate short

Acceptable DSL Loop Resistance (far end short)

HDSL: 900 maximum

ADSL: 1300 maximum

Capacitance (with far end open) checking for a Balanced Pair

T/G should be about equal toR/G MeasurementOtherwise, the pair may not be balanced

Use DMM-CAP Meter to measure


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Physical Layer: DC and AC

POTS Voltage: Power Influence:

Check for presence of POTS voltage

Tip to Ring +48 VDC

Ring to Ground -48 VDC

DMM-DCV Meter

Check for power influence

60 Hz AC induced voltage (T/R) from adjacent power

lines

AC Volt Meter


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March 2000 12

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TDR Signatures

Open/Load Coil

Bridge Tap

Short

How a TDR Works:

Launches a pulse and measures the time of its reflection

Reflections are caused by changes in cable impedance

Cable Faults cause these impedance change

Determines distance based on the Velocity of Propagation

VP = speed electricity in cable / speed of light

VP of 0.50 means electricity travels at .5 speed of light

The Polarity of the reflection reveals what type of Fault is on theCable

Open or Load Coil (Upwards Spike)

Short (Downwards Spike)

Bridge Tap (Downwards Spike followed by an Upwards Spike)

The use of a TDR is not an exact science, its an artform


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March 2000 13

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Using the TDR Function

E S C A P E E N T E R

F1 F 2 F3 F4

BA T T E R Y

FRAME A L A R M

x D S L xTU- C xTU- RP O W E R

F1 F 2 F3 F4

R E S Y N CERR INJ

A U T OHIST O R Y

1 2 3

4 5

S H IF T

7 8 9

*

0

6

A B C D

P O W E R

#

D M M T D R LINE

T 1 / E 1 S I G L P 1 S Y N C L P 2 S Y N C

A I S

E R R O R S P A T S Y N CB P V / C O D E

H OL D R E S P O N D B IT E RR

G R A P H I C

Su n Se t xD SL

Function Keys

Display shows function per screen

Uses:Checking for Excessive Bridge Taps

Locating Loading Coils

Finding Loop Length

Arrow Keys

Up, Down: Gain

Left, Right: Cursor

TDR Setup Screen

The TDR must be configured to provide accurate distance results:

Gauge: Select the gauge of wire in use

VP: Velocity of Propagation (defaults per gauge selected)

AVG: Number of pulses to average before a screen update

TDR Function Keys

There are a Number of Function keys on our SunSet xDSL Test

Set that control the TDR. They are:

Page_Left, Page_Right

Shifts the display on the screen half of the total distanceshown either to the left or to the right

Zoom_In, Zoom_Out

Zooms In half of the distance shown on the screen, orZooms Out twice the distance shown on the screen

Centers on the Cursor (Solid line)

[nnnn] at lower left and right the span currently viewed

+Offset, -Offset

Moves the pulse line up (+) or down (-) the screen display

O: Display shows current offset (bottom right)


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March 2000 14

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TDR Display

MarkerCursor

O: Offset

H: Horizontal (Zoom)

V: Vertical (Gain)

DISTANCE

TDR Display

The TDR Screen includes a number of useful tools anddimensions.

Cursor and Marker work together. They can be moved

independently to help locate faults, and the length of the fault.Cursor: Solid blank vertical line

Marker: Dotted vertical line

DISTANCE: Feet from 0 (TDR) to Cursor

MARKER: Feet from Marker to Cursor

O: Offset - from reference Positive = higher, Negative = lower

H: Horizontal Shows Zoom factor ( 1 512)

512 shows the entire cable span


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Bridge Taps

n From lots of testing in the field, Bridge Taps

have been found to be the most serviceaffecting for ADSL Circuits.

n The shorter the Bridge Tap, and the closer

the Bridge Tap is to the customer, the moreservice affecting it is. Why?

Central Office Customer

Premises

xTUxTU--CCD S L A MD S L A M x T Ux T U-- RR

B r i d g e T a pB r i d g e T a p

Bridge Taps

Unused and Unterminated lines to customers

Additional signal paths are OK for POTS but can hinder ADSL

transmission

The shorter the Bridge Tap and the Closer it is to either Modem,the worse they affect service

Bridge Taps under 120 may not impact service

Metallic Testing Criteria:

Bellcore TA-NWT-001210 (HDSL)

Sum of all Bridge taps must be < 2500 feet

No single Bridge tap can be > 2000 feet

Use the TDR to locate bridge taps


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Aerial Cable Bridge Taps

n Aerial Cable: This is the most problematic type

of cable for ADSL, and probably the mostprevalent. Why?

B-Box/

CrossBox

Aerial Cable Challenges

Existing cable installed on poles are the most common source ofBridge taps.

A bundle of pairs is normally installed from the B-Box across a

series of poles to service a number of customers.Drops are installed as customers order service

Many drops begin mid-span in the cable

Remaining length of cable in the bundle is not cut or

terminated

Instant Bridge Tap!

Strategies

Cut the wires in the bundle at the drop

Limits available pairs at the end of the cable

Not repaired when customer terminates/moves

Terminate pairs with special signal traps

Still experimental

Suggest that the customer move to the end of the street

Not likely


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TDR Techniques

n There are many methods available for

using the TDR:Manual

l Old (pre 1.03) Auto Gain Feature

l Examine Sections (Zoom and Pg Right)

Automatic

l Auto Search (1.03 and above)

Scans cable in increasing increments

Finds first fault

l Load Coil Detection

The next few slides will discuss these techniques


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TDR: Auto Gain Feature

n A Bridge Tap about 800 ft. away, and 250 feet long

1. 2.

3. 4.

Auto Gain Feature (pre-1.03)

An example of moving the marker and increasing the gain

1. Marker at 188.9 ft

Slight peak near the center of the screen


Gain increased to show peak


First good view of dip then peak


Cursor (dotted line) at 835.9 feet

Shows bridge tap clearly

Limitations

The Auto GAIN function could affect the way the screendisplayed the fault on the page, or seem to make faultsdisappear


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March 2000 19

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TDR: Zoom In and Zoom Out

n The same Bridge Tap at different Zoom References

1.

3.

2.

4.

Zoom Feature

An example of moving the marker and increasing the gain

1. Move Cursor to the area of interest

2. Screen shows from 10 to 13763 feet across

Sharp peak near the left of the screen

3. One Zoom (right end now half the distance 6886 feet)

Cursor and Marker stay relatively constant

4. Two Zooms (right end now half again 3448 feet)

First good view of dip then peak

5. Three Zooms (right end now half again 1729 feet)

Shows bridge tap clearly at about 835 feet

6. Adjust cursor to estimate length if necessary

PG_RGT

For a closer look at small segments, Zoom In, or start with a selectedspan (870 for example)

Use Gain (up arrow) to look for signatures if necessary

PG_RGT will move half screen width per press (about 435)


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Auto Search Feature

Before Search Found at 970.2 feet

Auto Search Feature

The new xDSL Software (1.03) has an Auto Search function whichallows the set to search for the first fault on the cable pair

Warning: This function is not fail proof. There may be times that

it will not detect a fault, and possibly times that it will detectsomething when there is no fault.

It is still up to the techs determination and interpretation if thefault exists or not, so manual control of the TDR is very importantas well.

SEARCH function Operation

The SEARCH key, F3, is present on every screen in the TDR

You will get a message that says Searching in the middle of

the screen, and if a fault is found, it will show FOUND in the topright hand corner of the screen

If no fault is found, then it will say No Fault Detected, and returnto the initial screen


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Finding Multiple Faults

1.

3.

2.

4.

Bridge Tap at 1600 Open at 6800

Finding Multiple Faults

This shows an example of how to find multiple faults

1. A Bridge Tap at 1600 ft. that is about 400 ft. long

2. An Open on the end of the Cable at about 6800 ft. only using

the ZOOM_OT


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Finding Multiple Faults (2)

Bridge Tap at 1600

Open at 6800

Multiple Faults (another example)

This example shows the same circuit, but instead of using theZOOM_OT key, I used the PAGE_RT key to simply look at thenext section of cable. Then I added a ZOOM_OUT hit of GAIN

when I saw the Open which was the end of the cable.


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March 2000 23

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TDR and Load Coil Detection

1. Detection

2. Locate with TDR

WARNING:Disconnect DSLAM First

What are Loading Coils?

As loop length increases, natural capacitance between tip, ring, andground builds up.

AC signal quality falls apart at higher voiceband frequencies

Counteract excess capacitance by applying inductance to loop

Load coils are inductorswhich reduce attenuation at the upper voice band

Provides a flat loss characteristic over 300 to 4000 Hz band.

Severely attenuates signals at frequencies above 4 kHz

ADSL transmission fails with load coils

TDR and Load Coil Detection

Load Coils are placed regularly in long POTS circuits

1st coil is ~ 3000 ft. from local COAdditional coils every 6000 ft.

Last coil between 3000 to 9000 ft. from customer

So, a home that is 20,000 ft. from the CO may have 3 load coils, ormore, on the cable pair.

Load Coil Detector verifies load coil presence

TDR provides distance to load coil for removal


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DSL Interferers

Center Frequency (kHz)

DDSISDN

HDSL

T1

28 40 196 772

AM Radio560+

Identifying Potential Interferers

Interferers are major barriers of xDSL transmission

Sources are:

Other digital services (T1, ISDN, HDSL)

Ambient Noise: AM radio, electrical equipment

Crosstalk can occur at common frequencies if signals close toeach other

Proximity in cable bundles to interferer sources can causecrosstalk

Some installations attempt to provision non-interferingservices by bundle.

AM Radio Transmits above 600k Hz, can induce noise into cablebundles when transmitters are nearby cable plant, or when

equipment is not properly groundedCrosstalk can occur at common frequencies if signals close to

each other

Power Spectral Density (PSD) Background Noise Measurement usedto detect interferers

Measures noise in 4 kHz to 1.6 MHz band

Covers both DMT (to 1.1Mhz) and CAP (to 1.6Mhz) ranges


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Background Noise Screens

Bridge Mode with Template

Term Mode

Background Noise Screens

On the left is the BG Noise running in BRIDGE Mode and on the right itis in TERM Mode

Bridge Mode:

You are Bridge onto an active circuit terminated by otherdevices

Term Mode:

You are one of the terminating devices

The frequency spectrum is scanned up to 1.6Mhz

ADSL DMT only uses to 1.1 KHz

ADSL CAP uses up to 1.6Mhz

Masks and Templates

You have different Masks for Bridge Mode and templatesfor TERM mode

G.dmt ATU-C, G.dmt ATU-R,

ETR 152 CAP: 1-Pair and 2-Pair

ETR 152 2B1Q: 392k, 584k, 1160k

CLASS 1 PSD, CLASS 2 PSD, CLASS 3 PSD

ATU-C Tx, ATU-R Tx


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Controller & Responder

E S CA P E E NT E R

S u n S e t I

F1 F2 F3 F4

BATTERY

P UL S ES E RR /A LM

SIGNAL ACT/RDY ERROR POWER

F1 F2 F3 F4

R ES YN C E RR IN J

AUTOHISTORY

1 2 3

4 5

SHIFT

7 8 9*

0

6

A B C D

POWER

#



ST

UL

UT

PULSES E RR /A LMFRMSYNC

L1

L2

PATSYNC BITERROR RINGING

GRAPHIC

07:37:00> LOW LOW IDLE IDLE