Post on 09-Apr-2018
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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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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
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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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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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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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
2. Marker at 387.3 ft
Gain increased to show peak
3. Marker at 595.1 ft
First good view of dip then peak
4. Marker at 188.9 ft
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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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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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
#
SIGNAL ACT/RDY ERROR
SIGNAL ACT/RDY ERROR
ST
UL
UT
PULSES E RR /A LMFRMSYNC
L1
L2
PATSYNC BITERROR RINGING
GRAPHIC
07:37:00> LOW LOW IDLE IDLE