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DSL Troubleshooting: Intermittent Faults
CommTech14 May 2009
Jim Wahl Aware, Inc.
Slide 2
AgendaAgenda
1. Identifying Intermittent Troubles Lines change Stability metrics
2. Troubleshooting Impulse Noise Do I really have an impulse noise problem? What can I do about it?
3. Troubleshooting Other Noise DSLAM-based noise mitigation techniques? When is dispatch-required?
Slide 3
Aware Inc.Aware Inc.
• Background 1986: Biometrics and Image Compression
Products• Fingerprint compression, facial imaging OEM products
1993: DSL Intellectual Property Licensing• Silicon customers: Infineon, Ikanos/ADI
2002: Test and Diagnostics Products • Hardware and Dr. DSL diagnostics software
• Products Test Equipment OEM:
• UDMT (ADSL2/VDSL2 CO/RT) modem modules• with Dr. DSL diagnostics
Dr. DSL Line Diagnostics Platform Technician Dr. DSL
• Diagnostics Customers TELUS, Spirent, Tollgrade, 3M, JDSU, Fluke,
Sunrise, Occam, Alcatel, and many others. Bedford, MA Corporate Office
Slide 4
DSL and Data RateDSL and Data Rate
• DSL data rate is based on the "space" between the signal attenuation and the noise floor.
• This is the signal-to-noise ratio (SNR).
• The greater the area between the signal and noise floor, the higher the SNR, and the higher data rate.
Slide 5
Loop Length and SNR / Data Rate Loop Length and SNR / Data Rate
7,000ft loop
6,000ft loop
Slide 6
Noise and SNR / Data Rate Noise and SNR / Data Rate
Slide 7
Intermittent DSL TroublesIntermittent DSL Troubles
Changes in crosstalk levels:
Slide 8
Count of Impulses per HourCount of Impulses per Hour
Test Results by Hour
0
5
10
15
20
25
30
35
40
20:00 21:00 22:00 23:00 0:00 1:00 2:00 3:00 4:00 5:00 6:00 7:00
Hour of Day
Nu
mb
er
of
Imp
us
les
Moderate to low amount
of impulses throughout
the night
Slide 9
Count of Impulses per HourCount of Impulses per Hour
Test Results by Hour
0
20
40
60
80
100
120
140
1:00 2:00 3:00 4:00 5:00 6:00 7:00 8:00
Hour of Day
Nu
mb
er
of
Imp
uls
es
Very quiet in early
morning hours with large
jump beginning at 5am
and lasting until 8am.
Slide 10
Count of Impulses per HourCount of Impulses per Hour
Test Results by Hour
0
10
20
30
40
50
60
2:00 3:00 4:00 5:00 6:00 7:00 8:00 9:00 10:00 11:00 12:00
Hour of Day
Nu
mb
er o
f Im
pu
lses
Moderate level of
impulses with three
highest totals at 3am,
7am, 11am
Slide 11
What’s a “bad” DSL line?What’s a “bad” DSL line?
• Depends on service. … High-speed internet service vs anything else. IPTV dramatically increases error-free QoS requirement. VoIP increases low-latency QoS requirement.
• PHY-layer IPTV Benchmarks Example 1: < 1 resync / week; 1 CRC / hour. Example 2: < 1 resync / day; 1 CRC / 4hrs. BBF WT-126: < 1 CRC / 30 min for SD; < 1 CRC / 4hrs for HD.
• Why the Discrepancy? “QoE” dependent on many factors, including content and
compression ratios. PHY-layer errors can be corrected or mitigated at higher
layer. Some packets (I- and P-frames) more important than others
(B-frames).
Slide 12
What’s a “bad” DSL line?What’s a “bad” DSL line?
One that a generates poor customer experience.
• If a customer hasn’t complained, yet. … Identify lines that are likely to generate complaints. To identify these lines, we need metrics.
• If a customer is already complaining. Improve service. To measure improvements, we need metrics.
Slide 13
Key Stability MetricsKey Stability Metrics
• MTBE (Mean Time Between Error) Average time between two CRCs (or Code Violations). Expressed in seconds or bits.
• MTBR (Mean Time Between Resyncs) Average time between resyncs. Excludes user-generated events (diagnostic test,
consumer powering off modem, …).
• BAT-V (Bit Allocation Table Variation) Average number of minor and major “bit-swap” events. A general metric for how much the underlying physical
layer is changing—more coming. …
All of the above metrics are calculated non-intrusively.
Slide 14
Where do I get them? Where do I get them?
•Not Test Equipment Stability metrics are collected over time. Test equipment provides snapshots.
•DSLAM? Partly. DSLAMs have the raw data, but not the analysis.
•Analysis Tools Collect data from multiple DSLAMs. Normalize and calculate metrics. Compare to other lines in node, neighborhood, network. Categorize: 1) DLM, 2) Dispatch, 3) Downgrade, 4)
Upgrade
Slide 15
Troubleshooting Faults
Slide 16
Sources of Intermittent FaultsSources of Intermittent Faults
• Impulse noise• External or in-home
• ADSL Crosstalk• T1 Crosstalk
• AM Radio (RFI)
1
1
2
2
3
3
4
4
• Corroded splice• Resistive fault
4
Slide 17
Impulse Noise
Slide 18
Impulse NoiseImpulse Noise
• Problems it causes: Video pixelization Audio hiccups
• Possible sources: Rotary tools
• Drills, power sanders Household appliances with motors
• Electric toothbrushes, older printers, garage door openers, older refrigerators, sewing machines.
Outdoor heavy equipment• Construction machinery
Slide 19
Possible SolutionsPossible Solutions
• Moving the modem away from suspected interference sources Problems:
• Not always possible to relocate modem• Interference may permeate throughout location
• Error Correction (INP, PHY-R, Delay) Problems:
• Introduces latency• Adds overhead data, reducing core line data rate• Often trial-and-error in the DSLAM profile.
Slide 20
Impulse Monitoring Sample #1Impulse Monitoring Sample #1
QLN
Slide 21
Impulse Monitoring Impulse Sample #1Impulse Monitoring Impulse Sample #1
Detected Impulse
Lasts about 50
microseconds
Peaks at more than
6mV
Slide 22
Impulse Monitoring Impulse Sample #2Impulse Monitoring Impulse Sample #2
Impulse affects a large portion of
the ADSL2 spectrum
QLN
Slide 23
Impulse Monitoring Impulse Sample #2Impulse Monitoring Impulse Sample #2
Impulse
Slide 24
Impulse Monitoring Impulse Sample #3Impulse Monitoring Impulse Sample #3
An impulse affecting the beginning and
middle of the ADSL 1 band
QLN
Slide 25
Impulse Monitoring Impulse Sample #3Impulse Monitoring Impulse Sample #3
Brief impulse
Slide 26
Impulse Monitoring Impulse Sample #4Impulse Monitoring Impulse Sample #4
Affects a large area
of the spectrum
QLN
Slide 27
-25
-20
-15
-10
-5
0
5
10
15
20
25
30
0 50 100 150 200 250 300
Impuls
e S
ignal [m
V]
Time [microseconds]
Time Domain
line 1
Impulse Monitoring Impulse Sample #4Impulse Monitoring Impulse Sample #4
Powerful, yet brief,
impulse
Slide 28
Impulse Monitoring Impulse Sample #5Impulse Monitoring Impulse Sample #5
Frequency Domain
Slide 29
Impulse Monitoring Impulse Sample #5Impulse Monitoring Impulse Sample #5
Impossible to see beginning or end in
time domain.
Slide 30
Troubleshooting ImpulsesTroubleshooting Impulses
• Voice-based Methods Limited Magnitude, duration, threshold counters miss important
events. Not usually clear how to set thresholds.
• Auto-thresholds, Frequency Domain Set thresholds at levels likely to impact DSL service. Time + frequency domain provide more insight into types
of events.
• Auto-analysis, Identification Recommend whether DSLAM profile can correct for
impulse events. Characterize impulses to help segment / troubleshoot.
Measuring improvements?
Slide 31
Wideband Noise
Slide 32
Characterizing NoiseCharacterizing Noise
1. Frequency Span How broad is disturber (T1 vs AM)
2. Rise Time How quickly does noise appear.
3. Magnitude
4. Coincident PSD How many different shapes are
present at the same time.
Slide 33
Characterizing NoiseCharacterizing Noise
Slide 34
Characterizing NoiseCharacterizing Noise
Type Frequency
Span Rise Time Magnitude
Coincident PSD
Crosstalk Wide
Slow: service penetrationModerate: capacitive coupling due to environmental factorsRapid: near-by service "flipped on"
Low to Mod Low
Impulse Noise Very narrow Rapid High Low
AM Radio Frequency Interference (RFI)
Narrow Moderate Moderate Moderate
Other RFI Narrow Rapid Moderate Low
Micro-cuts, resistive faults, corroded splices
Varies greatly Rapid Varies greatly Low
Slide 35
Mitigating NoiseMitigating Noise
1. Error Correction INP, PHY-R, Delay
2. Optimized Margin and Bit-swapping
3. Artificial Noise / Virtual Noise
4. Transmit PSD Masks
Each has advantages / disadvantages and
equipment dependencies.
Slide 36
Mitigation ToolsMitigation Tools
Slide 37
TakeawaysTakeaways
1. DSL Lines Vary Time of day, day of year. Stability metrics key for tracking
performance.
2. Two General Types of Disturbers Impulsive – troubleshoot with handhelds. RFI, crosstalk – troubleshoot with
centralized tools.
3. Remedy Impulse-free lines impossible, but minimize
below DSL impact. Advanced DSLAM profiles protect against
changes in RFI, crosstalk noise sources.
Jim WahlAware, Inc.www.aware.com