ppt

DSL Troubleshooting: Intermittent Faults

CommTech14 May 2009

Jim Wahl Aware, Inc.

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?

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

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.

Loop Length and SNR / Data Rate Loop Length and SNR / Data Rate

7,000ft loop

6,000ft loop

Noise and SNR / Data Rate Noise and SNR / Data Rate

Intermittent DSL TroublesIntermittent DSL Troubles

Changes in crosstalk levels:

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



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.



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

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).

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.

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.

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

Troubleshooting Faults

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

Impulse Noise

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

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.

Impulse Monitoring Sample #1Impulse Monitoring Sample #1

QLN

Impulse Monitoring Impulse Sample #1Impulse Monitoring Impulse Sample #1

Detected Impulse

Lasts about 50

microseconds

Peaks at more than

6mV


Impulse affects a large portion of

the ADSL2 spectrum

QLN


Impulse


An impulse affecting the beginning and

middle of the ADSL 1 band

QLN


Brief impulse


Affects a large area

of the spectrum

QLN

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


Powerful, yet brief,

impulse


Frequency Domain


Impossible to see beginning or end in

time domain.

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?

Wideband Noise

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.


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

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.

Mitigation ToolsMitigation Tools

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

[email protected]

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