Transcript DSAM Features - abegroup.org

Measuring Digital and DOCSIS
Performance
Randy Francis
JDSU
Today’s Agenda

Definitions / Background
−
−
−

Digital Testing/Troubleshooting
–
–
–

Voice Quality Measurements, E-Model, Delay, Packet Loss, Jitter
VoIPCheck Testing, PacketCable Testing
Return Path
–
–
–
2
Packet Loss, Throughput
VoIP Testing
–
–

MER, BER, QAM Ingress, Equalizer Stress,
C/N vs. BER vs. MER
Constellations / Symbols and Digital Bits, Phase Noise
DOCSIS & IP Testing
–

Digital Signals / QAM
DOCSIS 1, 1.1, 2.0. 3.0 (Data Over Cable Service Interface Spec.)
VoIP
Why monitor it?
RF Trending Results
Ingress & Leakage

Q and A

Quiz Time
© 2006 JDSU. All rights reserved.
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Acronyms & Terms








3
QAM
Symbols
Symbol Rate
I&Q
Constellation
MER
BER
FEC
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- Quadrature Amplitude Modulation
- Collection of Bits
- Transmission Speed
- Components of QAM data
- Graph of QAM Data
- Modulation Error Ratio
- Bit Error Rate
- Forward Error Correction
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
A Typical Home Installation
a typical home installation
TAP
House
TV
Drop Cable
GROUND
BLOCK
High Pass Filter
2-Way
Splitter
TV
3-Way
Splitter
TV
CABLE
MODEM
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Why Go Digital?
 Efficiency
– Source signals are digital
• Standard and High Definition TV (SDTV, HDTV)
– High Speed Data and Digital Video is more efficient than analog
• Transmit equivalent of 6 to 10 analog channels (VCR quality) over
one 6 MHz bandwidth
 Quality
– Better Picture and Sound Quality
– Less Susceptible to noise
• Error detection and correction is possible
 Flexibility
– Data easily multiplexed into digital signal
– Higher Data Security
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
What is Digital?
 Source and Destination is digital data
– Assign unique patterns of 1’s and 0’s
 Transmission path is via an analog carrier
– Choice of modulation is the one that optimizes
bandwidth (data versus frequency ‘space’) and resiliency
to noise
00
01
10
11
00
Generate Digital
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10
Receive Digital
Transmit Analog
6
01
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
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The Carrier, by the way, is ANALOG Modulation
Analog Content –
Analog Carrier
Digital Content –
Analog Carrier
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Some good news…and some bad news
 Good
 Bad
– Digital TV, DOCSIS, Digital
Voice all use SAME type of
RF Modulation – QAM
– Same measurements apply
– Signal level, Modulation Error
Ratio, Bit Error Rate, in-band
frequency response, are all
similar, if not identical.
8
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– Each service has different
threshold of impairments
– What is noticeable in Digital
voice may not be perceptible
in DOCSIS, what is
bothersome in DOCSIS is
different than digital video.
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
QAM and CATV
 16 QAM is part of the DOCSIS® 1.0/1.1 upstream
specifications
 64 QAM and 256 QAM is used for both digital video and
DOCSIS downstream, allowing more digital data
transmission using the same 6 MHz bandwidth
– Transmit equivalent of 6 to 10 analog channels (VCR quality) over
one 6 MHz bandwidth
 DOCSIS is the Standard for data over Cable
– Cable systems provide higher signal to noise ratios than over-theair transmission. A well designed and maintained cable plant meets
these QAM signal to noise requirements.
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
QAM Data Capacity (Annex B)
16 QAM
64 QAM
256 QAM
(Upstream)
(Downstream)
(Downstream)
5.0569
5.3605
(@ 6 MHz)
(@ 6 MHz)
Symbol
2.560
Rate (Msps) (@ 3.20 MHz)
10
Bits per
symbol
4
6
8
Channel
Data Rate
(Mbps)
10.24
30.3417
42.8843
Actual
Information
bit rate
(Mbps)
9.0
26.9704
38.8107
Overhead
12.11%
11.11%
9.5%
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QAM 256 allows
~ 2 more channels
vs. QAM 64
No Real Cost
Advantages
Why not go all QAM
256?
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS®
What is it?
What differs among versions?
DOCSIS Defined?
 DOCSIS Definition
– CableLabs® Certified Cable Modem Project
•
Data Over Cable Service Interface Specification
– Not required but highly adhered to standard – allows interoperability
 Data over a digital QAM signal
– Digital signal
• Downstream usually 64 or 256 QAM
• Upstream usually QPSK or 16 QAM
– Uses Data Packets
• Information broken into chunks
– Asymmetrical communication
• More Info Downstream
• Less Info Upstream
Digital “haystack”
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS® Versions at a Glance
DOCSIS 1.0 (High Speed Internet Access)
–23 million products shipped worldwide as of YE2002
–228 CM Certified, 29 CMTS Qualified
DOCSIS 1.1 (Voice, Gaming, Streaming)
–Interoperable and backwards-compatible with DOCSIS 1.0
–“Quality of Service” and dynamic services, a MUST for PacketCable™
–In the field NOW - 64 CM Certified, 22 CMTS Qualified
DOCSIS 2.0 (Capacity for Symmetric Services)
–Interoperable and backwards compatible with DOCSIS 1.x
–More upstream capacity than DOCSIS 1.0 (x6) & DOCSIS 1.1 (x3)
–Improved robustness against interference (A-TDMA and S-CDMA)
–Available NOW – Number of CM & CMTS Qualified growing
DOCSIS 3.0 (Channel Bonding)
–Interoperable and backwards compatible with DOCSIS 1.x & 2.0
–Specification released earlier this month – Devices not yet available
Good Resource Site: www.cablelabs.org
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS 1.1 Overview
 Interoperable with DOCSIS 1.0, plus more…
 Enhanced “Quality of Service” (QoS)
– Guarantees and/or limits for data rates
• (ex. Gold, Silver service levels)
– Guarantees for latency
 Improved security - designed to reduce possibility of “theft of
service, provide secure software downloading.” – BPI+
(Baseline Privacy Plus)
 Interoperability - DOCSIS 1.0 and DOCSIS 1.1 cable
modems and CMTSs on the same plant. Better operation and
OSS features
 Transmit Equalization - more robust transmission
 Max Modulation
– 256 QAM Downstream (~40Mbps)
– 16 QAM Upstream @ 3.2MHz (~10Mbps)
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS 2.0 Overview – Key Upstream Advances
 Symmetrical services are enabled by DOCSIS 2.0
– 1.5x greater efficiency
• operates at 64 QAM
– 2x wider channels
• new 6.4 MHz wide channel
 3x better upstream performance than V1.1
 6x better upstream performance than V1.0
 DOCSIS 2.0 widens the pipe for IP traffic, allowing cable providers to
create more and better services for voice, video, and data
– It does this by using enhanced modulation and improved error correction
 Superior ingress and impulse noise performance
 100% backward compatible with DOCSIS 1.0/1.1
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS 3.0 Overview
 Specification late last year
– DOCSIS 3.0 Interface Specifications (Released December 2006)
– CPE equipment in development stages
 Downstream data rates of 160 Mbps or higher
– Channel Bonding
– 4 or more channels
256QAM => ~40Mbps
4 x 256QAM => ~160 Mbps
 Upstream data rates of 120 Mbps or higher
– Channel Bonding
– 4 or more channels
64QAM => ~30Mbps
4 x 64QAM => ~120 Mbps
 Internet Protocol version 6 (IPv6)

16
– Current System (IPv4) is limited to 4.3B numbers
– IPv6 greatly expands the number of IP addresses
• Expands IP address size from 32 bits to 128 bits
• IPv6 supports 3.4×1038 addresses; 4923:2A1C:0DB8:04F3:AEB5:96F0:E08C:FFEC
• Colon-Hexadecimal Format
100% backward compatible with DOCSIS 1.0/1.1/2.0
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS 3.0 – Channel Bonding
Individual QAM 256 DOCSIS channel
Versions 1.0/1.1/2.0 used only one channel for
upstream and one channel for downstream
communications
256QAM => ~40Mbps
4 Bonded QAM 256 DOCSIS channels
DOCSIS v3.0 Spec requires devices to be able to bond a
minimum of 4 upstream channels into one and 4 downstream
channels into one for 4 times increased throughput in both
directions
The MSO does not have to use all 4 channels, but the devices
which are 3.0 compliant must have the ability to bond 4 or
more channels in both directions
4 x 256QAM:
4 x 40Mbps = 160 Mbps
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Digital Testing and
Troubleshooting
How?
What does it mean?
QAM Digital Measurements
 Spectrum & Digital Average Power Level
 MER QAM
 Pre/Post FEC BER
 Constellation Display
 Advanced Tests
–
–
–
–
19
QAM Ingress Under The Carrier
Group Delay
In-Channel Frequency Response
Equalizer Stress (Adaptive Equalization)
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
QAM Digital Measurements
 Measuring the quality of QAM digital carriers
is significantly different than measuring
analog carriers.
 Measurement of digital carriers is important
to determine how close the carrier is to failing
(how much margin) since there may be no
quality degradation.
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Digital Average Power Level Measurements
Digital Average Power Measurements
and Measurement Bandwidth
 The spectrum analyzer view is an
excellent tool to see discreet RFcarriers.
– Caution is needed when viewing digital
modulated signals (haystack). The
signal’s level measurement is derived
from the selected measurement
bandwidth (resolution bandwidth). At an
RBW of 300 kHz, a 64QAM - 6 MHz
wide digital signal reads in the spectrum
analyzer trace 3 dB too low.
 The Average Power principle takes
small slices from the integrated RFenergy, summing them together to one
total power reading in the LEVEL-mode.
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Analog and digital (broadcast) QAM signal.
The recommended delta in level should be
6 to 10 dB.
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Measuring the Digital “Haystack”
Non measured area
based on 280 kHz step
size within 6 MHz total
Bandwidth
Digital carrier under
test (6 MHz BW)
IF Measurement
Bandwidth =
280 kHz
Summing slices of the total integrated energy
0
-2.5 MHz
Frequency
+/- 140 kHz
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
+2.5 MHz
Measuring the Digital “Haystack”
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Measuring the Peak
Level of the Digital
Haystack
Measuring the Average
Level of the Digital
Haystack
280 kHz
Bandwidth power
6 MHz
Bandwidth power
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Digital – more than just dB’s
MER and Pre and Post BER measurements are
key to insuring Digital Quality
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Modulation Error Ratio (MER)
 Analogous to S/N
 A measure of how symbols (I vs. Q) are actually
placed, compared to ideal placement
 MER(dB) = 20 x log RMS error magnitude
average symbol magnitude
 Good MER
– 64 QAM: 28 dB MER
– 256 QAM: 32 dB MER
Average
symbol
magnitude
RMS error
magnitude
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
More MER
 Modulation Error Ratio (MER) in digital systems is
similar to S/N or C/N used in analog systems
 MER determines how much margin the system has
before failure
– Analog systems that have a poor C/N show up as a
“snowy” picture
– A poor MER is not noticeable on the picture right up to
the point of system failure - “Cliff Effect”
 Can’t use the TV as a piece of test equipment
anymore
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Digital TV Waterfall Graph
Picture Quality Vs. Impairments
Picture Quality ==>
Much Better
Still Great
Good
Until it
Crashes
Good
Digital
Analog
Beginning to get
Customer
Complaints
Customer
Rebellion
Increasing Impairments ==>
27
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
C/N vs. BER vs. MER
No FEC
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Lets have some MER review…
 “MER” is to Digital, what signal
to noise is for analog
 MER is affected by high noise,
low signal
 Also ANY other continuous
impairments
 MER readings are relatively
immune to “brief bursty”
interference
 MER is a predictor of BER
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© 2006 JDSU. All rights reserved.
Some Guidelines for MER
–64 QAM - 28dB or better
–256 QAM - 32dB or better
Try to add at least 3db to above
figures to allow headroom.
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Bit Error Rate (BER)
 Bit errors result when the receiver interprets
the wrong symbol and hence the wrong bits
 The number of bit errors versus the number
of bits transmitted is Bit Error Rate (BER)
 The more bits that are incorrect, the more
the signal will be affected
 Good signal: BER 10-10 (1.0 E-9)
 Threshold for visible degradation: BER 1E-6
– One error in every 1,000,000 bits
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
More on BER
 “BER” is HOW many
 Some amount of errors
errors per HOW many bits
can be corrected by
of data
digital receivers. (PRE
errors)
 BER is affected by bursty
interference
 POST errors are
uncorrected errors –
 Also, any other
always unacceptable!!!
impairments will
adversely affect BER
 Measurement reads in
scientific notation
 1.0 E-9 is the best on a
handheld test meter
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Pre and Post FEC BER
 Number of bad bits for every good bit.
 Forward Error Correction when working
will output >10-11
– 1 error in 100 billion bits
– 1 error every 42 minutes
– MPEG-2 likes good BER
 FEC will work to about 10-4
– 1 error in 10,000 bits
– 1 error every 276 uses
QAM256  42Mb/s
 FEC can correct BER from 1-E-4 to 1E-9
 FEC causes Cliff Effect
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
BER Example
 A 256QAM channel transmits at a symbol rate of 5M
symbols per second
 Bit rate = 8 bits per symbol X 5M symbol per second
=40M bits per second
 Error Incident = Bit rate X BER = Errors Per Second
BER
10-12
10-11
10-10
10-9
10-8
10-7
10-6
10-5
10-4
10-3
33
Error Frequency
1 in 1 Trillion bits
1 in 100 Billion bits
1 in 10 Billion bits
1 in 1 Billion bits
1 in 100 Million bits
1 in 10 Million bits
1 in 1 Million bits
1 in 100 Thousand bits
1 in 10 Thousand Bits
1 in 1 Thousand bits
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Error Incident
25000 secs between errs (6.94 hrs)
2500 secs between errs (41.67 mins)
250 secs between errs (4.167 mins)
25 seconds between errors
2.5 seconds between errors
4 errors per second
40 errors per second
400 errors per second
4000 errors per second
40000 errors per second
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
What Causes MER and BER to degrade? “ Noise”
 The most common problem with digital-TV
and cable modem services is interference
under the digital carriers and Noise(stated by
most larger operators with experience in digital
transmission services).
 Most common sources are:
– Ingress due to off-air UHF TV channels
– Intermittent ingress due to pager transmitters
or two-way radio base stations.
– Cracked or broken cables
– CSO/CTB-intermodulation –
• due to unterminated TAPs or impedance mismatch
– Laser Clipping
34
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
QAM Constellations
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Constellations, Symbols and Digital Bits
 Each “dot” on constellation represents a unique symbol
 Each unique symbol represents unique digital bits
 Digital data is parsed into data lengths that encode the symbol
waveform
Q
1011 1001 0010 0011
1010 1000 0000 0001
I
1101 1100 0100 0110
1111 1110 0101 0111
16 QAM
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Constellation Display
 This is a visual representation of the I and Q constellation
plots on a 16 QAM carrier
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
64 QAM and 256 QAM
 64 QAM has 8 levels of I and 8 levels of Q making 64
possible locations for the carrier.
 256 QAM has 16 levels of I and 16 levels of Q making
256 possible locations for the carrier.
 QAM 256 is somewhat less robust vs. QAM 64
8 Levels of Q
Channel
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
16 Levels of Q
Channel
16 Levels of I
Channel
8 Levels of I
Channel
Effects of Noise and Interference
 Noise and Interference moves the carrier
away from its ideal location causing a
spreading of the cluster of dots.
Ideal
Locations
40
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Decision Boundaries
 Data that falls outside the decision boundaries will be
assumed to be the adjacent data.
Data Received
Correctly
Q
I
Decision
Boundary
41
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Error
Constellation
 Typical errors which originate from the headend:
– Phase Noise
• The constellation appears to be rotating at the extremes
while the middle ones remain centered in the decision boundaries.
Phase Noise is caused by headend converters.
– Gain Compression
• The outer dots on the constellation are pulled into the center
while the middle ones remain centered in the decision
boundaries. Gain Compression is caused by filters,
IF equalizers, converters, and amplifiers.
– I Q Imbalance
• The constellation is taller than it is wide. This is a difference
between the gain of the I and Q channels. I Q Imbalance is
caused by baseband amplifiers, filters, or the digital modulator.
– Carrier Leakage
42
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
System Noise and/or Leakage Issues
 A constellation displaying significant noise
 Dots are spread out indicating high noise and most likely
significant errors
– An error occurs when a dot is plotted across a boundary and is
placed in the wrong location
 Meter will not lock if too much noise present
Dots are
spread out
showing error
43
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Phase Noise
 Display appears to rotate at the extremes
 HE down/up converters can cause phase noise
 Random phase errors cause decreased
transmission margin
 Caused by transmitter symbol clock jitter
Rotation
Constellation with Phase Noise
Rotation
Zoomed Constellation with Phase Noise
Key Cause: Converter within the HE (garbage in produces garbage out)
44
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Gain Compression
 If the outer dots are pulled into the center while
the middle ones are not affected, the signal has
gain compression
 Gain compression can be caused by IF and RF
amplifiers and filters, up/down converters and IF
equalizers
– May be caused by line amps or laser clipping
– Not a customer issue – maintenance techs issue
Outer edges
pulled in
45
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Coherent Interference
 If the accumulation looks like a “donut”, the
problem is coherent interference
– CTB, CSO, Off-Air Carriers (ingress)
 Sometimes only a couple dots will be misplaced
– This is usually laser clipping or sweep interference
Circular
“donuts”
46
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS & IP Testing
What is important?
What can be done?
DOCSIS® Testing - Levels
Verify proper receive level at cable modem
Should be seeing: 0dBmV
Pass/Fail
Indicator:
Downstream
Information
Frequency,
Modulation type,
Channel
MER shows that
downstream is
clean and clear
with margin
BER shows
that
downstream
is clean and
clear of
impulse
noise
48
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Upstream
Frequency,
Modulation type,
Channel BW,
DOCSIS version
Shows that upstream is properly
aligned and CMTS has “ideal”
receive level with margin to spare.
Recommend: (35dBmV – 45dBmV)
JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
DOCSIS – Packet Loss Testing
Check Packet
Loss and
determine if
upstream is
good
Overall loop
information,
Up+Downstreams
View Downstream performance
49
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Upstream Signal
to Noise Ratio
DOCSIS – Throughput Testing
Check Throughput for proper speeds
Config.
File
Ensure customer can get what they pay for
50
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Correlate Data & Voice with RF
Compare Node Outages with your Return Path performance history to
QUICKLY identify the cause of the problem – RF or Data layer!
HSD Node Outage Report Courtesy of Auspice Corporation
51
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
VoIP Testing
Why is it important?
What can be done?
VoIP – Bullet Train Analogy
 Ideal World:
– Packets like train Cars through a station – 1 at a time,
evenly spaced, and Fast
• Bullet Train…
• Cold & Refreshing…
53
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
VoIP – Train Analogy
 Real World
– VoIP Packets don’t always do what you want…
• Multiple railcars taking different directions, uneven spacing
• Missing railcars… all = TRAIN WRECK!
54
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Voice Quality - Measures
 Models to Answer: “Does it ‘Sound Good’?”
– International Telecommunications Union (ITU) has
multiple models
• Subjective (Opinion of users)
– Mean Opinion Score (MOS)
»
»
»
»
Defacto standard ‘score’
“Judged” summary of call quality
In-service test, can be used on live network
Algorithms have replaced but mimic opinionated scores
• Objective (Not an opinion)
– Perceptual Analysis/Measurement System (PAMS) and
Perceptual Evaluation of Speech Quality (PESQ)
» Not based on network issues: Packet loss, Jitter and Delay
» Do not factor in end-to-end delay
» Require HW probes
» (Not used for in-service testing)
– E-Model, R-Value
» Addresses all issues with PAMS and PESQ particularly as it places
focus on Packet loss, Jitter, and Delay
55
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Voice Quality - E-Model
 E-Model  R-Value
– Most complete, objective test
R = Ro + A – Ls – Ld – Le
•
•
•
•
•
Ro: Basic SNR or circuits – handsets, environment
A: Advantage Factor – GSM poor service, example.
Ls: Simultaneous impairment factor – loudness/volume,
hissing, sidetone, quantization distortion
Ld: Talker/listener echo, Overall Delay
Le: Equipment impairment factor – Packet loss, Jitter
Things you will take action on!
56
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Delay, Packet Loss and Jitter- What is it?
 Delay
X-Time
X-Time
D C B A
Network
Point A
Point B
– Time it takes a packet to ‘transverse’ the network
• Point A to Point B
– Time it takes is shown here as ‘X’
– Too much delay affects the quality of a call
• See this on PSTN with international calls (Over-talk)
• Echo (More delay = more potential echo)
– Usually an architecture (traffic/capacity) issue
• Longer distances and more routers add delay
• Too many calls/downloads add delay (slows routers)
• Generally not a HFC issue with equipment such as amplifiers
57
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JDSU CONFIDENTIAL & PROPRIETARY INFORMATION
Delay, Packet Loss and Jitter- What is it?
 Packet Loss
∞ -Time
X-Time
D C B A
Network
Point A
Point B
– Packet did not arrive (Point B) or out of sequence
• VoIP telephony is different than data
– If it was out of order, computer reorders for data
– If lost, just retransmit data
– Shown here as infinite time – did not show up
– Worse if it is ‘bursty’, many lost in a row – “lossy”
– Can be architecture or physical layer
• Ingress (especially upstream)
• Routers over capacity (too full to hold any more)
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Delay, Packet Loss and Jitter- What is it?
 Jitter
X-Time
D C B A
Point A
Network
D
A
B
C
Point B
Slower than X
Slower than X
Faster than X
– Packets not arriving at the same time (different from XTime) – time between packets is different
• Different than data
– You never notice with Data, doesn’t matter how the information arrives, just care
that it shows up
– VoIP is Real-Time
• Key Causes
– IP based equipment having packet routing issues
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Packet Knowledge
D C B A
Point A
Point B
FULL
Z
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Jitter – A problem?
 Creates two problems
– Direct speech quality
• Part is FF, part is normal, part is ‘Slow’
– Could result in Packet Loss
• Walk on top of packets if some are slow and consecutive ones are fast.
D
CB
A
X-Time
Slower than X
Faster than X
Slower than X
– Could be capacity, physical or core architecture
• Routers spitting out packets at varying intervals
• Packets take different routes to destination
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Jitter – Solution?
 Jitter Buffer
– Add a place to store the data, and ‘spit’ it out at a constant
rate
• If packets come in fast – slow them down
• Packets are slow – OK, we have added time to allow for this
Jitter Buffer
D
Point B
Slower than X
CB
A
D
X-Time
Slower than X
Faster than X
C
B
Y-Delay
• Very similar to Windows
Media Player
Buffer bar
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A
Jitter Buffer – Problem Solved?
 Yes and No
– We have taken care of jitter (that isn’t extreme), but have
added delay: Y (buffer size)
• If packets come in fast – slow them down
• Packets are slow – OK, we have added time to allow for this
X-Time
X-Time
D C B A
Y-Delay
D C B A
Just Right
 Dynamic Jitter Buffer
 Self adjusting buffer, based on
system conditions which optimizes the
jitter buffer
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D
C
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B
A
Jitter Buffer – Tradeoffs
 Too large
– Too much delay
 Too small
– Have jitter
– Have packet loss
 Just Right
– Dynamic Jitter Buffer
• Can set this option so the system will monitor and optimize the jitter
buffer
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VoIPCheck™ over DOCSIS®
 HFC Performance
– Packet Statistics
• Packet loss
• Delay
• Jitter
– VoIP Quality
• MOS
• R-Value
– Test Result Totals
•
•
•
•
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Current
Min
Max
Average
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VoIPCheck™ over DOCSIS®
 Segment HFC and IP
impairments
 Quick quality (MOS)
verification of VoIP over
DOCSIS® channel
 Good to check VoIP
packet statistics
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VoIP Testing Based on CableLabs™
PacketCable™ Standard
Place and test the Quality
of a PacketCable®
Based VoIP Call to
validate In-Service
performance
Measure Performance
Statistics
–
–
–
–
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Packet Loss
Total “End to End” Delay (ms)
Jitter
MOS & R-Value
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Segmented Error Budgets
Test Critaria
Measurement
Goal
Start Seeing
Degradation on Call
Quality
Delay (1-way)
< 100 ms
> 150 ms
Jitter
Packet Loss
R-Value
< 10 ms
< 1%
> 70
> 15 ms
> 2%
< 58
MOS
> 3.6
<3
MER
30dB(64), 33dB (256)
25dB(64), 28dB(256)
RF
At Home
PRE-FEC BER
Rx - Level
Tx - Level
MER
1.00E-09
-5~+5 dBmV
35 ~45 dBmV
32dB(64), 35dB (256)
1.00E-07
<-10dBmV or >+10dBmV
< 30dBmV or >50dBmV
28 dB(64), 31dB(256)
RF
At Node
PRE-FEC BER
Freq Response
Upstream SNR
1.00E-09
< 4dB
>35dB
1.00E-07
> 5dB
<25dB
CMTS
CMTS Loading
<50%
>80%
Service Level Test
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Wrap-Up
Troubleshooting is “Back to the Basics”








Majority of problems are basic physical layer issues
Most of the tests remain the same
Check power
Check forward levels, analog and digital
Check forward / reverse ingress
Do a visual check of connectors / passives
Replace questionable connectors / passives
Tighten F-connectors per your company’s installation
policy
– Be very careful not to over tighten connectors on CPE (TVs,
VCRs, converters etc.) and crack or damage input RFI
integrity
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Common problems typically identified in
outside plant
 Damaged or missing end-of-line terminators.
 Damaged or missing chassis terminators on
directional coupler, splitter or multiple-output amplifier
unused ports.
 Loose center conductor seizure screws.
 Unused tap ports not terminated. This is especially
critical on lower value taps.
 Unused drop passive ports not terminated.
 Use of so-called self-terminating taps (4 dB two port;
8 dB four port and 10/11 dB eight port) at feeder
ends-of-line. Such taps are splitters, and do not
terminate the line unless all F ports are properly
terminated.
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Common problems typically identified in
outside plant
 Kinked or damaged cable (including cracked cable,
which causes a reflection and ingress).
 Defective or damaged actives or passives (waterdamaged, water-filled, cold solder joint, corrosion, loose
circuit-board screws, etc.).
 Cable-ready TVs and VCRs connected directly to the
drop. (Return loss on most cable-ready devices is
poor.)
 Some traps and filters have been found to have poor
return loss in the upstream, especially those used for
data-only service.
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Quiz Time
Quiz Time
1. Define DOCSIS and QAM
2. What is the visual difference in an analog channel
vs. digital channel on a spectrum analyzer. How
many channels can you get in a digital 6MHz b/w?
3. When would someone use QAM256 modulation vs.
QAM64? Why not use all QAM256 modulation?
4. What’s the data rate expected for DOCSIS 3.0?
Distinguish between Downstream and Upstream.
What new technology enables this larger data rate?
Please explain.
5. How many IP addresses will IPv6 provide?
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Quiz Time con’t.
6. Why doesn’t Peak Level Power = Ave Digital Power?
7. Define MER and BER. Why do we use Constellation
Diagrams?
8. What are 2 good DOCSIS Tests?
9. What are 3 things to check when testing VoIP? What are
2 accepted “scorings” used in the industry?
10. What type testing should be done on the Reverse Path?
11. Why is Leakage testing important? Name 2 ways that it
can it be performed?
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Questions
 Thank you for your time…
 Any Questions???
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