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Rev.A00
DOCSIS 3.0
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CATV Market Dynamics
DOCSIS 3 Overview
DOCSIS 3 Benefits
Preparing for DOCSIS 3
What you need to test
How VeEX can help you
Troubleshooting Summary
Essential Technical Terms
DOCSIS 3.0 Confidential & Proprietary Information of VeEX Inc. 2
Agenda & Discussion Points
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Media Convergence
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Market Trends
Source: Future services on HFC networks: 33th PIKE Conference, 14 October 2008, Zakopane, Poland
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User Profiles & Applications
Digital
Photos
Gaming
MP3
WMV
DVD
Blu-ray
SDTV
HDTVMobile
Video
iPod
Walkman
You Tube
VOD
DVR/PVR
Data &
VoIP
Home
Networks
Web 2.0
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CATV Operators Need DOCSIS 3.0!
CustomerDemand
IPTV, Netflix,Blockbuster, SIP
Video, Gaming, You
Tube (HD), VideoPhone (HD) ...
CompetitorOffering
FTTx, GPON,VDSL2, FiOS,
Wireless
BusinessServices
T1/E1 solutions
HD VideoConferencing
IP Addressesneeded
IPv6
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Competition is extremely active Telcos are deploying VDSL2, GPON, FIOS and FTTx (USA & Europe)
Consumer’s have an insatiable demand for new services
HDTV, VoD, PVR, interactive DTV etc
To meet the growing challenge cable operators have to:
Expand network capacity in cost effective and timely manner
Evolutionary steps - incremental investments in current technology
Revolutionary steps – need to decide if and when to implement a Next Generation HFC
network
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CATV Operators Feeling Pressure
DOCSIS 3.0
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Verizon Beats Back Cable With YouTube Tilt April 27, 2010
Verizon Communications Inc. (NYSE: VZ) will
soon use FiOS TV's ability to feed in thousands of
YouTube videos as a key selling point in TV spots
aimed at drawing cable and satellite TV
subscribers to its completely fiber-fed platform.
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An Ongoing Battle for Customers
DOCSIS 3.0
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DOCSIS Overview
DOCSIS 3.0 Benefits
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DOCSIS Milestones
DOCSIS 1.0 (1999)• 1st products certified (CableLabs started project in 1996)
• Open standard for high-speed data over cable
• Modest security, Best-effort service
DOCSIS 1.1 (2000)
• Quality-of-Service (QoS) service flows• Baseline Privacy Interface (BPI+) Certificates
• Improved privacy & encryption process
DOCSIS 2.0 (2002)
• Improved throughput & robustness on Upstream
• 64/128 QAM modulation & higher symbol rates with FEC
• Programmable interleaving to upstream channels
DOCSIS 3.0 (2006)
• Channel bonding (4U/4D) for increased capacity
• IPv6 support
• Improved security (AES)
DOCSIS 3.0
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DOCSIS 3.0 Quick Overview
• Bonded downstream channels• 56Mbps (RAW) each, 222Mbps Total
Increased DownstreamThroughput
• Bonded upstream channels, 5-85MHz
• 27Mbps (RAW) each, 122Mbps TotalIncreased Upstream
bandwidth
• IPV6 will allow for 3.4x1038 IP addresses
• Address shortcomings with NAT devicesIPv6 Support
• Existing DOCSIS 1.0, 1.1 and 2.0 systemsBackwards compatibility
• Bronze and silver certifications phased out with onlyfull certification available now
CMTS qualification
• Full certification (CableLabs & Euro CableLabs)Modem certification
• Early Authentication and Encryption (EAE) or
• AES 128bit encryption which is more secure Additional network
security
DOCSIS 3.0
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Notes: Downstream bandwidths assuming QAM-256 modulation
Upstream bandwidth assuming QAM-64 modulation
Maximum synchronization speed and (Maximum usable speed)
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DOCSIS Throughput Compared
EuroDOCSIS
Version
Date Rates – Annex A
Downstream Upstream
1.x ~ 55.62 (50) Mb/s 10.29 (9) Mb/s
2.0 ~ 55.62 (50) Mb/s 30.72 (27) Mb/s
3.0 (4 Channels) ~ 222.48 (200+) Mb/s 122.88 (108+) Mb/s
3.0 (8 Channels) ~ 444.96 (400+) Mb/s 122.88 (108+) Mb/s
DOCSIS 3.0
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DOCSIS 3.0 Channel Bonding
Additional Upstream and Downstream Channels
“Bonded”
together forhigher
aggregatespeed andcapacity
Can bedeployed
incrementally
4D/4U =
150Mb/sdownstream
120Mb/supstream
No upperlimit to # ofchannels
HFC sub-split
effectivelylimits #
upstreamchannels
ExistingDOCSISmodems
sharechannelswith nonegativeimpact
DOCSIS 3.0
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Physically the same as DOCSIS 2.0 signals Consist of multiple QAM signals bonded logically together
Carry data of mutual relevance
Bonded channels can be contiguous or non-contiguous:
Contiguous - consist of frequency consecutive signals Non-contiguous - interspersed in the spectrum with other
carriers
MPEG-2 transport for downstream signals
QAM transport for upstream signals
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What do we know?
DOCSIS 3.0 Signals
DOCSIS 3.0
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DOCSIS 3.0 Preparation
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Preparing for DOCSIS 3.0
RFBandwidth
Availability
Headendand CoreNetwork
EquipmentPreparation
VerifyQAM64
UpstreamTxmission
VerifyQAM256
DownstreamTxmission
DOCSIS 3.0Modem
Emulation
IP/EthernetTesting
(Ping, FTP,RFC2544,
Web)
DOCSIS 3.0
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Obtaining the Required Bandwidth
DOCSIS 3.0requires a minimum
of 4 to16 downstreamchannels
Use SwitchedDigital Video to
reclaimbandwidth
Launch DigitalSimulcast andmigrate selectedanalog channels
Launch digitalonly systems
Expand the plantto 860MHz or
1GHz Use unusableold analogbroadcastchannels
Move testcarriers toalternate
frequencies
CMs are able toreceive 4 DS
channels spreadacross a 60MHz
window
DOCSIS 3.0
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Frequency Spectrum Changes
Today 870MHz Soon 1GHz
Reclaiming bandwidth:
• Switched Digital Video
• MPEG 4 video• Analog Video Reclamation
• Higher order modulation
Test requirements:
• Downstream expanding to 1GHz
• Bonded channels need verification• Return Path filling up rapidly impacting
traditional sweep and ingress test methods
• In-service testing where possible
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How much gain?
Upstream Expansion
Extension of the US band from 65 MHz to 85 MHz• DOCSIS technology becoming available
• FM band is compromised
• Large network investment is required
Extension of the US band beyond 85 MHz
• Not in current DOCSIS recommendations
New upstream band 900 – 1000 MHz
• Adaptation of DOCSIS (RF up converter)
• Ingress noise issue “solved”
• 862 to 1000 GHz is considered as DS extension band
• Big investment in diplex filters and return amplifiers
New upstream band above 1000 MHz
• Adaptation of DOCSIS (RF up converter)
• Ingress noise issue solved
• Quality concern regarding passives and cables
• Investment in diplex filters and return amplifiers
250Mb/s
500Mb/s
1000Mb/s
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Operators have strong differences in opinion with regard to options:
Solutions are typically driven by specific technical, geographical or local market factors
A combination of solutions often determines the preferred option
Confidential & Proprietary Information of VeEX Inc. 19
Expanding HFC Network Capacity
Source: Michiel Peters, TNO - Benelux Chapter SCTE , 15 September 2008, Amsterdam
DOCSIS 3.0
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DOCSIS 3.0
Plant Qualification & Test Methods
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Typical DOCSIS Network
DOCSIS 3.0
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Plant Qualification
• Generate QAM signal in RP to verify attenuation, level, Tilt, MER and BER
• Frequency response, Group delay, Constellation and Adaptive equalization
• Check spectrum for ingress, noise, CPD and laser clipping
• Check for modems transmitting excessive levels due to high value taps
Upstream Testing
• Forward Sweep (Sweepless), frequency response, amplifier tilt
• MER, CNR, Group Delay, Constellation, BER pre/post errors
• MPEG-2 Video Signal Analysis
Downstream Testing
• Qualify the plant on a node by node basis
• Cable drops should be Tri or Quad shielded
• Check for leakage & improve thresholds (< 5uV/m is recommended)
• Use the “divide-and-conquer” technique to locate problems
• Avoid downstream/upstream frequencies near the band edges/roll off
• Avoid downstream/upstream frequencies susceptible to ingress/interference
Useful Tips
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23
Setup
Upstream Test – Part 1
Configure the Upstream Generator (USG):
Frequency, level, modulation, bandwidth, and
symbol rate
Transmit the QAM-64 signal upstream to a
CX180+, CX350 or CX380 located in the
Headend or Hub.
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Basic
Upstream Test – Part 2
At the Headend or Hub, check:
Digital signal level (dBmV, dBµV)
Modulation Error Ratio (MER)
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Spectrum
Upstream Test – Part 3
At the Headend or Hub, check:
Upstream spectrum (5-65MHz) for Ingress,CPD, and other interference
Check below 5MHz and above 65MHz all theway to 200MHz if possible
A QAM-64 signal requires a clean upstreampath!
DOCSIS 3.0
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Still Having Problems?
Level and
MER lookOK?
A Signal Level Meter (SLM) and Spectrum
Analyzer are great application specific tools, but
they can be limited in telling you everything you
need to know about advanced digital signals
Downstream and upstream (DOCSIS) signals
can be impaired by other factors not easily
viewed using conventional test methods
Look for the “needle inside the QAM haystack”
to figure out what is going on!
DOCSIS 3.0
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Advanced
Upstream Testing – Part 4
For the Upstream, you need to check:
MER (equalized and un-equalized)
Pre and Post FEC
Frequency response (in-channel)
Group delay (in-channel)
Constellation diagram
Adaptive equalizer results
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Advanced
Downstream Testing – Part 5
For the Downstream, you need to check:
Digital Power Level
MER (equalized and un-equalized)
Pre and Post FEC
Frequency response (in-channel)
Group delay (in-channel)
Constellation diagram
Adaptive equalizer results
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Downstream QAM Parameters
BER
Pre/Post FEC
MER
64-QAM: 27 dB min256-QAM: 31 dB min
Constellation
Pre/Post Errorred Seconds (PRES/POES)
The number of seconds with at least one corrected codeword
Severely Errorred Seconds
The number of seconds with at least one uncorrectable codeword
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Impairments
Thermal noise is a basic physical phenomenon which cannot be avoided
Random voltage variation proportional to temperature, bandwidth and resistance.
At room temperature, in 6 MHz bandwidth and 75 ohms circuit, the thermal noise is
approximately -60dBmV. After amplification, the noise level can get much higher.
All the other impairments are “human made”, they depend on the design, implementation
and operation of all the elements in the signal chain
It is convenient to group all impairments into 2 categories:
Linear distortions and Non-linear distortions.
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Transmitted phase noise & Low carrier-to-noise ratio
Non-linear distortions (CTB, CSO, XMOD, CPD…)
Linear distortions (micro-reflections, amplitude ripple, group delay)
Severe impedance mismatches aka linear distortions
Improperly aligned or defective amplifiers
In-correct modulation profiles
Incorrect signal levels
In-channel ingress
Data collisions
Laser clipping
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What Degrades MER?
DOCSIS 3.0
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What MER is Acceptable?
Output of QAM Modulator – 40 dB
Input to Lasers – 39 dB
Output of Nodes – 37 dB
Output of Subscriber Taps – 35 dB
At the input to the subscriber’s receiver – 34 dB
The absolute minimum is 31db
MER is expressed in dB derived as follows:
RMS error magnitude Average symbol magnitude
10 log
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Downstream PerformancePre/Post FEC BER
What the results are telling you:
Level, MER and Constellation are OK
Pre/Post FEC BER indicate a problem
What to look for:
Interference from a sweep transmitter
Downstream laser clipping
Up-converter problem in the Headend
Loose connections or CPD
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Notes on FEC
To have an accurate idea of the BER performance you need to know both pre
and post FEC bit error rate
Forward error correction (FEC) is a digital error checking system that sendsredundant information with the payload so the receiver can repair corrupted dataand eliminate the need to retransmit.
By using the same Reed Solomon decoder at the receiving end, bit errors can be
detected – these are called Pre-FEC errors
Pre FEC BER is the error rate of the incoming signal prior to being corrected bythe FEC circuitry - a minimum of 1x10-7 is expected, but FEC may be able tocorrect errors as high as 1x10-6.
Post-FEC errors cause poor TV quality or DOCSIS data retransmission
Post FEC Bit errors are not acceptable and should be corrected
The FEC decoder needs a BER of >1x10-6 to operate properly
Both Pre and Post FEC BER need to be verified in order to determine if the FECcircuitry is working to correct errors and if so how hard.
DOCSIS 3.0
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Modulation Error Ratio
MER = 10log (avg symbol power/avg error power )
Average symbolpower
I
Q
Average error
power
Source: Hewlett-Packard
I
Q
I
Q
A large “cloud” of
symbol points means
low MER—this is not
good!
A small “cloud” of
symbol points
means high MER—
this is good!
N
j j j
N
j j j
Q I
Q I MER
1
22
1
22
10log10
DOCSIS 3.0
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Forward Path Modulation
Modulation
Type
Std. Symbol
Rate (MHz)
Max data rate
(Mbps)
Annex A
(8MHz)
QAM64 6.952 41.4
Annex A
(8MHz)
QAM256 6.952 55.2(220 max 4 channel
bonding)
Annex B
(6MHz)
QAM64 5.057 38
Annex B
(6MHz)
QAM256 5.361 43(160 max 4 channel
bonding)
QAM 64 or QAM 256 are most commonly used
DOCSIS 3.0
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Return Path Modulation – DOCSIS
DOCSIS (Data-Over-Cable Service Interface Specifications)
Reverse Path / Upstream Data Rate
Standard symbol rate (bandwidth): 1.28 (1.6), 2.56 (3.2), 5.12 (6.4) MHz
DOCSIS Bandwidth
(MHz)
Modulation
type
Max data rate
(Mbps)
1.0 3.2 QPSK 5.12
1.1 3.2 QPSK
QAM16
5.12
10.24
2.0 6.4 QAM16
QAM64
10.24
30.72
3.0 6.4 QAM64
QAM128
120(4 channel bonding)
DOCSIS 3.0
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Constellation Display
Learn to interpret the
constellation display – it
tells you a lot of the signal
Symbol points should be
small and well-defined
DOCSIS 3.0
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Every MPEG2 digital receiver has an Adaptive Equalizer
The Equalizer typically cascades two digital filters:
Feed Forward Equalizer (FFE) - reference tap is the last of 16 taps
Decision Feedback Equalizer (DFE) - output is fed back to input, 108 taps long
Compensates for Linear distortions (Amplitude imperfections & group delay)
The Equalizer uses MER as a tool to adaptively cancel these Linear distortions
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The Adaptive Equalizer
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Adaptive Equalizer Test Functions
ImpairmentResults
Frequency Response &
Group Delay Graphs
Tap Expert
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What the key measurements are telling you!Hum
– Low frequency disturbances of the digital carrier e.g.
switching power supplies
Phase Jitter
– Instability of the QAM carrier seen at the demodulator
– Phase changes of oscillators e.g. the up-converter – Introduces a back and forth rotation of the
constellation where some symbols will eventually
cross the decision boundaries and cause an error in
transmission
EVM (Error Vector Magnitude)
– A measure of how far constellation points deviate fromtheir ideal locations.
– Ratio of RMS Constellation Error Magnitude to peak
Constellation symbol magnitude
Symbol Rate Error
– Should be less than +/- 5pm
Linear Distortions – a closer look (1)
DOCSIS 3.0
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Linear Distortions – a closer look (2)
What the measurement is telling you!
Frequency Response
Frequency response of the digital carrier
Micro-reflections can cause amplitude ripple in thefrequency response
Should be less than 3dB (peak-to-peak)
Group Delay Different frequencies travel through the same medium at
different speeds (see supporting slide)
Worse near band edges and diplex filter roll-off areas
Group Delay variation is usually expressed in ns for theDownstream and in “ns / MHz” for the Upstream
Should be < 50ns peak-to-peak
General Notes:
• Amplitude and Group Delay responses help visualize the effects of filters, diplexers, traps, suck-outs in the
signal path, from (and including) the QAM modulator up to the point of test.
• The frequency span of the calculated responses is directly related to sampling period of the Equalizer
Symbol period. For QAM-64, the span response is 5.05 MHz, while for QAM256 the span is 5.36 MHz
DOCSIS 3.0
( )
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Linear Distortions – a closer look (3)
What the measurement is telling you!
Echo Margin
Echoes are micro-reflections
The tallest vertical bar is the incident signal (reference tap)
Smallest difference between any coefficient and the
DOCSIS template defined by CableLabs
Safety margin when getting too close to the “cliff effect” Should ideally be > 6dB
Equalizer Stress
Derived from all the Equalizer coefficients
Indicates how hard the Equalizer is working to cancel out the
Linear distortions
Global indicator (the higher the figure, the less stress)
Noise Margin
Generally, the lower the MER, the larger the probability of
errors in transmission (Pre-FEC and Post FEC)
Amount of noise that can safely be added to degrade the
Equalized MER before losing the signal (cliff effect)
DOCSIS 3.0
Li Di t ti
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Micro-reflection at about 2.5 µs (2500 ns):
Assume ~1 ns per ft., 2500/2 = 1250 ft
(actual is 1.17 ns per ft: (2500/1.17)/2 = 1068 ft)
Frequency response ripple ~400 kHz p-p:
Distance to fault = 492 x (.87/.400) = 1070 ft.
Linear Distortions
DOCSIS 3.0
O ti l RF L l
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DOCSIS recommends that the digitally modulated
signal’s average power level be set 6 dB to 10 dB
below what the visual carrier level of an analog TV
channel on the same frequency would be
This ratio should be maintained throughout the entire
cable network
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Operational RF Levels
DOCSIS 3.0
DOCSIS 3 0 CM E l ti Li k U
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After link up, power level onforward and return paths are
measured.
Step-by-step CM link up
process to clearly identify any
failed steps
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DOCSIS 3.0 CM Emulation Link Up
DOCSIS 3.0
DOCSIS 3 0 CM IP T t (1)
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DOCSIS 3.0 CM – IP Tests (1)
Complete server connection
status indicates any IP
problems
Once the CM is on-line, a fullrange of IP tests including
Ping test can be performed
DOCSIS 3.0
DOCSIS 3 0 CM IP T t (2)
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DOCSIS 3.0 CM – IP Tests (2)
Throughput (FTP) Download
and Upload should be verified
at the CM service location.
Web Test and Web Browserprovide bandwidth and visual
indications of performance
DOCSIS 3.0
DOCSIS 3 0 CM V IP T t (1)
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VoIP Expert generates
industry standard wave files
to verify MOS and R-Factor of
upstream and downstream
and includes packet jitter,
packet loss, and delay.
Real-time of subjective voicequality evaluation (MOS and
R-factor) using the Telchemy
Algorithm and test method is
provided
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DOCSIS 3.0 CM – VoIP Tests (1)
DOCSIS 3.0
DOCSIS 3 0 CM V IP T t (2)
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Detailed Packet statistics
provide a complete insight to
transport and IP layer
impairments
Jitter performance is checkedusing the Inter Packet Delay
Variation (IPDV) method per
RFC3393 recommendations
Confidential & Proprietary Information of VeEX Inc. 51
DOCSIS 3.0 CM – VoIP Tests (2)
DOCSIS 3.0
DOCSIS 3 0 Ethernet Tests (1)
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Ethernet Testing is important to
validate business services, E1
circuit emulation or Wireless
backhaul applications (E1/T1/IP)
Copper (10/100/1000BaseT)& Fiber (1000BaseX) based
Ethernet service should be
verified
Confidential & Proprietary Information of VeEX Inc. 52
DOCSIS 3.0 – Ethernet Tests (1)
DOCSIS 3.0
DOCSIS 3 0 Ethernet Tests (2)
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RFC2544, BERT, & Throughput
test modes are used to test
Ethernet circuits running at the
subscriber premise or in the core
network at Headend locations
Advanced traffic generationand detailed analysis is used
to check and benchmark all
types of Ethernet service
offered at customer locations.
Confidential & Proprietary Information of VeEX Inc. 53
DOCSIS 3.0 – Ethernet Tests (2)
DOCSIS 3.0
DOCSIS 3 0 Pre Qualification
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DOCSIS 3.0 Pre-Qualification
Spectrum analysis (Upstream & Downstream)
Bonded channel statistics (Upstream & Downstream)
Constellation analysis (Upstream & Downstream)
Equalizer measurements (Upstream & Downstream)
Group delay and Frequency response (Upstream & Downstream)
Cable Modem Emulation (Bonding, Encryption, BPI certificates, etc)
IP & Ethernet Tests (Ping, Throughput, Web Browser, VoIP, RFC2544)
DOCSIS 3.0
How Many Testers Do You Need?
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How Many Testers Do You Need?
SignalLevelMeter
DOCSIS 3.0CM Analyzer
QAM-64USG
Source
DigitalSpectrumAnalyzer
Groupdelay &
frequencyresponse
tester
Adaptiveequalizer
tester
EthernetTester
CX350
can do
it all
CX380
can do
it all
DOCSIS 3.0
RF Test Checklist
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Constellationdisplay
Low MER orCNR
Phase noise
I-Qimbalance
Coherentinterference
(ingress)
Gaincompression
Laserclipping
Sweeptransmitterinterference
Pre / PostFEC BER
Sweeptransmitterinterference
Laserclipping
Looseconnections
CPD
Low MER orCNR
LinearDistortions
Adaptiveequalizer
graph
In-channelfrequencyresponse
In-channelgroup delay
Constellationdisplay
Unequalized
MERUnequalized
Signal Levelproblems
Analogchannel
signal level
Digitalchannelpower
Upstreamtransmit level
Constellationdisplay
TransientImpairments
Pre/Post-FECBER
Constellationdisplay
Upstreampacket loss
EqualizerGraph
Micro-reflections
Confidential & Proprietary Information of VeEX Inc. 56
RF Test Checklist
DOCSIS 3.0
Troubleshooting
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Verify correct average power level
Integrated up-converter RF output should be set in the DOCSIS-specified +50 to
+61dBmV range
Typical levels are +55 to +58dBmV
Also check BER, MER and constellation
Confidential & Proprietary Information of VeEX Inc. 57
Integrated Up-converter
CMTS
88-860 MHz downstream
RF output
+50 dBmV to +61 dBmV
Attenuator
(if required)
To headend downstream
combiner
Troubleshooting
DOCSIS 3.0
Troubleshooting
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Verify correct Power level, BER, MER and Constellation
CMTS downstream IF output
External up-converter IF input
External up-converter RF output
Confidential & Proprietary Information of VeEX Inc. 58
External Up-converter
CMTS
RF upconverter
88-860 MHz downstream
RF output to CATV network
(+50 dBmV to +61 dBmV)
Attenuator 44 MHz downstream
IF output
(e.g., +42 dBmV +/-2 dB)
44 MHz IF input to
upconverter
(typ. +25 dBmV to +35
dBmV)
Troubleshooting
DOCSIS 3.0
Combiner Output and Fiber Link
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Check signal levels and BER at downstream laser input and node output
Bit errors present at downstream laser input but not at CMTS or up-converter
output may indicate sweep transmitter interference, loose connections or
combiner problems
Bit errors at node output but not at laser input are most likely caused by
downstream laser clipping
Confidential & Proprietary Information of VeEX Inc. 59
Combiner Output and Fiber Link
DOCSIS 3.0
Troubleshooting Tips
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Confidential & Proprietary Information of VeEX Inc. 60
Residential wirelessnetworks may limit
DOCSIS 3.0performance benefits
Routers, Switches,and Ethernet cards
can limit bandwidth to100Mbps or 10Mbps
PC performance caneffect or limitthroughput
Hardware settingscan effect bandwidth
e.g. MTU size
Speed test serverscan skew throughput
results
Ensure TestEquipment has
sufficient bandwidth
to perform highthroughput test
Troubleshooting Tips
DOCSIS 3.0
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Essential Technical Terms
to Remember
61Confidential & Proprietary Information of VeEX Inc.
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QAM Measurement Terms (2)
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Hum
Low frequency disturbances of the digital carrier
Same as hum on analog carriers, if the level is the same, it’s the system, if higher on the
digitals then it’s probably the QAM modulator
Symbol Rate Error Should be < +/- 5ppm
Echo Margin
A measurement in dB of how far the taps are from the template with the time equalizer
measurement. Caused by impedance mismatches in the system.
Should be > 6dB
Confidential & Proprietary Information of VeEX Inc. 63
QAM Measurement Terms (2)
DOCSIS 3.0
QAM Measurement Terms (3)
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Group Delay
Different frequencies travel through the same medium at different speeds. So the lower
the lower frequencies of the same carrier arrive at the receiver at different timing than
the higher frequencies.
Should be < 50ns peak-to-peak
Frequency Response
Frequency response of the digital carrier
Should be < 3dB peak-to-peak
Carrier Offset
Carrier frequency test.
Should be no more than +/- 25KHz
Confidential & Proprietary Information of VeEX Inc. 64
QAM Measurement Terms (3)
DOCSIS 3.0
Group Delay - Return Path
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5 MHz
10 MHz
15 MHz
20 MHz
25 MHz
30 MHz
40 MHz
35 MHz
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
Confidential & Proprietary Information of VeEX Inc. 65
Group Delay Return Path
5 MHz
10 MHz
15 MHz
20 MHz
25 MHz
30 MHz
40 MHz
35 MHz
HFC
(Filters,
Taps)
t
HFC
(Filters,
Taps)
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
5 MHz
10 MHz
15 MHz
20 MHz
25 MHz
30 MHz
40 MHz
35 MHz
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
DOCSIS 3.0
Linear Distortions
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In-Depth Understanding
ECHO MARGIN
The Coefficients of the Equalizer will also reveal the presence of an Echo, (a.k.a. micro-reflections). The Equalizerwill cancel such an echo, and in doing so, the equalizer coefficient which corresponds to the delay of the echo will
be much higher than the surrounding ones, “it sticks out of the grass”. The relative amplitude of this coefficient is
an indication of the seriousness of the echo, and its position gives the delay of the echo, hence its roundtrip
distance.
The Echo Margin is the smallest difference between any coefficients and a template defined by Cablelabs, as a
safety margin before getting too close to the “cliff effect”. It is normal to notice relatively high coefficients close
to the Reference as this corresponds to the filters in the modulator / demodulator pair and to the shape of QAMsignal.
EQUALIZER STRESS
The Equalizer Stress is derived from the Equalizer coefficients and indicate how much the Equalizer has to work to
cancel the Linear distortions, it is a global indicator of Linear distortions. The higher the figure, the less stress.
NOISE MARGIN
We all know that the lower the MER, the larger the probabilities of errors in transmission (Pre-FEC and then Post-
FEC); the MER degrades until errors are so numerous that adequate signal recovery is no more possible (cliff
effect). As Noise is a major contributor to the MER, we define Noise Margin as the amount of noise that can be
added to a signal (in other words, how much we can degrade MER) before get dangerously close to the cliff effect.
Noise is chosen because on the one hand it is always present, and on the other hand it is mathematically
tractable. Other impairments, such as an Interferer, are not easily factored into error probabilities.
Linear Distortions
DOCSIS 3.0
Linear Distortions
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In-Depth Understanding
EQUALIZED MER vs. UN-EQUALIZED MER
The MER (Modulation Error Ratio) is the ratio of the QAM signal to Non-Linear distortions of the incoming QAMsignal. The MER should have included the Linear distortions to indicate the health of the signal; but the QAM
demodulator cannot operate properly without the Equalizer and the Equalizer uses the MER as a tool to
adaptively cancel the Linear distortions. Consequently it is convenient to distinguish the MER (non-linear
distortions only) from an Un-equalized MER (non-linear and linear distortions), the Un-equalized MER is
calculated from the MER and Equalizer Stress.
The Un-equalized MER is always worst than the MER. A small difference between the two indicates little Linear
distortions, a large difference shows that there are strong Linear distortions. Even if the Linear distortions arecancelled by the Equalizer, we have to keep in mind that the Equalization is a dynamic process as it tracks Linear
distortions by trial and error even after converging. The larger the Linear distortions the larger the tracking
transients are, hence more probability of transmission error (pre-FEC or Post-FEC BER).
PHASE JITTER
Phase Jitter is caused by instability of the carrier of the QAM signal at the demodulator. This instability could be
found at the QAM modulator and up-converter or in the QAM receiver (Local Oscillators used in frequency
conversions). The phase jitter introduces a rotation of the constellation, where the symbols clusters elongate and
get closer to the symbol’s boundary. Eventually some symbols will cross the boundary and cause an error in
transmission. The QAM demodulator has a Phase lock loop to track phase variations of the carrier; it tracks easily
long term drift as well as some short terms variations (up to 10 or 30 kHz) but it cannot track very fast variations
above its loop response. So in a QAM demodulator, the wideband jitter is more damageable than short term
jitter.
Linear Distortions
DOCSIS 3.0
Recommended Reading
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Hranac, R. “Digital Troubleshooting, Part 1” Communications Technology, June
2006
www.cable360.net/ct/operations/testing/15092.html
Hranac, R. “Troubleshooting Digitally Modulated Signals, Part 2”
Communications Technology, July 2006
www.cable360.net/ct/operations/testing/18539.html
Hranac, R. “Linear Distortions, Part 1” Communications Technology, July 2005
www.cable360.net/ct/operations/testing/15131.html
Hranac, R. “Linear Distortions, Part 2” Communications Technology, August 2005
www.cable360.net/ct/operations/testing/15170.html
Confidential & Proprietary Information of VeEX Inc. 68
Recommended Reading
DOCSIS 3.0
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Thank You. Any questions?