DOCSIS 3.0
Rev.A00
Agenda & Discussion Points

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
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2
Market Trends

Media Convergence
Source: Future services on HFC networks: 33th PIKE Conference, 14 October 2008, Zakopane, Poland
DOCSIS 3.0
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3
User Profiles & Applications
Web 2.0
Digital
Photos
Home
Networks
Data &
VoIP
Gaming
MP3
WMV
VOD
DVR/PVR
DVD
Blu-ray
You Tube
SDTV
HDTV
DOCSIS 3.0
Mobile
Video
iPod
Walkman
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4
CATV Operators Need DOCSIS 3.0!
DOCSIS 3.0
Customer
Demand
Competitor
Offering
IPTV, Netflix,
Blockbuster, SIP
Video, Gaming, You
Tube (HD), Video
Phone (HD) ...
FTTx, GPON,
VDSL2, FiOS,
Wireless
Business
Services
IP Addresses
needed
T1/E1 solutions
HD Video
Conferencing
IPv6
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5
CATV Operators Feeling Pressure

Competition is extremely active


Consumer’s have an insatiable demand for new services


Telcos are deploying VDSL2, GPON, FIOS and FTTx (USA & Europe)
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
DOCSIS 3.0
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6
An Ongoing Battle for Customers

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.
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
Increased Downstream
Throughput
• Bonded downstream channels
• 56Mbps (RAW) each, 222Mbps Total
Increased Upstream
bandwidth
• Bonded upstream channels, 5-85MHz
• 27Mbps (RAW) each, 122Mbps Total
IPv6 Support
Backwards compatibility
• IPV6 will allow for 3.4x1038 IP addresses
• Address shortcomings with NAT devices
• Existing DOCSIS 1.0, 1.1 and 2.0 systems
CMTS qualification
• Bronze and silver certifications phased out with only
full certification available now
Modem certification
• Full certification (CableLabs & Euro CableLabs)
Additional network
security
• Early Authentication and Encryption (EAE) or
• AES 128bit encryption which is more secure
DOCSIS 3.0
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DOCSIS Throughput Compared

Date Rates – Annex A
EuroDOCSIS
Version
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
Notes:

Downstream bandwidths assuming QAM-256 modulation

Upstream bandwidth assuming QAM-64 modulation

Maximum synchronization speed and (Maximum usable speed)
DOCSIS 3.0
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DOCSIS 3.0 Channel Bonding
Additional Upstream and Downstream Channels
“Bonded”
together for
higher
aggregate
speed and
capacity
DOCSIS 3.0
4D/4U =
Can be
150Mb/s
deployed
downstream
incrementally
120Mb/s
upstream
No upper
limit to # of
channels
HFC subsplit
effectively
limits #
upstream
channels
Confidential & Proprietary Information of VeEX Inc.
Existing
DOCSIS
modems
share
channels
with no
negative
impact
12
DOCSIS 3.0 Signals
What do we know?

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
DOCSIS 3.0
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DOCSIS 3.0 Preparation
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Preparing for DOCSIS 3.0
RF
Bandwidth
Availability
DOCSIS 3.0
Headend
and Core
Network
Equipment
Preparation
Verify
QAM64
Upstream
Txmission
Verify
QAM256
Downstream
Txmission
DOCSIS 3.0
Modem
Emulation
Confidential & Proprietary Information of VeEX Inc.
IP/Ethernet
Testing
(Ping, FTP,
RFC2544,
Web)
15
Obtaining the Required Bandwidth
Expand the plant
to 860MHz or
1GHz
Launch digital
only systems
Use unusable
old analog
broadcast
channels
Launch Digital
Simulcast and
migrate selected
analog channels
Use Switched
Digital Video to
reclaim
bandwidth
DOCSIS 3.0
Move test
carriers to
alternate
frequencies
DOCSIS 3.0
requires a minimum
of 4 to16 downstream
channels
Confidential & Proprietary Information of VeEX Inc.
CMs are able to
receive 4 DS
channels spread
across a 60MHz
window
16
Frequency Spectrum Changes
Today 870MHz
Soon 1GHz
Reclaiming bandwidth:
• Switched Digital Video
Test requirements:
• Downstream expanding to 1GHz
• MPEG 4 video
• Bonded channels need verification
• Analog Video Reclamation
• Return Path filling up rapidly impacting
traditional sweep and ingress test methods
• Higher order modulation
• In-service testing where possible
DOCSIS 3.0
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Upstream Expansion
How much gain?
Extension of the US band from 65 MHz to 85 MHz
250Mb/s
• 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
•
•
•
•
500Mb/s
1000Mb/s
Adaptation of DOCSIS (RF up converter)
Ingress noise issue solved
Quality concern regarding passives and cables
Investment in diplex filters and return amplifiers
DOCSIS 3.0
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18
Expanding HFC Network Capacity

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
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
Upstream Testing
• 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
Downstream Testing
• Forward Sweep (Sweepless), frequency response, amplifier tilt
• MER, CNR, Group Delay, Constellation, BER pre/post errors
• MPEG-2 Video Signal Analysis
Useful Tips
• 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
DOCSIS 3.0
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22
Upstream Test – Part 1
Setup

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.
DOCSIS 3.0
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23
Upstream Test – Part 2
Basic

At the Headend or Hub, check:

Digital signal level (dBmV, dBµV)

Modulation Error Ratio (MER)
DOCSIS 3.0
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24
Upstream Test – Part 3
Spectrum

At the Headend or Hub, check:



Upstream spectrum (5-65MHz) for Ingress,
CPD, and other interference
Check below 5MHz and above 65MHz all the
way to 200MHz if possible
A QAM-64 signal requires a clean upstream
path!
DOCSIS 3.0
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25
Still Having Problems?
Level and
MER look
OK?



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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26
Upstream Testing – Part 4
Advanced

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
DOCSIS 3.0
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27
Downstream Testing – Part 5
Advanced

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
DOCSIS 3.0
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28
Downstream QAM Parameters
Constellation
MER
64-QAM: 27 dB min
256-QAM: 31 dB min
BER
Pre/Post FEC

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
DOCSIS 3.0
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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.
DOCSIS 3.0
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30
What Degrades MER?

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
DOCSIS 3.0
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31
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:
10 log
DOCSIS 3.0
RMS error magnitude
Average symbol magnitude
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32
Downstream Performance
Pre/Post FEC BER

What the results are telling you:



What to look for:




DOCSIS 3.0
Level, MER and Constellation are OK
Pre/Post FEC BER indicate a problem
Interference from a sweep transmitter
Downstream laser clipping
Up-converter problem in the Headend
Loose connections or CPD
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33
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 sends
redundant information with the payload so the receiver can repair corrupted data
and 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 by
the FEC circuitry - a minimum of 1x10-7 is expected, but FEC may be able to
correct 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 FEC
circuitry is working to correct errors and if so how hard.
DOCSIS 3.0
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34
QAM – Constellation Diagram
Constellation Diagram
Each box in the Constellation diagram contains one symbol
QAM64: 6 bits per symbol, 64 boxes
QAM256: 8 bits per symbol, 256 boxes
DOCSIS 3.0
Quadrant 4
Quadrant 1
Quadrant 3
Quadrant 2
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35
Modulation Error Ratio
MER = 10log (avg symbol power/avg error power)
Q
2
 N  2
 


Q


j 
I j
j 1


MER  10 log 10 N
2 
2
 


    I j  Q j  
 j 1

Q
Average error
power
Average symbol
power
I
A large “cloud” of
symbol points means
low MER—this is not
good!
Q
I
I
A small “cloud” of
symbol points
means high MER—
this is good!
Source: Hewlett-Packard
DOCSIS 3.0
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36
Forward Path Modulation
QAM 64 or QAM 256 are most commonly used
DOCSIS 3.0
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
Annex B
(6MHz)
QAM64
5.057
38
Annex B
(6MHz)
QAM256
5.361
43
(220 max 4 channel
bonding)
(160 max 4 channel
bonding)
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37
Return Path Modulation – DOCSIS
DOCSIS (Data-Over-Cable Service Interface Specifications)
Reverse Path / Upstream Data Rate
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)
Standard symbol rate (bandwidth): 1.28 (1.6), 2.56 (3.2), 5.12 (6.4) MHz
DOCSIS 3.0
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38
Constellation Display
DOCSIS 3.0

Learn to interpret the
constellation display – it
tells you a lot of the signal

Symbol points should be
small and well-defined
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39
The Adaptive Equalizer




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
DOCSIS 3.0
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40
Adaptive Equalizer Test Functions
Impairment
Results
Frequency Response &
Group Delay Graphs
Tap Expert
DOCSIS 3.0
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41
Linear Distortions – a closer look (1)
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 from
their ideal locations.
– Ratio of RMS Constellation Error Magnitude to peak
Constellation symbol magnitude
Symbol Rate Error
– Should be less than +/- 5pm
DOCSIS 3.0
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42
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 the
frequency 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 the
Downstream 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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43
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
DOCSIS 3.0

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)
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Linear Distortions
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.
DOCSIS 3.0
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45
Operational RF Levels

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
DOCSIS 3.0
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46
DOCSIS 3.0 CM Emulation Link Up
Step-by-step CM link up
process to clearly identify any
failed steps
After link up, power level on
forward and return paths are
measured.
DOCSIS 3.0
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47
DOCSIS 3.0 CM – IP Tests (1)
Complete server connection
status indicates any IP
problems
Once the CM is on-line, a full
range of IP tests including
Ping test can be performed
DOCSIS 3.0
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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 Browser
provide bandwidth and visual
indications of performance
DOCSIS 3.0
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DOCSIS 3.0 CM – VoIP Tests (1)
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 voice
quality evaluation (MOS and
R-factor) using the Telchemy
Algorithm and test method is
provided
DOCSIS 3.0
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50
DOCSIS 3.0 CM – VoIP Tests (2)
Detailed Packet statistics
provide a complete insight to
transport and IP layer
impairments
Jitter performance is checked
using the Inter Packet Delay
Variation (IPDV) method per
RFC3393 recommendations
DOCSIS 3.0
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51
DOCSIS 3.0 – Ethernet Tests (1)
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
DOCSIS 3.0
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52
DOCSIS 3.0 – Ethernet Tests (2)
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 generation
and detailed analysis is used
to check and benchmark all
types of Ethernet service
offered at customer locations.
DOCSIS 3.0
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53
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
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How Many Testers Do You Need?
Signal
Level
Meter
DOCSIS 3.0
CM Analyzer
QAM-64
USG
Source
Digital
Spectrum
Analyzer
Group
delay &
frequency
response
tester
CX380
can do
it all
DOCSIS 3.0
Adaptive
equalizer
tester
Ethernet
Tester
CX350
can do
it all
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55
RF Test Checklist
Constellation
display
Pre / Post
FEC BER
Linear
Distortions
Signal Level
problems
Transient
Impairments
Low MER or
CNR
Sweep
transmitter
interference
Adaptive
equalizer
graph
Analog
channel
signal level
Pre/Post-FEC
BER
Phase noise
Laser
clipping
In-channel
frequency
response
Digital
channel
power
Constellation
display
I-Q
imbalance
Loose
connections
In-channel
group delay
Upstream
transmit level
Upstream
packet loss
Coherent
interference
(ingress)
CPD
Constellation
display
Unequalized
Constellation
display
Gain
compression
Low MER or
CNR
MER
Unequalized
Equalizer
Graph
Microreflections
Laser
clipping
Sweep
transmitter
interference
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Troubleshooting
Integrated Up-converter

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
CMTS
To headend downstream
combiner
Attenuator
(if required)
88-860 MHz downstream
RF output
(+50 dBmV to +61 dBmV)
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Troubleshooting
External Up-converter

Verify correct Power level, BER, MER and Constellation

CMTS downstream IF output

External up-converter IF input

External up-converter RF output
CMTS
44 MHz IF input to
upconverter
(typ. +25 dBmV to +35
dBmV)
44 MHz downstream
IF output
(e.g., +42 dBmV +/-2 dB)
DOCSIS 3.0
Attenuator
88-860 MHz downstream
RF output to CATV network
(+50 dBmV to +61 dBmV)
RF upconverter
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Combiner Output and Fiber Link

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
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Troubleshooting Tips
Residential wireless
networks may limit
DOCSIS 3.0
performance benefits
Routers, Switches,
and Ethernet cards
can limit bandwidth to
100Mbps or 10Mbps
PC performance can
effect or limit
throughput
Ensure Test
Equipment has
sufficient bandwidth
to perform high
throughput test
Speed test servers
can skew throughput
results
Hardware settings
can effect bandwidth
e.g. MTU size
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Essential Technical Terms
to Remember
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61
QAM Measurement Terms (1)


I/Q Gain and Phase

The phase and gain of both the I and Q carrier must be equal in order for the
constellation to be correct.

This impairment is caused by the QAM modulators.

The gain difference between the 2 carriers should be less than 1.8% and the phase
difference should be less than 1 degree.
Phase Noise

Jitter (changes in phase) of the oscillators, most likely the up-converter

The phase shift or jitter should be < 0.5 degrees
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QAM Measurement Terms (2)


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
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QAM Measurement Terms (3)



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
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Group Delay - Return Path
5 MHz
5 MHz
10 MHz
10 MHz
15 MHz
40 MHz
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
DOCSIS 3.0
15 MHz
20 MHz
25 MHz
35 MHz
10 MHz
15 MHz
20 MHz
30 MHz
5 MHz
20 MHz
25 MHz
HFC
(Filters,
Taps)
30 MHz
35 MHz
40 MHz
25 MHz
HFC
(Filters,
Taps)
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
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30 MHz
35 MHz
40 MHz
45 MHz
50 MHz
55 MHz
60 MHz
65 MHz
t
65
Linear Distortions
In-Depth Understanding
ECHO MARGIN
The Coefficients of the Equalizer will also reveal the presence of an Echo, (a.k.a. micro-reflections). The Equalizer
will 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 QAM
signal.
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 PostFEC); 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.
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Linear Distortions
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 QAM
signal. 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 are
cancelled 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.
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Recommended Reading

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
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Thank You.
Any questions?
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