road map docsis 3.0

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Rev.A00

DOCSIS 3.0



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



Media Convergence


Market Trends

Source: Future services on HFC networks: 33th PIKE Conference, 14 October 2008, Zakopane, Poland




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




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



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

Confidential & Proprietary Information of VeEX Inc. 6

CATV Operators Feeling Pressure

DOCSIS 3.0



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.


An Ongoing Battle for Customers

DOCSIS 3.0



DOCSIS Overview

DOCSIS 3.0 Benefits

8Confidential & Proprietary Information of VeEX Inc.


http://slidepdf.com/reader/full/road-map-docsis-30 9/69Confidential & Proprietary Information of VeEX Inc. 9

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



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



Notes: Downstream bandwidths assuming QAM-256 modulation

Upstream bandwidth assuming QAM-64 modulation

Maximum synchronization speed and (Maximum usable speed)


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



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



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


What do we know?

DOCSIS 3.0 Signals

DOCSIS 3.0



DOCSIS 3.0 Preparation




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



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



Confidential & Proprietary Information of VeEX Inc.

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

17DOCSIS 3.0




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

DOCSIS 3.0



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


Expanding HFC Network Capacity

Source: Michiel Peters, TNO - Benelux Chapter SCTE , 15 September 2008, Amsterdam

DOCSIS 3.0



DOCSIS 3.0

Plant Qualification & Test Methods





Typical DOCSIS Network

DOCSIS 3.0




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

DOCSIS 3.0



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.

Confidential & Proprietary Information of VeEX Inc.DOCSIS 3.0




Basic


At the Headend or Hub, check:

Digital signal level (dBmV, dBµV)

Modulation Error Ratio (MER)

DOCSIS 3.0




Spectrum


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




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




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

DOCSIS 3.0




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

DOCSIS 3.0




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

DOCSIS 3.0




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



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


What Degrades MER?

DOCSIS 3.0




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

DOCSIS 3.0




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

DOCSIS 3.0




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




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




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




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




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



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


The Adaptive Equalizer

DOCSIS 3.0




Adaptive Equalizer Test Functions

ImpairmentResults

Frequency Response &

Group Delay Graphs

Tap Expert

DOCSIS 3.0




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





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

( )





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




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



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


Operational RF Levels

DOCSIS 3.0

DOCSIS 3 0 CM E l ti Li k U



After link up, power level onforward and return paths are

measured.

Step-by-step CM link up

process to clearly identify any

failed steps


DOCSIS 3.0 CM Emulation Link Up

DOCSIS 3.0

DOCSIS 3 0 CM IP T t (1)




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)




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)



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


DOCSIS 3.0 CM – VoIP Tests (1)

DOCSIS 3.0

DOCSIS 3 0 CM V IP T t (2)



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


DOCSIS 3.0 CM – VoIP Tests (2)

DOCSIS 3.0

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 – Ethernet Tests (1)

DOCSIS 3.0

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 generationand detailed analysis is used

to check and benchmark all

types of Ethernet service

offered at customer locations.


DOCSIS 3.0 – Ethernet Tests (2)

DOCSIS 3.0

DOCSIS 3 0 Pre Qualification




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?




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



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


Unequalized

MERUnequalized

Signal Levelproblems

Analogchannel

signal level

Digitalchannelpower

Upstreamtransmit level


TransientImpairments

Pre/Post-FECBER


Upstreampacket loss

EqualizerGraph

Micro-reflections


RF Test Checklist

DOCSIS 3.0

Troubleshooting



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


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



Verify correct Power level, BER, MER and Constellation

CMTS downstream IF output

External up-converter IF input

External up-converter RF output


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



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


Combiner Output and Fiber Link

DOCSIS 3.0

Troubleshooting Tips




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



Essential Technical Terms

to Remember




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



DOCSIS 3.0




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



DOCSIS 3.0

Group Delay - Return Path



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


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




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




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



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


Hranac, R. “Linear Distortions, Part 1” Communications Technology, July 2005


Hranac, R. “Linear Distortions, Part 2” Communications Technology, August 2005



Recommended Reading

DOCSIS 3.0

http://www.cable360.net/ct/operations/testing/15092.html










Thank You. Any questions?

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