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HFC Plant Optimization for D3.0. John J. Downey Consulting Network Engineer Cisco Systems. ATDMA General Deployment Recommendations. After increasing channel width to 6.4 MHz, measure & document US MER (unequalized would be best) 25 dB or higher Unequalized MER is recommended

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Hfc plant optimization for d3 0 l.jpg

HFC Plant Optimization for D3.0

John J. Downey

Consulting Network Engineer

Cisco Systems

Atdma general deployment recommendations l.jpg
ATDMA General Deployment Recommendations

  • After increasing channel width to 6.4 MHz, measure & document US MER (unequalized would be best)

    • 25 dB or higher Unequalized MER is recommended

    • Check US MER as well as per CM MER

  • Document unequalized MER with test equipment at multiple test points in plant

    • PathTrak Return Path Monitoring System linecard

    • Sunrise Telecom Upstream Characterization toolkit

    • Trilithic

  • Pick freq < 30 MHz away from diplex filter group delay

  • Turn on Pre-Equalization

    • Can exclude specific Mac or OUI

Us mer snr issues l.jpg
US MER(SNR) Issues

  • Increasing ch width keeps same average power

    • Doubling ch width will drop MER by 3 dB or more

  • Equalized vs unequalized MER readings

  • Modulation profile choices

    • QPSK for maintenance, 64-QAM for Data, 16-QAM for VoIP?

    • Max output for 64-QAM is 54 dBmV

      • Cab up n power-adjust continue 6

  • Pre-EQ affect

    • Great feature in 1.1 & > CMs, but could mask issues

Pre eq l.jpg



Upstream 6.4 MHz bandwidth 64-QAM signal

Before Adaptive EQ:

Substantial in-channel tilt caused correctable FEC errors to increment. CMTS’s reported US MER (SNR) was 23 dB.

After Adaptive EQ:

DOCSIS 2.0’s 24-tap EQ—was able to compensate for nearly all in-channel tilt (with no change in digital channel power). Result: No correctable or uncorrectable FEC errors and the CMTS’s reported US MER (SNR) increased to ~36 dB.

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Post-Deployment Troubleshooting

  • MER per US with ability to drill-down for per-CM MER

  • Use US monitoring tools like PathTrak or Cisco Broadband Troubleshooter (CBT) to view 5-65 MHz for laser clipping

    • Need analyzer to read < 5 MHz for AM or ham radio ingress

    • New PathTrak card reads 0.5 MHz - 85 MHz & MacTrak

  • Cable Flap List monitoring for US or CM issues

  • Uncorrectable/Correctable FEC per US with ability to drill-down for per-CM counters

  • Bottom line is correctable & uncorrectable FEC

    • If correctable FEC is incrementing, then eventually it will lead to uncorrectable FEC, which equals packet drops

Cbt display l.jpg


CBT Display

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Impairment Increase vs Reporting

  • Ingress cancellation will cancel some CPD

  • CPD resembles AWGN when all DSs are digital

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Pre-eq Coeff Direct Load

  • CMs that drop > 3 dB MER in one SM period get direct load

  • CMTS support for Type 9 TLVs for DOCSIS 2.0 &> CMs

  • DOCSIS 1.1 CMs in this state re-register

  • Following message types added to "sh cab modem verbose"

    • Pre-Equalization Counters : 1205 good, 0 scaled, 24 impulse

    • Equalizer Coeffs Direct Load : 1 direct coeff loads

      • Significant change in freq response may create scaled count

      • When CMTS decides to return CM’s EQ taps to known state without direct load, an impulse value is sent

      • Each time Type 9 TLV is sent to CM, direct load counter will increase by 1

      • When CM goes offline, counters are zeroed

Determining per cm pre eq taps l.jpg
Determining per-CM Pre-EQ Taps

  • Poll pre-eq tap MIB directly from CM:

    • Raw values polled to determine red, yellow, green

  • Cablelabs Proactive Network Maintenance (PNM)

  • Charter’s “Node Slayer”

  • Comcast has “Scout Flux”

D3 0 us issues l.jpg
D3.0 US Issues

  • Frequency Stacking Levels

    • What is the max output with multiple channels stacked

    • Is it pwr/Hz & could it cause laser clipping?

  • Diplex Filter Expansion to 85 MHz

    • If amplifier upgrades are planned for 1 GHz, then pluggable diplex filters may be warranted to expand to 85 MHz on the US

    • Still must address existing CPE equipment in field & potential overload

    • RFoG could be perfect scenario (maybe even 200 MHz split)

  • CM must be w-online (requires 1.1 cm file) for US bonding

  • Monitoring, Testing, & Troubleshooting

    • Just like DOCSIS 2.0, test equipment needs to have D3.0 capabilities

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US Frequency and Level Issues

  • Freq assignments

    • 5 to 42, 55, 65, 85 MHz ?

      • Diplex filters, line EQs, step attenuators, CPE overload

  • Max Tx for D2.0 64-QAM for 1 ch is 54 dBmV

  • D3.0 US single ch max power

    • 57 dBmV (32 & 64-QAM)

    • 58 dBmV (8 & 16-QAM)

    • 61 dBmV (QPSK)

  • Max Tx per ch for 4 freqs stacked at 64-QAM ATDMA is only 51 dBmV & 53 for S-CDMA

    • When stacking, level will not change unless max is reached

Channel placement l.jpg
Channel Placement

  • Each US channel used for bonding is individual channel

  • Transmitters (channels) are separate

    • Can have different settings; modulation, ch width, tdma or scdma, etc.

  • Frequencies do not need to be contiguous

  • Wise to keep relatively close so attenuation and tilt don’t cause issues

  • CMs have some dynamic range to allow few dB difference between channels

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Total Power

  • Was only one US ch, now up to 4 chs Txing at same time

    • Possibly 6.4 MHz each; nearly 26 MHz US channel loading

  • Lots of power hitting US laser

  • Probability of laser clipping is increased, especially if using legacy Fabry-Perot (FP) lasers

    • Distributed Feedback (DFB) lasers have more dynamic range

  • Use US monitoring system capable of looking above 42 MHz to see second and third order harmonics

  • Any burst noise above diplex filter (i.e. 42 MHz) coming out of return path receiver is usually indicative of laser clipping

Laser clipping l.jpg
Laser Clipping

  • Noise above ~40 MHz (~65 MHz in a Euro-DOCSIS network) is most likely caused by laser clipping

Laser clipping16 l.jpg
Laser Clipping

  • Blue trace shows case of strong laser clipping

  • Green line represents flat US laser noise floor with no clipping

  • Note that this US has four US bonded channels

Laser clipping artifacts l.jpg
Laser Clipping Artifacts

  • 1.5 MHz AM causing Laser Clipping

  • Possibly getting in at power insertion port of node

Us load balance isolation example l.jpg

Fiber Optic

Rx 1



@ 24 MHz



@ 31 MHz

Fiber Optic

Rx 2



@ 24 MHz


US Load Balance & Isolation Example

  • Attempting to “share” one US port across two other US ports

    • Can cause isolation issues

    • Load balance issues (ambiguous grouping)

    • Low Tx CM in HE/field can overcome isolation and show up on wrong ports

      • Exacerbated with wide-open power adjust continue window

  • Note: D3.0 CMs in mtc-mode do not load balance on US

System levels reverse l.jpg

CS(CEQ) tap

FEQ w/ US pad










1.5 dB

Step Attenuator or EQ tap




43 dBmV







level @ the tap

PIII .5” cable

.40 dB @ 30 MHz

System Levels Reverse

  • Less noise from low value taps

  • Reduces potential “bleed-over”/ isolation issues

  • Note: pad creates grp delay at cutoff , whereas EQ does not

A total design variation of ~14 dB!

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Transmit Level Possibilities

  • Running D3.0 CM in low mod scheme allows higher power

  • Use D3.0 CM in 2.0 mode

    • Single frequency on D3.0 CM offers 3 dB higher power

  • Minimum level of 20 dBmV could cause issues in lab or HE test CM

    • Pmin = +20 dBmV, 2560 ksym/s

    • Pmin = +23 dBmV, 5120 ksym/s

  • Sample ATDMA Mod Profile

Us summary l.jpg
US Summary

  • Targeted insertion of D3.0

    • Leverage existing US chs while adding more US capacity

    • Load balance 1.x/2.0 and enable D3.0 when needed

  • Leverage D3.0 bonding for D2.0 tiers & services

    • Better stat-mux efficiency

  • Account for phy connectivity, not just ch capacity

    • Not advantageous to combine noise to satisfy connectivity

  • Fix Max Tx issues now

    • Design for tight “bell-curve” (43-48 dBmV), if possible

  • Good News – ECR to increase US Tx levels

    • 61 dBmV max, with 3 dB typical

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Reasons CM Does Not Bond on Intended USs

  • CM not in w-online mode or maybe using 1.0 cm file

  • Mtc-mode off

  • Mtc-mode required-attribute & no attribute in cm file

  • No BG configured or incorrect fiber node config

  • CM not set for bonding or firmware issue

  • All US chs not “sta”

    • US(s) shut

    • Max or Min Tx issues

    • Poor MER, plant issues, mis-wired

  • Oversubscribed CIR

    • Call signaling (nRTPS), min US guaranteed speed,

    • Could have multiple single ch bonding groups

  • Note: US service flows like UGS & RTPS assigned to single ch bonding

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DS Questions & Potential Concerns

  • Why it’s Needed

    • Competitive pressure, offering higher tiers of service, more customers signing up

  • Frequency Stacking Levels & Placement

    • What is the e-qam max output with four channels stacked

    • Do channels have to be contiguous?

  • Isolation Concerns

    • Applications w/ different service grps lead to overlaid networks

    • Signals destined for one node could “bleed” over to another

  • DS Frequency Expansion to 1 GHz

    • Amplifier upgrades are occurring now. It’s best to make the truck roll once, so think about diplex filters, spacing, taps, etc.

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Impairments That Could Affect DOCSIS 3.0

  • Isolation

  • Off-air Ingress

  • Attenuation

  • Freq assignments

    • Spectrum allocation

    • Plant limits

Difficult architecture for narrowcast l.jpg


DS 0

US 0

US 1

US 2

US 3


Difficult Architecture for Narrowcast

  • Optical splits create large service group (SG) sizing

  • Small narrowcast area or big mxn domain for large SG?

    • Small narrowcast area = small targeted area, but costly node splits

    • Large SG = better stat muxing & sharing, but more spectrum needed

M cmts 100 mbps service tier l.jpg
M-CMTS = 100 Mbps Service Tier

  • 4 DS freqs

  • 2 US freqs





3.2 MHz

6.4 MHz

  • 5, 4x4 MAC domains with ATDMA & TDMA USs

  • DS connector overlaid for 2 nodes

Docsis 3 0 ds considerations l.jpg
DOCSIS 3.0 DS Considerations

  • Frequency assignments

    • CMTS may be limited to 860 MHz or 1 GHz

    • Legacy CMs (1.x & 2.0) limited to 860 MHz bandedge

    • E-qam limited to contiguous 24 MHz or 4 channel slots

    • CMs may be limited to 50 or 60 MHz passband

  • M-CMTS architecture requires DTI and local USs

    • Distance limitation, time offset differences, level differences

  • Resiliency is another topic to address

    • If one DS frequency goes bad in field, how do CMs recover or react?

  • E-qam licensing?

  • CM requires 1.1 config file

Docsis 3 0 ds considerations cont l.jpg
DOCSIS 3.0 DS Considerations (cont)

  • More DS = more US

  • Testing and maintaining multiple DS channels

    • Physical chs have not changed for DOCSIS 3.0

    • Test equip with built-in CMs need to support bonding

      • May need to exclude from LB and other feature like pre-eq

  • DS ch bonding max power with 4 freqs stacked

    • Four chs stacked on 1 connector limited to 52 dBmV/ch

      • DOCSIS 1.x/2.0 DS is 61 dBmV max output

  • DS isolation issues

M cmts ds overlay and isolation issues l.jpg

DSs 0-3 = 603 MHz

DS 0

Overlay = 609, 615 & 621 MHz

Potential Isolation Path

DS 1

DS Combiner


DS 2

DS 3


M-CMTS, DS Overlay and Isolation Issues

  • E-QAM with DTI

    • DS Licensing?

    • Contiguous QAMs?

    • Level granularity?

  • Load balance between local & remote DSs could have timing issues

Isolation amp l.jpg


Isolation Amp

  • Can this device handle 50 dBmV input with 4-8 ch loading?

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Design Rules & Restrictions

  • D3.0 spec goes to 1 GHz, some equipment may not

  • D3.0 CM spec requires 60 MHz capture window

  • DPC3000 capture of 96 MHz over most spectrum

    • 82 MHz max window supported over entire spectrum

  • TI 4x4 CM (60 MHz window)

  • Brcm 8x4 CM (2, 32 MHz bands or 1, 96 MHz band)

    • DS freqs must be contiguous within tuner block unlike 4x4 CMs

    • Can use RCC templates to setup both tuners

    • New feature called Split Tuner creatse 2 Rx modules and moves tuners automatically without RCC templates

  • Put voice call service flows on a primary DS

    • cable docsis30-voice downstream req-attr-mask 0 forb-attr-mask 80000000

Ds summary l.jpg
DS Summary

  • Targeted insertion of D3.0

    • Leverage existing US chs while adding more DS capacity

    • Load balance 1.x/2.0 and enable D3.0 when needed

  • Leverage D3.0 bonding for D2.0 tiers & services

    • Better stat-mux efficiency & improved consumer experience

  • Enable seamless upgrade to higher D3.0 tiers

    • Wire once & add QAM chs as tiers or service take-rates go up

  • Can also disable DS bonding

    • No cable mrc-mode

    • Per-CM exclude with vendor specific MIB or TLV

What does this bandwidth graph represent l.jpg
What Does This Bandwidth Graph Represent?






















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10 Points to Ponder

  • Many speed sites report at layer 3 of OSI model

    • Configure cm file for 5-10% higher than marketed

  • No control over actual frame size (64-1518 B)

    • Frame size overhead 18/64 (28%) vs 18/1518 (1.2%)

    • MTU affected by wireless, VPN, ….

  • Small frames = small DOCSIS pipes

    • Only 35 Mbps when all frames are DS VoIP of 229 B

  • PowerBoost™ can give perception of greater speed

    • Could cause issues when deciding to do node splits

    • How to control peak rate

10 points to ponder cont l.jpg
10 Points to Ponder (cont)

  • DS TCP requires US acks

    • US pipe could slow down DS speed tests

    • Small US acks make US pipe worth less

      • DOCSIS overhead usually 11 B per frame

      • 10.24 Mbps raw = 9 Mbps usable, but only 7.5 with acks!

  • More frames = more PPS = higher CPU usage

    • At some point CPU in modem could (will) be bottleneck

    • TCP (typically 2 DS per 1 US ack)

  • During congestion, you still want priority for VoIP signaling, maybe video acks, and CM registration

  • Load balancing is good, but what speed tier pushes customer to bonding?

    • Maybe >50% of linerate

10 points to ponder cont36 l.jpg
10 Points to Ponder (cont)

  • Netflix/Hulu TV are using ABR, which is TCP-based

    • Will cause US traffic in form of acks

    • New CMs may have ack suppression on by default

    • Typical US to DS TCP ratio of ~2%

    • With ack suppression, that can drop below 1%

      • Ack suppression doesn’t alleviate CM CPU

    • DS IP video of 3-7 Mbps and may make ack suppression inefficient

    • Implement PHS, but more testing needed

  • Many tweaks needed to get per-CM US speeds > 3 Mbps

    • Lots of concatenation leads to fragmentation

    • Fragmentation adds headers

    • Preamble & gaurdtime added to each fragment

    • D3.0 US bonding can do concatenation and keep < 2000 B

      • May not require fragmentation, so less overhead