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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

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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

4

Pre-EQ

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.


Post deployment troubleshooting l.jpg

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

6

CBT Display


Impairment increase vs reporting l.jpg

Impairment Increase vs Reporting

  • Ingress cancellation will cancel some CPD

  • CPD resembles AWGN when all DSs are digital


Pre eq coeff direct load l.jpg

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


Us frequency and level issues l.jpg

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


Sample upstream spectrum usage l.jpg

Sample Upstream Spectrum Usage

Euro Split

TV IF


Total power l.jpg

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

Amplifier

CMTS US0

@ 24 MHz

4-Way

CMTS US2

@ 31 MHz

Fiber Optic

Rx 2

4-Way

CMTS US1

@ 24 MHz

Filter

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

  • 17 dB at 5 MHz & 32 dB at 1 GHz

    • Eliminates max transmit CMs

    • Eliminates high DS tilt to TV

CS(CEQ) tap

FEQ w/ US pad

4

26

17

23

500’

600’

350’

2.5

2

1.5 dB

Step Attenuator or EQ tap

17

Input

38

43 dBmV

X

42

29

39.5

Reverse

transmit

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!


Transmit level possibilities l.jpg

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


Reasons cm does not bond on intended uss l.jpg

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


Ds questions potential concerns l.jpg

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.


Impairments that could affect docsis 3 0 l.jpg

Impairments That Could Affect DOCSIS 3.0

  • Isolation

  • Off-air Ingress

  • Attenuation

  • Freq assignments

    • Spectrum allocation

    • Plant limits


Difficult architecture for narrowcast l.jpg

CMTS

DS 0

US 0

US 1

US 2

US 3

LC1x4

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

Bonding

Primary

16-QAM

64-QAM

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 Tx

DS 2

DS 3

Edge-QAM

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

W

Isolation Amp

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


Design rules restrictions l.jpg

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?

Mbps

38

37

36

35

34

33

32

31

30

29

28

27

26

25

24

23

22

21

20

Time


10 points to ponder l.jpg

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


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