triple-play service testing carlene gardner strategic marketing manager jdsu cable networks division...
TRANSCRIPT
Triple-Play Service TestingCarlene Gardner
Strategic Marketing ManagerJDSU Cable Networks Division
For CCTA Training, San Juan PR, August 26, 2009
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Outline
Problems in reverse path– What to look for to check carrier health – What causes performance to degrade
DOCSIS testing– Recap of process to get online– Tips for testing to check where the problem is
VoIP testing– Specific parameters for voice service– Why it is different
Future services
Upstream Test Parameters
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Metric: Carrier to Noise, Signal to Noise
Noise
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Metric: Carrier to Noise, Signal to Noise
C/N and SNR, versus the noise floor, are somewhat predictive of BER and MER
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Metric: MER
Modulation Error Ratio (MER) on downstream QAM carrier is similar to S/N or C/N
MER on upstream QAMs is similar to SNR read at CMTS MER determines how much margin the system has before
failure
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Metric: MER
MER math
RMS errormagnitude
Average symbol
magnitude
Indicates how close symbols are to ideal point MER(dB) = 10 x log RMS error magnitude
average symbol magnitude
Upstream channel fails if MER is not better than– QPSK: 18 dB MER– 16 QAM: 24 dB MER– 64 QAM: 27 dB MER
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Metric: MER of constellation
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Metric: MER degradation due to noise
Symbols cannot reach ideal point due to noise
Ideal points
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What Causes MER and BER to degrade?
Noise Ingress in Upstream Cause:
– Often originating from subscriber’s homes, loose RF connectors, or faulty coax cable
– Upstream noise is worse at headend due to “funneling”– A little bit of noise from many locations becomes a lot of noise at the
headend
Effect– When noise ingress gets too high, data symbols start falling outside
their constellation boundaries– The CMTS can no longer correctly determine good data from bad– FEC is no longer able to correct corrupted data packets– The CMTS discards packets with too much data corruption – Web & email traffic will re-transmit and may eventually get through– VoIP traffic is lost forever!
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What Causes MER and BER to degrade?
Note that the higher noisefloor causes theMER measurementTo degrade
Noise
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What Causes MER and BER to degrade?
Coherent Ingress
Cause– Coherent Ingress are carrier waves or other constant carrier signals
that exist in the HFC network– Ingress: Often originating from subscriber’s homes, loose RF
connectors, or faulty coax cable– Internally-generated impairment: Common Path Distortion (CPD)
Effect– Within a DOCSIS upstream channel, can cause intermittent,
complete data loss or cause a complete DOCSIS outage– Subscribers may complain about a problem that comes and goes
• 24x7 monitoring of ingress is the only way to confidently identify intermittent ingress problems
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What Causes MER and BER to degrade?
Compression – Laser / RF Amplifier Cause
– Excessive input levels into an active device causing the laser or amplifier to “clip” or not be able to transmit the highest amplitude voltage signals
– Not rebalancing upstream devices after a change such as adding another DOCSIS channel
Effect– Laser or amplifier clipping causes data loss because the highest
amplitude symbols (bits) are pushed into other symbol decision boundaries
– Data loss can be continuous or sporadic depending upon system and device conditions
– Web traffic can be re-transmitted, but VoIP traffic is permanently lost
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What Causes MER and BER to degrade?
Group Delay
Cause– Linear changes in phase of a signal– Inherent difficulty of maintaining even speed of propagation through
multiple devices, long amplifier cascades – Group delay can also occur due to amplitude changes throughout
the upstream band
Effect– Frequencies propagate at different speeds through the HFC plant– Group delay affects cable modem signal quality and thus MER– If group delay is bad enough, the CMTS will be unable to recover
the transmitted signal and data will be lost (<200 nsec/MHz is spec)– Data and VoIP communications will be impacted or lost completely
DOCSIS Testing
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DOCSIS Test – Initialization of a Modem
Scan and synchronize to downstream Obtain upstream parameters Range
Send device class Establish IP connectivity Establish time of day Get operational parameters
“Ranging”
“Registration”
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Qualify the Drop
Verify receive level at cable modem
MER shows that downstream is clean and clear with margin
BER shows that downstream is clean and clear of impulse noise
Upstream is properly aligned and CMTS has “ideal” receive level
Packet Loss
Throughput
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Registration
Full list of registration addresses, Service Identifier (SID) and config file assigned
Error messages detail where a failed test halted, can help locate source of problem
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IP or RF?
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IP Impairments
Traffic Congestion– CMTS Over-utilization– Switch / Router Over-utilization– Viruses, Worms or just General Killer Apps
Routing Errors– Cable Modem routes– MTA routes– IP Gateways
Provisioning Issues– Subnet Rules– Configuration Files such as TFTP Files– SNMP – BPI & PacketCable Certificates
VoIP Testing
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VoIP – Bullet Train Analogy
Ideal World:– Packets like train Cars through a station – 1 at a time,
evenly spaced, and Fast
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VoIP – Train Analogy
Real World– VoIP Packets don’t always do what you want…
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Metric: MOS Score, R-Factor
Test the HFC Performance– VoIP Quality
• MOS• R-Value
– Processing• Packet Loss• Jitter• Delay
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Metrics: Delay, Packet Loss and Jitter
Delay
– Time it takes a packet to ‘transverse’ the network
– Too much delay affects the quality of a call• Over-talk and Echo • Usually an architecture (traffic/capacity) issue• Generally not a HFC issue with equipment such as
amplifiers
C B AD
X-Time
X-Time
Point A Point B
C B AD Network
Aim for 100ms or better
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Metrics: Delay, Packet Loss and Jitter
Packet Loss
– Packet did not arrive (Point B) or out of sequence– Worse if it is ‘bursty’, many lost in a row – “lossy”– Can be architecture or physical layer
• Ingress (especially upstream)• Routers over capacity (too full to hold any more)
C B AD
X-Time
∞ -Time
Point A Point B
C B AD Network
Aim for 1% or better
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Metrics: Delay, Packet Loss and Jitter
Jitter
– Packets not arriving with the same timing (different from X-Time) – time between packets is different
– You never notice with Data, doesn’t matter how the information arrives, just care that it shows up but VoIP is Real-Time• Key Causes are IP packet routing, IP based equipment
C B AD
X-Time
Point A Point B
C B AD
Slower than XFaster than X
Slower than X
CBADNetwork
Aim for 10ms or better
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Use Metrics to Segment HFC and IP layer
Segment HFC and IP impairments– Identify if issues are
occurring in HFC Plant or in the IP network
Check MOS of VoIP over DOCSIS channel
Check VoIP packet statistics– Noise and Ingress on plant
are major causes of Packet Loss
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eMTA-CABLE MODEM
7 dB TAP
Drop Cable
High Pass Filter
GROUND BLOCK
3-WaySplitter
DIGITAL SET-TOP
House
2-Way Amplifier
Testing the Home for Ingress Contribution
VoIP
OLDER TV SET
Return Equalizer
ONLINE GAMING
WIRELESS LAPTOP
COMPUTER
ETHERNET
INGRESS SPECTRUM MEASUREMENTS
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Adjust Goals Per Location
Test Criteria Measurement GoalDegradation
Threshold
Delay (1-way) < 100 ms > 150 msJitter < 10 ms > 15 ms
Service Level Test Packet Loss < 1% > 2%R-Value > 70 < 58
MOS > 3.6 < 3MER 30dB(64), 33dB (256) 25dB(64), 28dB(256)
RF PRE-FEC BER 1.00E-09 1.00E-07At Home Rx - Level -5~+5 dBmV <-10dBmV or
>+10dBmVTx - Level 35 ~45 dBmV < 30dBmV or >50dBmV
MER 32dB(64), 35dB (256) 28 dB(64), 31dB(256)RF PRE-FEC BER 1.00E-09 1.00E-07
At Node Freq Response < 4dB > 5dB
Future
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DOCSIS® 3.0 – Channel Bonding
256QAM = ~40Mbps (38.8 Mbps)
Individual 256 QAM DOCSIS channel
Versions 1.0/1.1/2.0 used only one channel for upstream and one channel for downstream communications
4 x 256QAM
4 x ~40Mbps = ~160 Mbps
BONDED
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Trends in the upstream
Expanding digital environment– Upstream becoming
increasingly crowded– More carriers
More challenging than before– Wider carriers offer a wider
target for interference– Higher modulation has more
sensitivity to ingress
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Use constellations to view impairments
Microreflections etc.
Microreflection pattern over time Cloud pattern over time
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Back to the Basics
Most problems are still physical layer issues Most of the test strategy remains the same
– Divide and conquer
Check forward and return RF levels– analog and digital
Check for leakage Sweep the forward / reverse to detect issues Replace questionable connectors / passives Tighten F-connectors … but not too tight Robust plant will be ready for the next great thing
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Future Services
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Thank you!
Carlene Gardner
JDSU Cable Networks Division