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Triple-Play Service Testing
Carlene Gardner
Strategic Marketing Manager
JDSU Cable Networks Division
For CCTA Training, San Juan PR, August 26, 2009
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
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Upstream Test Parameters
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
 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
Average
– 16 QAM: 24 dB MER
symbol
magnitude
– 64 QAM: 27 dB MER
RMS error
magnitude
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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?
Noise
Note that the
higher noise
floor causes the
MER measurement
To degrade
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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
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DOCSIS Testing
DOCSIS Test – Initialization of a Modem
“Ranging”
 Scan and synchronize to downstream
 Obtain upstream parameters
 Range
“Registration”
 Send device class
 Establish IP connectivity
 Establish time of day
 Get operational parameters
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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
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VoIP Testing
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
X-Time
X-Time
D C B A
Point A
Network
Point B
– 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
Aim for 100ms or better
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Metrics: Delay, Packet Loss and Jitter
 Packet Loss
∞ -Time
X-Time
D C B A
Point A
Network
Point B
– 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)
Aim for 1% or better
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Metrics: Delay, Packet Loss and Jitter
 Jitter
X-Time
D C B A
Point A
Network
D
A
B
C
Point B
Slower than X
Slower than X
Faster than X
– 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
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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Testing the Home for Ingress Contribution
7 dB TAP
Return
Equalizer
House
Drop Cable
OLDER TV SET
WIRELESS LAPTOP
DIGITAL SET-TOP
COMPUTER
GROUND
BLOCK
2-Way
Amplifier
VoIP
High Pass
Filter
ETHERNET
3-Way
Splitter
eMTA-CABLE MODEM
INGRESS SPECTRUM MEASUREMENTS
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ONLINE GAMING
Adjust Goals Per Location
Test Criteria
Service Level Test
RF
At Home
RF
At Node
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Degradation
Threshold
Measurement
Goal
Delay (1-way)
Jitter
Packet Loss
R-Value
MOS
MER
PRE-FEC BER
Rx - Level
< 100 ms
< 10 ms
< 1%
> 70
> 3.6
30dB(64), 33dB (256)
1.00E-09
-5~+5 dBmV
Tx - Level
35 ~45 dBmV
> 150 ms
> 15 ms
> 2%
< 58
<3
25dB(64), 28dB(256)
1.00E-07
<-10dBmV or
>+10dBmV
< 30dBmV or >50dBmV
MER
PRE-FEC BER
Freq Response
32dB(64), 35dB (256)
1.00E-09
< 4dB
28 dB(64), 31dB(256)
1.00E-07
> 5dB
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Future
DOCSIS® 3.0 – Channel Bonding
Individual 256 QAM DOCSIS channel
Versions 1.0/1.1/2.0 used only one channel for
upstream and one channel for downstream
communications
256QAM = ~40Mbps (38.8 Mbps)
BONDED
4 x 256QAM
4 x ~40Mbps = ~160 Mbps
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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
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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





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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
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