Download Latency - (SCTE) - New Jersey Chapter

VoIP Testing
SCTE New Jersey Chapter
9/13/07
My Business Card
Larry Jump
Regional Sales Engineer
Sunrise Telecom
814.692.4294
[email protected]
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Today’s Agenda
 DOCSIS Troubleshooting Pyramid
 RF Impairments
 DOCSIS Problems
 VoIP Impairments
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Troubleshooting Methodology
Troubleshooting any DOCSIS network begins with a BOTTOM UP approach.
Today, typically 80% of the problems are RF and 20% are IP/DOCSIS related.
VoIP Lives Here
Communications between CMs,
MTAs, subscribers & IP servers
Communications between
CMTS & CMs only
What we fix today
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Building Blocks of Troubleshooting
Evolution of RF Troubleshooting
•
RF impairments have typically been viewed as the root cause
of all network problems in cable networks
•
Since two-way data services have been introduced, cable
operators have evolved the RF plant to higher and higher
standards
•
Although RF impairments are still readily present in cable
networks, it is important to acknowledge that other
impairments exist which must be addressed
DOCSIS & IP Protocols > 20% of
Impairments
•
•
DHCP / TFTP / ToD / DNS / CMS / etc. servers
Modem and MTA configuration files, CMTS configs
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Build a Solid RF Foundation for the Pyramid
1. Sweep/balance for alignment
2. Return path node certification and maintenance
3. Home installation quality of workmanship and
materials
4. Conform to DOCSIS Specifications for signal
quality
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Forward Sweep
Headend or Hub Site
Forward Signal Path
H
R
L
 Ensures Unity Gain in the Forward
path. Unity Gain minimizes
distortions on all forward signals.
 Ensures that entire frequency band
arrives at the customer’s receive
site with equal quality.
 Ensure good sweep and 95% of the
other problems go away!
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Return Sweep
Return Signal Path
H
R
L
 Ensures Unity Gain in the Return
path. Unity Gain minimizes
distortions on all return signals.
 Ensures that entire frequency band
from all subscribers arrives at the
headend or hub site with equal
quality.
 Ensure good sweep and 95% of the
other problems go away!
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Downstream Measurements
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Modulation Error Ratio
 MER is used as the single figure of merit for DVBC standards.
 It includes distortions such as CCN, CSO, CTB,
laser compression, etc…. The sum of all evils.
 A 256 QAM picture tiles at 28dB
 A minimally good MER is 31 dB for 256 QAM at
the back of the customer’s set.
H
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Modulation Error Ratio (MER)
 Modulation Error Ratio
(MER) is a measure of the
phase and voltage variation.
 RMS error magnitude

MER(dB)  10 log 

average symbol magnitude 
Symbol
Error

And expressed mathematically by:
 N 
2
2 

I

Io

Q

Qo
j
j
j
  j

j 1

MER (dB)  10  log 
dB
N


Io 2j  Qo 2j



j 1



 


Each point represents one symbol (2-bits)
in QPSK of data or one phase position.
The distance from the circle is the error.


• Finally, aggregate MER is just the sum of
multiple MER measurements:
 1 NN MERi / 10 
MERT  10  log  10
dB
NN
i 1


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DOCSIS Compliant Downstream
 Analog Measurements
 CNR
 CSO
( ≥35 dB per DOCSIS spec)
( ≥41 dB per DOCSIS spec)
 CTB
( ≥41 dB per DOCSIS spec)
 Analog (Digital) Measurements
 BER (post-FEC 10-8 or less per DOCSIS
spec)
 Modulation Error Ratio (MER)
 Constellation Analysis
 Digital Channel Power
1 error in 100 Million
bits
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Typical BER/MER Requirements
BER
64 QAM
MER
10-10 >35
10-8 27-34
10-6 23-26
10-5 <23
256 QAM
MER
Quality
>35 Excellent
31-34
Good
28-30 Marginal
<28
Fail
NOTE: Set-top boxes can tolerate some Post
FEC errors, but cable modems
cannot.
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Downstream Digital Measurements
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Constellation Analysis
Noise
CW Interference
Phase Noise
I/Q Gain Error
I/Q Phase Error
Compression
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Equalizer Stress
 Digital receivers use adaptive equalizers to negate the effects of




signals arriving other than the desired signal.
Signals can arrive ahead of or after the desired signal. In a
cable system, the majority of signals are reflections and microreflections that arrive after the desired signal.
Cable modems and digital set top boxes must be able to handle
pre and post (DELAYED) signals at levels defined by DVB
standards. If the equalizer is pushed beyond those limits, errors
will occur.
By using the Velocity of Propagation, the distance to the source
of the reflection can sometimes be located. If the reflections
occur before the next upstream amplifier, they are simply
amplified and passed downstream thereby eliminating the
ability to perform fault detection based on reflection time.
Equalizer stress is used more as a figure of merit for the margin
available to the set top box or cable modem.
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Cable Modem Downstream
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Downstream Frequency Response
DOCSIS Specifies <.5 db peak to valley per MHz
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Intermittents
 While you can’t measure a problem that isn’t happening, there
may be clues
 Any reading that is not normal for your system may be suspect
 Example -Lower than normal MER/BER
 Erodes headroom and error margin – any degradation will cause issue
 Higher than usual ingress/noise
 Check Ingress in Reverse AND Forward Path
 Any CPD
 CPD levels are often variable – may be minor now – major tomorrow
 Use Statistical Measurements to monitor over time
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Cable Modem Downstream
 Stats Mode
Measurements
Graphed over time
 MER and Pre and
Post BER measured
over time
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Upstream Certification
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DOCSIS Compliant Upstream
 Linear Impairments such as:
• Micro-reflections
(per DOCSIS spec.)
• Amplitude ripple
-10 dBc @ <= 0.5 µsec
-20 dBc @ <= 1.0 µsec
-30 dBc @ > 1.0 µsec
(0.5 dB/MHz per DOCSIS spec.)
• Group Delay
(200 ns/MHz per DOCSIS spec.)
 Non-linear Impairments such as:
• Common Path Distortion (CPD)
• Return Laser Clipping
 Transient Impairments such as:
• Ingress & Impulse Noise (CNR > 25 dB per DOCSIS)
 MER / Pre & Post-BER Channel Characterization
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Effects of Over-Driving a Laser
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CPD – Note the 6 MHz Marker Delta!
Because the channels in the forward system are 6 MHz
apart, the sum and difference frequencies occur at 6
MHz intervals as well.
6 MHz
6 MHz
6 MHz
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Upstream Characterization
SPECTRUM ANALYZER
with QAM DEMOD
16-QAM
TRANSMITTER
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Upstream Laser Compression Constellation
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Micro-Reflections
Micro-reflections are an indication
of a mismatch in the network.
 A problem because the mismatches reflect the
incident signal back towards the source causing
standing waves (or ripples) in the amplitude of
the frequency response where these 2 signals
collide
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Group Delay
 Group delay occurs at the roll-off points of the diplex filters
and its effect gets worse with more filters in the cascade.
(remember there are 2 for every active)
 Group delay affects MER; to achieve minimum MER levels for
16-QAM group delay must be reduced to a certain level
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Which means?
As different frequencies pass through a Cable System,
some will move faster than others—
Group Delay
5 MHz
5 MHz
10 MHz
10 MHz
15 MHz
20 MHz
25 MHz
30 MHz
35 MHz
40 MHz
SYSTEM
Filters &
Traps
15 MHz
20 MHz
25 MHz
t
5 MHz
10 MHz
SYSTEM
Filters &
Traps
15 MHz
20 MHz
25 MHz
30 MHz
35 MHz
40 MHz
30 MHz
35 MHz
40 MHz
T I M E
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Group Delay
 DOCSIS specifies 200 nSec/MHz
• But <70 nSec/ MHz is recommended for VoIP in 16-QAM
modulation
240.6 ns/MHz
at 35 MHz!!!
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16-QAM with Group Delay
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Time Lapsed Analysis of US Channel
MER
Pre-FEC
Post-FEC
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Return Path Monitoring
Early Fault Detection and Faster Node characterization for
VoIP and other return services
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Moving Up the Troubleshooting
Pyramid
 Now we can move from RF diagnostics to DOCSIS
CALL SIGNALING
& VOICE TRAFFIC
DQoS–SERVICE FLOWS
And DOCSIS Measurements
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Cable Modem Connect
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Cable Modem Detail
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Signaling and Service Flows
 There are 2 types of traffic in a VoIP call, signaling and





the actual voice transmissions
The signaling sets up the call path and tears it down
after the call is completed. Signaling also provides dial
tone, ring, and ring back.
Service flows are simply a system of prioritizing digital
transmissions. Some service flows take priority over
others.
Service flows only exist between the CMTS and the CM.
Many systems use best effort data transmission for
signaling and QoS for voice.
A QoS service flow always gives voice traffic priority and
allocates separate bandwidth for that traffic.
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Call Signaling and Service Flows
MTA
CM
CMTS
CMS
CMTS
CM
MTA
Call Signaling (best effort)
Call Signaling (best effort)
Service Flow Add
Service Flow Add
RTP Call Flow (QoS)
Call Signaling (best effort)
Call Signaling (best effort)
Service Flow Del
Service Flow Del
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Some Common “DOCSIS” Call
Preventers?
Call Signaling Fails to Go Through
• “Best Effort Service” competes with other traffic
• Usually TCP/IP signaling will go through, but customer may not wait for dial tone or
digits
• CMS receives excessively delayed digits from DOCSIS contention region – REQuest
– Grant period used by other best effort services such as Vonage, gamaing, etc.
• Remedy  Establish dedicated QoS for Call Signaling with (10 kbps) per eMTA,
drawback is uses US BW
Call Disconnects After Ring
• eMTA and CMTS unable to establish DQoS
– Bad eMTA, not PacketCable certified or bad PacketCable certificate in eMTA
– eMTA CODEC or configuration file mis-configured
– CMTS out of Service Flows – Failure to delete inactive SIDs
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Test Best Effort & Service Flow
Channels
STEP 1
Test the IP Best Effort Service Flow
- Packet Loss, Latency & Jitter
- Best Effort Verification for Call Signaling
STEP 2
 Test the Upstream &
Downstream Voice Quality
– VoIP MOS & R-Factor tests
– Using DOCSIS Service Flow (QoS)
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Testing Best Effort Call Signaling
 Test the performance of your
systems “Best Effort” services
 Good for testing network
performance for call signaling
 Will your eMTA establish
communication with the CMS?
 Measures
Packet loss
Latency
Jitter
Courtesy Sunrise Telecom Broadband
Note: Does not use QoS (DOCSIS Service Flow)
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Moving Up the Troubleshooting
Pyramid
 Now we can move from DOCSIS to IP
CALL SIGNALING
& VOICE TRAFFIC
DQoS–SERVICE FLOWS
And DOCSIS Measurements
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Building Blocks of Troubleshooting
IP traffic is quickly becoming an
“impairment” when it reaches excessive
levels, overwhelming the Cable Modem
Termination System (CMTS)
 Excessive CMTS utilization causes VoIP subscribers to
experience poor call quality, data users experience
slow downloads/uploads, and in worst case scenarios
the data network comes to a halt!
 Conventional RF monitoring tools cannot identify IP
traffic utilization
 Capacity planning tools become essential
 How do we manage the traffic on the network and our
subscribers before the CMTS becomes oversubscribed?
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Throughput Test
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Two Categories of VOIP Testing
1. Network
Components
Latency
Jitter
Lost Packets
2. Voice Quality
Mean Opinion Score
R Factor
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Latency Measurement
Latency is simply the transit time between one
network element to another
End to end delay
LATENCY = TIME
Toll Quality <150 mSec
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Types of Delay or Latency
 Transportation Delay
 Delay caused by the packet to get through the network components
such as routers and gateways. This is controlled by the system
architecture.
 Propagation Delay
 Simply the time it from one place to the other.
 Packetization Delay
 This is the time it takes to build the voice packets in the MTA
 Jitter Buffer Delay
 Delay caused by the jitter buffer
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Latency Causes
 Voice must be digitized, optionally compressed, processed for
echo canceling, and packetized.
 Voice packets may take multiple hops.
 Result is that voice over IP networks has more delay than
traditional circuit switched approaches.
 Solutions
 Run with very small packetization period (10ms).
 Minimize processing delays in system components.
 Minimize number of hops from source to destination.
 Give voice packets priority.
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How is Latency Measured?
Network
Entity
Directing ping packets through a UGS
pipe (service flow) provides a good
platform for testing latency to the
network side of the CMTS
PING
PING
QoS Service Flow
PING
DOCSIS
Analyzer
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VoIP Issue - Jitter
Jitter is simply variations in Latency
Delay in routers vary with current traffic load.
Voice packets can take different routes.
Net result is a variability in delay, this is called jitter. In other
words the packets don’t always arrive in the order they were sent.
 Voice is a real time communication so the packets must play out in
the order they were transmitted
 Solution.




 Jitter buffer at the playout side.
 Received packets are placed here first before playout.
 Builds in a standard delay to allow packets to arrive, get buffered up, then
played out.
 User hears no gaps between delayed packets.
 Give voice packets priority.
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Jitter = Change in Latency
Packet Sequence In
1
2
3
4
5
Packet Sequence Out
UGS Service Flow
3
2
4
1
5
Transmitted packets evenly spaced in time
Received packets unevenly spaced in time
Packets are experiencing changes in transit
time (Latency)
Delta Latency = Jitter
Most gateways use buffering to compensate
for jitter, but buffering can contribute to
latency
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Jitter Buffer
Incoming voice packets
3
2
4
1
5
 Voice packets have
sequence numbers
so a buffer can
reconstruct the
incoming traffic
 Buffer plays out
packets in sequence
order
 Jitter Buffers cause
delay
1
2
3
4
5
To Listener
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What’s Good? What’s bad?
Latency - Numbers vary but…..
Preferred <60 mSec
Maximum 150 mSec
Very annoying about 250 mSec
Jitter
Preferred equal to or <20 mSec
Maximum should be <30 mSec
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VoIP Issue - Lost Packets
 There is no time for retransmission!
 Packet loss can occur due to traffic or
physical layer problems
 Solutions – PLC Packet Loss Concealment
 If single packet is lost, replay the last
packet because the human ear is very
forgiving.
 After one replay, play white noise.
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VOIP & Lost Packets
VOIP is very sensitive to lost
packets
A 1% packet loss will cause the call
to break up
A 3% packet loss will drop the call
completely
Some telcos spec 0.5% packet loss
or better for High Speed data/voice
circuits
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Voice Quality Tests
MOS – Mean Opinion Score
Actual listeners grade performance on a scale
of 1-5
A more modern method uses a MOS server and
send voice data packets to the server for
analysis.
The server also sends voice data back to the
instrument which also performs analysis
Using this method, both upstream and
downstream are analyzed separately.
R Factor is essentially the same thing only rated
on a different scale based on 100 instead of 5
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Factors Affecting Voice Quality
 Latency, Jitter and Lost
Packets
 Noise
 Delay
 Echo
 Intelligibility
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Cable Modem VoIP Measurements
 Cable Modem or Ethernet VoIP Tests
 Downstream
 Upstream
 CMTS Round Trip
 Select Test Length
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Knowledge Check
What is the primary source of
impairments in a DOCSIS network
NCS signaling is sent in a DOCSIS 1.1 service
flow for guaranteed QoS. True or False
DOCSIS specifies that the CM / eMTA achieves
a Post-FEC BER to be less than or equal to what
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Thank You!
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