voip testing

VoIP Testing VoIP Testing

SCTE New Jersey Chapter9/13/07

2Confidential & Proprietary

My Business CardMy Business Card

Larry JumpRegional Sales EngineerSunrise [email protected]


Today’s AgendaToday’s Agenda

DOCSIS Troubleshooting Pyramid RF Impairments DOCSIS Problems VoIP Impairments


Troubleshooting MethodologyTroubleshooting Methodology

What we fix todayWhat we fix today

Communications betweenCMTS & CMs only

Communications betweenCMTS & CMs only

Communications between CMs,MTAs, subscribers & IP serversCommunications between CMs,MTAs, subscribers & IP servers

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


Building Blocks of TroubleshootingBuilding Blocks of Troubleshooting

• DHCP / TFTP / ToD / DNS / CMS / etc. servers• Modem and MTA configuration files, CMTS configs

• 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

Evolution of RF Troubleshooting

DOCSIS & IP Protocols > 20% of Impairments


Build a Solid RF Foundation for the PyramidBuild 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


Forward SweepForward Sweep

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!

H

LR

Forward Signal PathHeadend or Hub Site


Return SweepReturn Sweep

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!

H

LR

Return Signal Path


Downstream MeasurementsDownstream Measurements


Modulation Error RatioModulation Error Ratio

MER is used as the single figure of merit for DVB-C 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.


Modulation Error Ratio (MER) is a measure of the phase and voltage variation.

Modulation Error Ratio (MER)Modulation Error Ratio (MER)

Each point represents one symbol (2-bits) in QPSK of data or one phase position.

The distance from the circle is the error.

Each point represents one symbol (2-bits) in QPSK of data or one phase position.

The distance from the circle is the error.

SymbolSymbol

ErrorError

MER (dB) 10 log

I j Io j 2 Q j Qo j 2

j1

N

Io j2 Qo j

2 j1

N

dB

MER(dB) 10 logRMS error magnitude

average symbol magnitude

MERT 10 log1

NN10MERi / 10

i1

NN

dB

• Finally, aggregate MER is just the sum of multiple MER measurements:

And expressed mathematically by:


DOCSIS Compliant DownstreamDOCSIS Compliant Downstream Analog Measurements

CNR ( ≥35 dB per DOCSIS spec) CSO ( ≥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


Typical BER/MER RequirementsTypical BER/MER Requirements

64 QAM 256 QAMBER MER MER Quality

10-10 >35 >35 Excellent

10-8 27-34 31-34 Good

10-6 23-26 28-30 Marginal

10-5 <23 <28 Fail

NOTE: Set-top boxes can tolerate some Post FEC errors, but cable modems cannot.


Downstream Digital MeasurementsDownstream Digital Measurements


Constellation AnalysisConstellation Analysis

Noise CW Interference Phase Noise

I/Q Gain Error I/Q Phase Error Compression


Equalizer StressEqualizer 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 micro-reflections 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.


Cable Modem DownstreamCable Modem Downstream


Downstream Frequency ResponseDownstream Frequency Response

DOCSIS Specifies <.5 db peak to valley per MHz


IntermittentsIntermittents

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


Cable Modem DownstreamCable Modem Downstream

Stats Mode Measurements Graphed over time

MER and Pre and Post BER measured over time


Upstream CertificationUpstream Certification


DOCSIS Compliant UpstreamDOCSIS Compliant Upstream

Linear Impairments such as:• Micro-reflections -10 dBc @ <= 0.5 µsec

(per DOCSIS spec.) -20 dBc @ <= 1.0 µsec -30 dBc @ > 1.0 µsec

• Amplitude ripple (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


Effects of Over-Driving a LaserEffects of Over-Driving a Laser


CPD – Note the 6 MHz Marker Delta!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


Upstream CharacterizationUpstream Characterization

SPECTRUM ANALYZERwith QAM DEMOD

SPECTRUM ANALYZERwith QAM DEMOD

16-QAM TRANSMITTER

16-QAM TRANSMITTER


Upstream Laser Compression - Upstream Laser Compression - ConstellationConstellation


Micro-ReflectionsMicro-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


Group DelayGroup 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


Which means?Which means?

As different frequencies pass through a Cable System, some will move faster than others—

10 MHz

15 MHz

20 MHz

25 MHz

30 MHz

35 MHz

40 MHz

5 MHz

T I M E

t

SYSTEM

Filters &

Traps

20 MHz

15 MHz

10 MHz

5 MHz

25 MHz

30 MHz

35 MHz

40 MHz

SYSTEM

Filters &

Traps

5 MHz

10 MHz

15 MHz

20 MHz

25 MHz

30 MHz

40 MHz

35 MHz

Group Delay


Group DelayGroup 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!!!


16-QAM with Group Delay16-QAM with Group Delay


Time Lapsed Analysis of US ChannelTime Lapsed Analysis of US Channel

MER

Pre-FEC

Post-FEC


Return Path Monitoring

Early Fault Detection and Faster Node characterization for

VoIP and other return services

Early Fault Detection and Faster Node characterization for

VoIP and other return services


CALL SIGNALING& VOICE TRAFFICCALL SIGNALING& VOICE TRAFFIC

DQoS–SERVICE FLOWSAnd DOCSIS Measurements DQoS–SERVICE FLOWSAnd DOCSIS Measurements

Moving Up the Troubleshooting Moving Up the Troubleshooting PyramidPyramid

Now we can move from RF diagnostics to DOCSIS


Cable Modem ConnectCable Modem Connect


Cable Modem DetailCable Modem Detail


Signaling and Service FlowsSignaling 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.


Call Signaling and Service FlowsCall Signaling and Service Flows

Call Signaling (best effort)

MTAMTA CMCM CMTSCMTS CMSCMS CMCM MTAMTACMTSCMTS


Service Flow Add Service Flow Add



Service Flow Del Service Flow Del

RTP Call Flow (QoS)


Some Common “DOCSIS” Call Some Common “DOCSIS” Call Preventers?Preventers?

Call Signaling Fails to Go Through

Call Disconnects After Ring

• “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

• 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


Test Best Effort & Service Flow Test Best Effort & Service Flow ChannelsChannels

Test the Upstream & Downstream Voice Quality

– VoIP MOS & R-Factor tests– Using DOCSIS Service Flow (QoS)

STEP 1

STEP 2

Test the IP Best Effort Service Flow- Packet Loss, Latency & Jitter- Best Effort Verification for Call Signaling


Testing Best Effort Call SignalingTesting 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 lossLatencyJitter

Courtesy Sunrise Telecom Broadband

Note: Does not use QoS (DOCSIS Service Flow)


CALL SIGNALING& VOICE TRAFFICCALL SIGNALING& VOICE TRAFFIC

DQoS–SERVICE FLOWSAnd DOCSIS Measurements DQoS–SERVICE FLOWSAnd DOCSIS Measurements

Moving Up the Troubleshooting Moving Up the Troubleshooting PyramidPyramid

Now we can move from DOCSIS to IP


Building Blocks of TroubleshootingBuilding 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 over-subscribed?


Throughput Test Throughput Test


Two Categories of VOIP TestingTwo Categories of VOIP Testing

1. Network Components

Latency

Jitter

Lost Packets

2. Voice Quality

Mean Opinion Score

R Factor


Latency MeasurementLatency Measurement

LATENCY = TIME

Latency is simply the transit time between one network element to another

End to end delay

Toll Quality <150 mSec


Types of Delay or LatencyTypes 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


Latency CausesLatency 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.


How is Latency Measured?How is Latency Measured?

PING

PING

Network

Entity

PING

PING PING

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

DOCSISAnalyzerQoS Service Flow


VoIP Issue - JitterVoIP 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.


Jitter = Change in LatencyJitter = Change in Latency

UGS Service Flow

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

1 2 3 4 5

Packet Sequence In

123 4 5

Packet Sequence Out


Jitter BufferJitter Buffer

Voice packets have sequence numbers so a buffer can reconstruct the incoming traffic

Buffer plays out packets in sequence order

Jitter Buffers cause delay

123 4 5

1 2 3 4 5

Incoming voice packets

To Listener


What’s Good? What’s bad?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


VoIP Issue - Lost PacketsVoIP 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.


VOIP & Lost PacketsVOIP & 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


Voice Quality TestsVoice 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


Factors Affecting Voice QualityFactors Affecting Voice Quality

Latency, Jitter and Lost Packets

NoiseDelayEcho Intelligibility


Cable Modem VoIP MeasurementsCable Modem VoIP Measurements

Cable Modem or Ethernet VoIP Tests Downstream Upstream CMTS Round Trip

Select Test Length


Knowledge CheckKnowledge Check

What is the primary source of impairments in a DOCSIS network

What is the primary source of impairments in a DOCSIS network

NCS signaling is sent in a DOCSIS 1.1 serviceflow for guaranteed QoS. True or False

NCS signaling is sent in a DOCSIS 1.1 serviceflow for guaranteed QoS. True or False

DOCSIS specifies that the CM / eMTA achievesa Post-FEC BER to be less than or equal to what

DOCSIS specifies that the CM / eMTA achievesa Post-FEC BER to be less than or equal to what


Thank You!

voip testing

Documents

docsis network

docsis specifications

docsis specctb

docsis speccso

good mer

equal quality

rf plant

return path