Managing Cable Telephony Performance Albert Higashi, Telchemy

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Managing Cable Telephony PerformanceAlbert
Higashi, Telchemy
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Outline

• Basic network model
• Packet path problems
• VoIP over Cable potential sources of problems
• Service Management
• Active vs Passive Service Monitoring
• Troubleshooting
• Summary

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Voice over IP Basics

• Voice over Internet Protocol
• Characteristics
• Packetized voice
• Packets carried in IP frames
• Uses RTP over UDP
• RTP Real time service
• UDP unacknowledged datagrams
• Challenges
• Signalling
• Latency of IP Networks
• Quality of Service

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Components of an IP Telephony System
Softswitch/ Call Management
Trunking / Media Gateway
IP-POTS
IP
IP
IP Phone
POTS Phone
IP-IP
IP
IP Phone
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Inside an IP Phone or Gateway
Jitter buffer
CODEC
E T H E R N E T
Echo Control
RTP/ UDP/ IP TCP/ IP
IP Network
Call Signalling
Estimated Residual Echo Return Loss
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Codecs and PLC Algorithms
t milliseconds
Codec or Vocoder
1100101010
n samples
Translates a block of speech samples to a
codeword G.711 -gt 1 sample
-gt 8 bit codeword G.729A -gt 80
samples -gt 80 bit codeword G.723.1 5.3kbps -gt
240 samples -gt 160 bits
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Codec Performance
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Packet Loss Concealment

• Attempts to hide the effects of packet loss
• Techniques
• Silence substitution
• Inserts silent frame
• Noise substitution
• Inserts noise frame
• Packet repetition
• Repeats previous packet
• Packet interpolation
• Inserts packet based on previous and subsequent
  packets
• Frame interleaving
• Separates adjacent frames for transmission
• Forward Error Correction
• Transmits redundant information

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Jitter Buffers
On time
Jitter Buffer
Packets played out at regular intervals
Delayed
Late arriving packet may be discarded

• Absorb variations in latency (jitter)
• Out-of-sequence packets re-ordered
• Duplicate packets removed

• Introduce delay
• Discard late packets

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Inside an IP Phone or Gateway
Jitter buffer
CODEC
E T H E R N E T
Echo Control
RTP/ UDP/ IP TCP/ IP
IP Network
Call Signalling
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Packet Telephony - Architecture
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Downstream
Upstream
PSTN
Analog phone
Local Loop
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Packet Telephony - Architecture
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Packet Path
PSTN
Analog phone
Local Loop
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Packet Telephony - Architecture
IP Phone or Adaptor
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Packet Path
PSTN
Analog phone
Local Loop
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Packet Path
Asymmetric path
CODEC
CODEC
Packet loss Jitter Delay Re-ordering Outages Trans
ient problems
CODEC Extra delay Distortion
Jitter buffer Extra delay Late packets discarded
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Problems HFC network
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Downstream
Upstream
PSTN
Analog phone
Local Loop
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Problems HFC network

• Packet loss, delay, jitter
• Packet loss - discard errored packets
• Paths are asymmetric
• Downstream path
• Upstream path
• Ingress noise

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Problems IP network
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Downstream
Upstream
PSTN
Analog phone
Local Loop
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Problems IP network

• Route problem
• One way voice/ no voice
• Excessive delay
• Link failures
• Short break in voice path (one or two way)
• Periodic problems
• Route flapping
• Routing table updates
• Transient congestion
• Transient increase in jitter (and loss)
• Can be due to DoS attack
• Access link congestion
• Transient increase in jitter, COMMON problem

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Jitter
Average jitter level (PPDV) 4.5mS Peak jitter
level 60mS
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Effects of Jitter

• Low levels of jitter absorbed by jitter buffer
• High levels of jitter
• lead to packets being discarded
• cause adaptive jitter buffer to grow - increasing
  delay but reducing discards
• If packets are discarded by the jitter buffer as
  they arrive too late they are regarded as
  discarded
• If packets arrive extremely late they are
  regarded as lost hence sometimes lost packets
  actually did arrive

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Packet Loss
Average packet loss rate 2.1 Peak packet loss
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Packet Loss is bursty

• Packet loss (and packet discard) tends to occur
  in sparse bursts - say 20-30 in density and one
  second or so in length
• Terminology
• Consecutive burst
• Sparse burst
• Burst of Loss vs Loss/Discard

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Example Packet Loss Distribution
Consecutive loss
20 percent burst density (sparse burst)
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Loss and Discard

• Loss is often associated with periods of high
  congestion
• Jitter is due to congestion (usually) and leads
  to packet discard
• Hence Loss and Discard often coincide
• Other factors can apply - e.g. duplex mismatch,
  link failures etc.

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Example Loss/Discard Distribution
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Leads To Time Varying Call Quality
High jitter/ loss/ discard
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Packet Loss Concealment
Estimated by PLC

• Mitigates impact of packet loss/ discard by
  replacing lost speech segments
• Very effective for isolated lost packets, less
  effective for bursty loss/discard
• But isnt loss/discard bursty?
• Need to be able to deal with 10-20-30 loss!!!

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Effectiveness of PLC
Codec distortion
Impact of loss/ discard and PLC
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Call Quality Problems

• Packet Loss
• Jitter (Packet Delay Variation)
• Codecs and PLC
• Delay (Latency)
• Echo
• Signal Level
• Noise Level

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Problems PSTN Echo
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Downstream
Upstream
PSTN
Analog phone
Local Loop
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Effect of Delay on Conversational Quality
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Causes of Delay
External delay
Accumulate and encode
RTP
IP UDP TCP
CODEC
Echo Control
Network delay
Jitter buffer, decode and playout
RTP
IP UDP TCP
CODEC
Echo Control
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Cause of Echo
Gateway
IP
Echo Canceller
Acoustic Echo
Line Echo
Round trip delay - typically 50mS
Additional delay introduced by VoIP makes
existing echo problems more obvious Also -
convergence echo
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Echo problems

• Echo with very low delay sounds like sidetone
• Echo with some delay makes the line sound hollow
• Echo with over 50mS delay sounds like. Echo
• Echo Return Loss
• 55dB or above is good
• 25dB or below is bad

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Call Quality Problems

• Packet Loss
• Jitter (Packet Delay Variation)
• Codecs and PLC
• Delay (Latency)
• Echo
• Signal Level
• Noise Level

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Problems Loss Plan
Analog phone
Softswitch
Head end
MTA
CMTS
HFC network
Media Gateway
IP
Downstream
Upstream
PSTN
Analog phone
Local Loop
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Signal Level Problems
Amplitude Clipping occurs -- speech sounds loud
and buzzy
0 dBm0
-36 dBm0
Temporal Clipping occurs with VAD or Echo
Suppressors -- gaps in speech, start/end of
words missing
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Noise

• Noise can be due to
• Low signal level
• Equipment/ encoding (e.g. quantization noise)
• External local loops
• Environmental (room) noise
• From a service provider perspective - how to
  distinguish between
• room noise (not my problem)
• Network/equipment/circuit noise (is my problem)

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Measuring VoIP performance
Analog signal based
VoIP Specific
VQmon ITU G.107
ITU P.862 (PESQ)
Active Test - Measure test calls
VQmon ITU P.VTQ
ITU P.563
Passive Test - Measure live calls
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Gold Standard - ACR Test

• Speech material
• Phonetically balanced speech samples 8-10 seconds
  in length
• Test designed to eliminate bias (e.g.
  presentation order different for each listener)
• Known files included as anchors (e.g. MNRU)
• Listening conditions
• Panel of listeners
• Controlled conditions (quiet environment with
  known level of background noise)

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Example ACR test results

• Extract from an ITU subjective test
• Mean Opinion Score (MOS) was 2.4
• 1Unacceptable
• 2Poor
• 3Fair
• 4Good
• 5Excellent

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Packet based approaches
Test Call
VoIP Test System
VoIP Test System
IP
Measure call
Live Call
VQmon, G.107. P.VTQ
VoIP End System
VoIP End System
IP
Passive Test
Passive Test
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Signal based approaches
P.862 Tester
Test Call
VoIP End System
VoIP End System
IP
P.862 is an Active Test Approach
VoIP End System
VoIP End System
IP
P.563 Tester
P.563 is a Passive Test Approach
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Active vs Passive testing

• Active test
• Can be done at any time, scheduled or on-demand
• Sees the path between active test functions
• Can detect problems before customers are affected
• Ideal for pre-deployment, service assurance and
  troubleshooting
• Passive test
• Can only be done when there are live calls
• Sees the path from the source VoIP device to the
  monitoring point
• Measures what actually happened to customer call
• Ideal for service management, post-call problem
  diagnosis

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Deploying passive monitoring (PacketCable 1.5)
Analog phone
MTA
CMTS
HFC network
Media Gateway
IP
PSTN
Active monitor
Passive monitor
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Deploying active monitoring
Analog phone
MTA
CMTS
HFC network
Media Gateway
IP
PSTN
Active monitor
Passive monitor
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Management infrastructure
Analog phone
MTA
CMTS
HFC network
Media Gateway
IP
PSTN
Active monitor
Passive monitor
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Outline

• Basic network model
• Packet path problems
• VoIP over Cable potential sources of problems
• Service Management
• Active vs Passive Service Monitoring
• Troubleshooting
• Summary

Check out www.voiptroubleshooter.com