rajah cisco

© 2008 Cisco Systems, Inc. All rights reserved.

Optimizing Channel

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Optimizing Channel Change

Time

Rajesh Rajah (rajeshra @ cisco.com)

Cisco IPTV Practice


Optimizing Channel

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Agenda

� Introduction

� Components of channel change time

� Factors for Optimization

� Cisco complementary techniques

Visual Quality of Experience (VQE) solution

Buffer optimization on STB

� Summary


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Acronym Finder

Real Time Protocol/ Real Time Control ProtocolRTP/RTCP

Quality of ServiceQOS

Application Layer Forward Error CorrectionAL-FEC

Digital Subscriber Line Access Multiplexer DSLAM

MPEG Instantaneous Decoder RefreshIDR

Internet Group Management ProtocolIGMP

Entitlement Control/Management MessagesECM/EMM

MPEG Group of Pictures (IBP frames)GOP

MPEG Program Specific InformationPSI

Moving Picture Experts GroupMPEG

Protocol Independent Multicast Source specific MulticastPIM SSM

Set Top BoxSTB

Standard/High DefinitionSD/HD

User Datagram ProtocolUDP

Visual Quality of Experience (VQE)VQE

MPEG Program Map TablePMT

MPEG Program Association TablePAT

Conditional Access TableCAT

Advanced Video CodecAVC


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Channel Change/Zap Time

� Total duration from the time viewer presses the channel change button, to the point the picture of the new channel is displayed, along with corresponding audio

� Zap-time in IPTV customer deployments in Europe

Average zap time range from 1.5 to 2.8 seconds

Overall average of 1.99 seconds1.99 seconds

� This presentation is focused on Wireline IPTV network• Examines each factor in detail

• Concludes that IP multicast is a small component

• Explains the techniques for overall improvement of zap time


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AccessAggregation

Multicast Channel flow in an IPTV Network

STBHomeGateway

Eg:DSLAM

PE-AGG

Core Distribution/ regional

Home NetworkVideoHeadend

Same choices for all access technologies Different by access technology

IGMPsnooping

IGMPsnooping

IGMP:{Limits}

{Static-fwd}PIM-SSM

PIM-SSM

L3 Transport Options in clouds:Native: PIM-SSM or MVPN/SSM

MPLS: LSM / mLDP RSVP-TEOpt.Source

Redundancy

IGMP

PIM-SSM

All or Selected channels

Video Stream x

IGMP Static Joins

(<<X)

IGMP Join (say, 232.1.1.1)IGMP Join

IGMP JoinPIM Join

PIM JoinPIM Join

Video Stream x

(X)Video Channels


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Channel change Events Summary

STB MPEG

Network

STB Related to STB implementation

Related to network delays

Related to STB MPEG bufferNot to scale*

STB STBNetwork STB

User hits channel change on remote

SW starts channel change

STB sends IGMP leave (wire), clear old buffers

STB sends IGMP join (wire)

Leave/Join/Network Latency

STB MPEG Buffer

1st UDP packet arrives at STB

SW recognizes UDP pkt

Start filling jitter buffer

Jitter buffer full

Wait for arrival of PSI – PAT, PMT, CAT

Wait for arrival of I-frame

STB MPEG buffer processing complete

STB starts decode

Channel change complete

Video/Audio is played

* t=0


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Sample Channel change time calculationAVC/H.264 SD on IPTV DSL

13

12

11

10

9

8

7

6

5

4

3

2

1

~ 500 - 580 ms~ 125 msSTB MPEG bufferWait for PAT/PMT

~80 – 160ms~ 20 – 60msRouter/NetworkCore/Agg Network Latency

STB

STB MPEG buffer

STB MPEG buffer

STB MPEG buffer

STB

DSLAM/Network

DSLAM/Network

DSLAM/Network

DSLAM/Network

STB

STB

Device/Location

~ 20 - 40 ms< 10 msDSLAM gets Join for channel Y

< 10 msDSLAM gets Leave for channel

X

~ 50 ms

~ 1s to 2s

~ 250 ms to 2s

~ 125 ms

~ 300 ms

~ 10 ms

~ 30 – 50 ms

< 10 ms

< 10 ms

Typical Latency

~ 60 - 100 msDSL Latency (FEC/Interleave)

~ 50 – 90 msDSLAM stops channel X, and

sends Channel Y

Send IGMP Join for channel Y

Send IGMP Leave for channel X

~ 1.9s – 4.8sDecode

~ 1.8s – 4.7sMPEG buffer

~ 870 ms – 2.7sWait for I-frame

~ 620 - 700 msWait for ECM/CA

~ 380 - 460 msDe-jitter buffer

Cumulative Latency

Channel Change Latency Factor


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Zap time – Network factors

1. Access network latency (processing/propagation delays)

STB – IGMP Leave channel X, Join Y

DSLAM – Stop X, Start Y

DSL FEC/Interleave

IGMP features used (version, fast leave, snooping etc)

Availability of the channel (Channel replication point)

2. Core/Aggregation network latency

Multicast routing mechanisms used (PIM Source specific mcast –SSM etc)

Availability of the channel (Channel replication point)

Small component in the overall zap time ~50 – 200 ms

Network Quality of Service (QoS) to ensure minimal jitter, latency and packet drop


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Zap time – MPEG acquisition

1. Parse meta data to locate MPEG PSI Info

Wait for and parse PAT

Wait for and parse PMT

Typical frequency – 100ms to 500ms

2. Obtain conditional access keys (ECMs) to decrypt channel

Wait for ECMs – part of PMT – 100ms to 500ms

Key changes/rotation – implementation dependant

EMMs typically sent using CAT (PID 1)

3. Obtain MPEG key frame (aka Random access point)

I-frame (MPEG2) or IDR frame (H.264)

One Index frame (full picture) per GOP – 12 to 30 (IBP) frames

Typical frequency of I-frame – 500ms.

Long GOP structure (2-4 seconds) saves bandwidth, but can cause significant channel change latency

Probability of hitting I-frame as the first frame (>20%)


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Zap time – STB Buffering/Decode

1. Jitter buffer fill (De-jitter)Prevent under-run for Network-introduced Jitter – 100 to 200ms

Additional buffer for AL-FEC support (say, COP3)

Network-related jitter – recommended max of 50 ms

2. MPEG Buffer (Encoder buffer fullness model)Typical latency – 750ms to 2s

Higher buffer for H.264/AVC than MPEG2

Slightly longer delay for MPEG2/AVC HD compared to SD flavors

3. Decode/Display delayTypically about 50 ms

Video buffer delay is usually higher than audio buffer delay


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Optimizing Channel change time – Page 1

� GOP length tuning� Tuning PAT/PMT intervals (if supported)

Encoder

� Tuning of ECM intervals (PMT)

� Key rotation timeframeConditional Access

� Cache PAT/PMT� Buffer optimization and play-out techniques

STB

� Tuning IGMP timers � Video-optimized QoS config

#Residential Gateway

(RG)

Optimization FactorsDevice

# Not a direct contributor to reduce zap time. But, helps reduce

response variability and enables better treatment for Video


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Optimizing Channel change time – Page 2

� IGMP static joins for popular channels� Video-optimized QoS config

#

� Secured control plane #

Distribution/Aggregation

Network Elements

� IGMP Fast/Immediate leave

� Tuning IGMP timers (Query time etc)� Explicit IGMP Host tracking (IGMPv3)� Video-optimized QoS config

#

� Secured control plane #

Access Network Elements

(DSLAM/MetroE

switch/PON)

� Video-optimized QoS config #(marking, scheduling

etc)Headend Router

� Secured control plane #(PIM/IGMP limits, Control

plane policing, Hardware rate-limiters etc)� Video-optimized QoS config

#Core Network Elements

Optimization FactorsDevice

# Not a direct contributor to reduce zap time. But, helps reduce

response variability and enables better treatment for Video


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Complementary Solutions for accelerating Channel change

Two Independent Cisco solutions

� I-frame caching and Unicast burst to STB (Cisco VQE appliance)

� STB buffer optimization technique


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Cisco Visual Quality of Experience (VQE)

VQE-S

Aggregation Router

DSLAM

STB

VQE-CEncoder

Multicast StreamUnicast Stream

Head-End

Video MUX

� Enables I-frame caching and Unicast burst to STB

� VQE server typically positioned at Aggregation layer

� Also, provides Error repair by retransmitting missed video packets to STB


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Cisco VQE—Rapid Channel Change (RCC)Fast Fills STB Video Buffer from Broadcast Cache

VQE

STB

IGMP Leave

VQE Signaling – New channel request

Unicast Video Burst To STB

IGMP Join

Multicast Stream

Unicast Stream

Control Messages

New Multicast Stream

Tim

e

VQE supports standards based RCC using standard Real-Time Transport Control

Protocol (RTCP)Open-source client for 3rd party STB

STB Merges theUnicast/Multicast Streams

and Discards the Duplicates

Sub-second Channel Change Time. Over 50% improvement

Works with both MPEG-2 & MPEG-4 AVC video streams


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• Current channel change times average ~2.2 seconds

• Cut channel change time by over 50%

• When used with VQE rapid

channel change (which

guarantees I-frame within short time window) can improve even

further

Cisco Buffer Optimization on STBImproves viewer experience without adding upstream complexity

Analysis Sample Size:N early start =150N normal = 150

Average Improvement: ~1.2 sec

1.2 Sec

~1 second channel change – 2X faster

No network equipment required

Does not require 30% bandwidth overhead

Can pair with VQE for error correction/channel change


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Trade-offs and Factors to consider

� Tradeoffs

– Bandwidth

– Viewer-experience (Quality of Experience)

– Implementation complexity

– CPU/Memory/Buffer requirements

� Factors to consider for high-quality channel change

– Seamless channel switchover (avoid artifacts/blocking, jerkiness etc)

– Video and Audio sync (Lip-sync)

– Concurrent channel change factor - Peak % (10 to 25%)


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Key Take-Away Points

� Random access delay, MPEG buffer delay etc are significant contributors to zap time

� IP Multicast is a small component in the overall zap time

� Creative mechanisms to accelerate zap time are available

� Cisco VQE enables rapid channel change (RCC) using bursting of cached I-frames to STB

� Intelligent Buffer optimizations can lead to faster zap time


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Additional Resources

� VQE specification on CCO

http://www.cisco.com/en/US/prod/collateral/video/ps7191/ps7127/product_data_sheet0900aecd806c0bfb.html (Currently, supports RTP retransmission and FEC)

� VQE client (VQE-C) reference implementationftp://ftp-eng.cisco.com/ftp/vqec/

� Sample Benchmark study of several European providers (Overall average of ~2 seconds)http://www.01net.com/outils/fai.php?rub=4667&type=television

� Optimizing Video Transport Networkhttp://www.cisco.com/en/US/prod/collateral/routers/ps368/prod_white_paper0900aecd80478c12_ns610_Networking_Solutions_White_Paper.html


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Reference slides


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Zap time for various systems

~ 1.5 – 3sMPEG2 over IP Multicast

~ 2 – 3sH.264 over QAM

~ 3 – 5sH.264 over QPSK

~ 1.7 – 3sH.264 over IP Multicast

~ 2 – 4sMPEG2 over QPSK

~ 3 – 4sMPEG2 over 8VSB

~ 1.2 – 3sMPEG2 over QAM

~ 1 – 3sAnalog (off-air)

~ 1sAnalog (Cable)

Typical Latency System Type (SD)

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