Application Note Contents
Title Managing IPTV Performance Overview.......................................................... 1
Series IP Video Performance Management IPTV Services .................................................. 1
Date February 2008 Factors Affecting the Performance of IPTV.... 2
Video Impairments .......................................... 4
Video Performance Metrics ................................ 5
IPTV, Internet TV, and Video on Demand
provide exciting new revenue opportunities IPTV Performance Management ........................ 6
for service providers. This Application Note
Summary .......................................................... 8
describes some of the typical issues and
problems affecting IPTV service quality, and
introduces a cost-effective approach to service
IPTV Services make it publicly available.
IPTV offers exciting new opportunities for service • Video on Demand (VOD) is a service that
providers to introduce integrated voice, video, and provides access to movies or other video
data services over broadband. A number of video content on demand. This could be part
service types can be delivered over IP: of an IPTV service or could be a service
offered independently over the Internet.
• “IPTV” is generally used to refer to a closed
Video over IP service with a broad range In some cases, IPTV services are not seen as
of content delivered by a service provider. directly competing with existing cable or satellite
Some definitions of IPTV would suggest service but may provide some specialization; for
that the broadband connection to the home example, a VOD service or a service with content
would also be part of this service; however, in some particular language.
within the context of this Application Note
we regard the two as independent. Many different models can be used for delivery
of content over IP. Real-time streaming services
• “Internet TV” is generally a more open, play video content over IP using either the UDP or
Web-like service in which “stations” can TCP protocol and the video stream is decoded and
broadcast on the Internet. This potentially played in near-real time. Other approaches include
allows anyone to generate video content and the download of entire movies or large “chunks” of
Application Note Managing IPTV Performance February 2008 1
Telchemy Application Note
video to a disk drive or memory. encoded from the first block in the slice; an error
makes this information unusable for the remainder
This Application Note discusses some of the factors of the slice. Some errors may damage the frame
affecting the performance of IPTV and Internet TV structure and render the whole frame unusable.
services that use a streaming model, and explains
how performance monitoring can provide real-time For inter-frame or motion-based coding, motion
feedback to service providers. vectors are determined for each block and encoded.
As for intra-frame coding, errors can render a whole
Factors Affecting the Performance of slice or frame unusable. In simple inter-frame
IPTV coding systems, the loss of one frame can make all
subsequent frames unusable until the next I frame
Codec, Bit Rate, and Video Content is received, resulting in a significant period of
degraded, frozen, or blank video.
Video content is typically encoded and compressed
using MPEG-2, MPEG-4 Part 10 /H.264, Microsoft In most cases, the standards for video coding
WMV9/VC1, or other codecs. Video codecs provide considerable flexibility to both encoder
typically support a wide range of compression rates, and decoder, allowing a range of cost/performance
allowing a trade-off between quality and bandwidth. tradeoffs to be made. This can make it difficult to
precisely assess the impact of network impairments
Much of this compression comes from the use without knowledge of the exact implementation.
of inter-frame difference encoding—rather than
sending every video frame, only the difference Limited Bandwidth
between one frame and the previous frame is sent.
This works well if there is relatively little change Bandwidth limitations often occur in the access
in the image; however, if there is considerable link—typically a DSL or cable connection. If there
movement across much of the image, then either is insufficient bandwidth for the video stream, then
the bandwidth will increase or quality will reduce. some packets may be discarded in router buffers,
Many video codecs allow either a constant bit rate leading to quality degradation.
(in which case quality may vary) or variable bit rate
(in which case quality will vary less). A more subtle problem can occur due to the
highly variable rate of packet transmission due to
Common video codecs use a combination of dissimilar sized I, B, and P frames. The peaks in
intra- and inter-frame coding. For intra-frame packet transmission rate that occur during I frames
encoding, the image frame (I frame) is divided can lead to packet loss and hence quality degration.
into blocks, a Discrete Cosine Transform is used
to convert each block to a set of coefficients, and Packet Loss and Loss Concealment
then variable length coding applied. A group of
blocks is combined into a single entity (slice), Packet loss may occur for a variety of reasons,
sometimes carried within a single packet. If a including network congestion, link failure,
transmission error occurs, then the whole group insufficient link bandwidth, and transmission errors.
may be lost, creating a “stripe" within the decoded Packet loss is often bursty, with periods of high loss
image. This may occur because, for example, the occurring when network congestion levels are high.
DC coefficients within each block are predictively
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Telchemy Application Note
The type of quality degradation that occurs due to For UDP-based video streams, packet loss can
packet loss will depend on the protocol being used cause sections of frames, or complete frames, to be
to carry video: corrupted. As a frame often spans multiple packets,
and typical video streams include interpolated
• If UDP is being used, packet loss will frames, a given packet loss rate can result in a
directly impact image quality, as some parts frame loss rate six times higher (Figure 1 below).
of the video stream will be missing.
• If Reliable UDP is used, then lost
packets can be retransmitted; however, if Not all problems are due to IP impairments—if
retransmitted packets are lost, they will servers are not provisioned to support the maximum
not be "re-retransmitted" and hence image number of expected users, then server congestion
quality will suffer. can occur. This would generally lead to pauses in
video playback as the playout buffer level is too
• FEC may be used with UDP to replace lost low.
packets; however, if the packet loss rate is
too high (e.g., during bursts of loss), then The use of protocols such as UDP Multicast
FEC will not be as effective. can help with reducing server loading; however,
these rely on a large proportion of the subscribers
• If TCP is being used, then packet loss will watching the same content at approximately the
lead to retransmission, which can in turn same time.
lead to the playout buffer in the set-top
box being starved and to pauses in video
■ Figure 1. Impact of Packet Loss Rate on MPEG Frame Loss/ Error Rate
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Telchemy Application Note
Jitter and Timing Drift edges.
Network jitter is short term variation in packet • Blur is a loss in detail that occurs around
arrival time, typically due to network congestion. edges, typically due to the quantization of
IPTV set-top boxes or playback software typically transform coefficients representing higher
buffer received video for 5-20 seconds before frequencies.
playout, which in essence means that typical
network jitter levels have little effect. Larger delay Modern video codecs are very effective, but can
variations, for example due to server congestion, still suffer from degraded quality when operating at
can cause problems due to playout buffer starvation. low bit rate where there is considerable movement
in large areas of the video sequence.
Timing drift occurs when the sending and receiving
end clocks are running at slightly different rates. Transmission-related Impairments
This may require the receiving end to adjust its
clock rate in order to avoid occasional problems For IP video transmitted over the UDP protocol,
due to buffer underflow or overflow. packet loss can lead to significant quality
degradation. A simple non-robust video stream can
Core Network, Access Network, and Home be severely degraded with even low levels of packet
Network loss, due to the error propagation effects described
The IP transmission path typically starts at a video
server and ends at a set-top box. This means that Video quality is often represented in terms of PSNR
packets traverse multiple networks, often owned by (Peak Signal to Noise Ratio), which is a measure
different service providers. Core IP networks are of the root mean square (RMS) error between
often high capacity optical networks that operate the original and reconstructed video sequences.
well below congestion levels, and hence when Generally a PSNR of under 20dB is regarded as
problems occur, they are often located within the unwatchable, and this level is reached for MPEG-2
access network or home network. with a loss rate of under 1 percent.
Video Impairments Error mitigation algorithms are being increasingly
applied to help to compensate for packet loss.
Encoding-related Impairments Methods include:
Video encoding can introduce a number of different • Forward Error Correction - redundancy is
impairments, often related to the bit rate used for applied to the data stream to allow some
encoding and to the characteristics of the video proportion of lost or errored packets to be
sequence. Common problems include: replaced.
• Blockiness - typical video codecs process • Interleaving - in which the video stream is
images in small blocks, and quantization of split into alternate frames and each frame
parameter or coefficient values can lead to encoded separately.
discontinuities between blocks. This can
be exacerbated by the Mach Band effect, • Macroblock error concealment - spatially
in which the human eye tends to sharpen corresponding macroblocks are copied from
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Telchemy Application Note
the previous frame. latter provides a rating of the ability of the IP packet
transport to support video.
These approaches are used in some modern video
codecs and can help considerably with tolerance to Estimated Perceptual Quality Scores (MOS)
VQmon/HD's estimated Mean Opinion Scores
Video Performance Metrics (MOS) provide a 1-5 rating (5 being best) of user-
perceived quality, and incorporate some subjective
Video performance can be assessed in many ways. factors such as content dependency factors. MOS
Full Reference (FR) methods such as PSNR compare scores provided by VQmon/HD include:
the output video sequence with the input and
measure the level of distortion that has occurred. • Video MOS (MOS-V) - a score that
This comparison is fairly computationally intensive considers the effect of the video codec,
and requires access to the video streams at both frame rate, packet loss distribution, and GoP
ends of the connection. Zero Reference methods, structure on viewing quality.
such as the algorithms used in VQmon/HD, look at
the received IP video stream and estimate quality • Audio MOS (MOS-A) - a score that
based on the type of video codec, loss rate, loss considers the effect of the audio bit rate,
distribution, and other parameters. sample rate, and packet loss on viewing
VQmon/HD produces estimated perceptual quality
scores (MOS) And a video transmission quality • Audiovisual MOS (MOS-AV) - a score that
rating (VSTQ). The former provide estimates of considers the effect of picture and audio
the user-perceived quality of the video, and the quality and audio/video synchronization on
■ Figure 2 - Comparison of VQmon/HD MOS Score with Full Reference VQM Score
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Telchemy Application Note
the overall user experience. whereas Class D algorithms are very sensitive to
even low rates of loss.
• Burst MOS scores (Burst MOS-V, Burst
MOS-A) - scores indicating picture/ Figure 2 on page 5 shows how VQmon/HD’s
audio quality during "bad" periods when MOS-V score compared to MOS scores estimated
significant degradation is occurring. by the full reference NTIA VQM tool for a range
of video files and test conditions.
• Gap MOS scores (Gap MOS-V, Gap
MOS-A) - scores indicating picture/audio Content Detection
quality when little or no degradation is
occurring. VQmon/HD’s content dependency factors allow
more accurate MOS scores to be determined for
Video Transmission Quality Rating (VSTQ) different types of video content. If low levels
of impairments occur in a video sequence with
VQmon/HD's Video Service Transmission Quality considerable activity—for example, a sporting
(VSTQ) score is a 0-50 rating that indicates the even—subjective quality would not be impacted
ability of the IP network to reliably transmit video. as severely as it would if the video sequence were
VSTQ considers packet loss rate, the distribution of slow-moving and contained many fine details.
lost packets (i.e., burstiness), and the type and bit High levels of impairment, however, would cause
rate of codec. the video stream to break up or freeze, and hence
would be annoying in any video sequence.
PLC Effectiveness Calculation
IPTV Performance Management
In addition to modeling the behavior of a wide
variety of video codecs, VQmon/HD also models Performance Monitoring
four classes of packet/frame loss concealment,
which covers the range of typical implementations. IPTV services can benefit from real time non-
Class A loss concealment algorithms are very intrusive performance monitoring. Lightweight
robust and able to tolerate high levels of loss, agents, such as VQmon/HD, can be directly
IP Video Stream
VOD Server Residential IPTV
IP Network Gateway Set-top Box
Service Monitoring Function
■ Figure 3 - Example of Embedded IPTV Service Quality Monitoring Function
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Telchemy Application Note
integrated into set-top boxes and residential Impact of Content Scrambling/Encryption
gateways to provide service providers with
real-time feedback on service quality, and can Scrambling or encryption is often used to protect
potentially provide feedback to servers to enable video content (often called Digital Rights
optimization of the video streaming algorithm. Management or DRM). This means that IPTV
Extensions to RTCP XR (RFC3611) are being performance measurement systems cannot decode
developed to support the transport of metrics during the picture stream. Telchemy's VQmon/HD has
video sessions. been specifically designed to work with encrypted
content, and is able to extract information related to
Typical feedback metrics would include: both video content and I/B/P frame loss.
• MOS-V, MOS-A, and MOS-AV scores, Packet Loss vs. Perceptual Quality Scores
providing estimates of user-perceived
picture, audio, and audiovisual quality Some service providers attempt to measure video
quality using packet loss metrics (such as MDI).
• Packet loss rate, burst density, burst length, If packet loss affects I frames, the resulting errors
gap density, and gap length, providing some will extend through the entire Group of Pictures
insight into the impact of IP impairments and be very obvious to the viewer. If the same
packet loss rate affects B or P frames, the resulting
• I, B, and P frame statistics, including the error typically lasts for just one frame (e.g., 1/60th
packet loss rate within each type of frame second) and the viewer may not even notice the
event. Video performance monitoring algorithms
such as VQmon/HD are able to measure the impact
of each packet loss event on specific frame types,
and hence more accurately measure performance.
PLC Packet Loss Concealment
PSNR Peak Signal-to-Noise Ratio
ADSL Asymmetric Digital Subscriber Line
QoS Quality of Service
DC Discrete Cosine
RFC Request for Comments
DCT Discrete Cosine Transform
RMS Root Mean Square
DRM Digital Rights Management
RTCP XR RTP Control Protocol Extended Reports
DSL Digital Subscriber Line
RTP Real Time Protocol
FR Full Reference
SLA Service Level Agreement
GoP Group of Pictures
TCP Transmission Control Protocol
IETF Internet Engineering Task Force
UDP User Datagram Protocol
IP Internet Protocol
VDSL Very high speed/bit rate DSL
LAN Local Area Network
VOD Video On Demand
MDI Media Delivery Index
VoIP Voice Over Internet Protocol
MOS Mean Opinion Score
VQM Video Quality Metric
NTIA National Telecommunications Information
VSTQ Video Service Transmission Quality
WLAN Wireless Local Area Network
Application Note Managing IPTV Performance February 2008 7