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VideoStreamingConcepts.pdf 流媒体技术概述 。
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Video Streaming: Concepts, Algorithms,
and Systems
John G. Apostolopoulos, Wai-tian Tan, Susie J. Wee
Mobile and Media Systems Laboratory
HP Laboratories Palo Alto
HPL-2002-260
September 18
th
, 2002*
E-mail: [japos, dtan, swee}@hpl.hp.com
video
streaming,
video
delivery,
streaming
media
content
delivery
networks,
video
coding,
error-
resilient;
multiple
description
coding
Video has been an important media for communications and entertainment for many
decades. Initially video was captured and transmitted in analog form. The advent of digital
integrated circuits and computers led to the digitization of video, and digital video enabled
a revolution in the compression and communication of video. Video compression became
an important area of research in the late 1980’s and 1990’s and enabled a variety of
applications including video storage on DVD’s and Video-CD’s, video broadcast over
digital cable, satellite and terrestrial (over-the-air) digital television (DTV), and video
conferencing and videophone over circuit-switched networks. The growth and popularity
of the Internet in the mid-1990’s motivated video communication over best-effort packet
networks. Video over best-effort packet networks is complicated by a number of factors
including unknown and time-varying bandwidth, delay, and losses, as well as many
additional issues such as how to fairly share the network resources amongst many flows
and how to efficiently perform one-to-many communication for popular content. This
article examines the challenges that make simultaneous delivery and playback, or
streaming, of video difficult, and explores algorithms and systems that enable streaming of
pre-encoded or live video over packet networks such as the Internet.
We continue by providing a brief overview of the diverse range of video streaming and
communication applications. Understanding the different classes of video applications is
important, as they provide different sets of constraints and degrees of freedom in system
design. Section 3 reviews video compression and video compression standards. Section 4
identifies the three fundamental challenges in video streaming: unknown and time-varying
bandwidth, delay jitter, and loss. These fundamental problems and approaches for
overcoming them are examined in depth in Sections 5, 6, and 7. Standardized media
streaming protocols are described in Section 8, and additional issues in video streaming are
highlighted in Section 9. We conclude by describing the design of emerging streaming
media content delivery networks in Section 10.
* Internal Accession Date Only Approved for External Publication
Copyright Hewlett-Packard Company 2002
1
1
VIDEO STREAMING: CONCEPTS, ALGORITHMS,
AND SYSTEMS
John G. Apostolopoulos, Wai-tian Tan, Susie J. Wee
Streaming Media Systems Group
Hewlett-Packard Laboratories
Palo Alto, CA, USA
{japos,dtan,swee}@hpl.hp.com
1. INTRODUCTION
Video has been an important media for communications and entertainment
for many decades. Initially video was captured and transmitted in analog
form. The advent of digital integrated circuits and computers led to the
digitization of video, and digital video enabled a revolution in the
compression and communication of video. Video compression became an
important area of research in the late 1980’s and 1990’s and enabled a
variety of applications including video storage on DVD’s and Video-CD’s,
video broadcast over digital cable, satellite and terrestrial (over-the-air)
digital television (DTV), and video conferencing and videophone over circuit-
switched networks. The growth and popularity of the Internet in the mid-
1990’s motivated video communication over best-effort packet networks.
Video over best-effort packet networks is complicated by a number of factors
including unknown and time-varying bandwidth, delay, and losses, as well
as many additional issues such as how to fairly share the network resources
amongst many flows and how to efficiently perform one-to-many
communication for popular content. This article examines the challenges
that make simultaneous delivery and playback, or streaming, of video
difficult, and explores algorithms and systems that enable streaming of pre-
encoded or live video over packet networks such as the Internet.
We continue by providing a brief overview of the diverse range of video
streaming and communication applications. Understanding the different
classes of video applications is important, as they provide different sets of
constraints and degrees of freedom in system design. Section 3 reviews
video compression and video compression standards. Section 4 identifies the
three fundamental challenges in video streaming: unknown and time-varying
bandwidth, delay jitter, and loss. These fundamental problems and
Chapter 1
2
approaches for overcoming them are examined in depth in Sections 5, 6, and
7. Standardized media streaming protocols are described in Section 8, and
additional issues in video streaming are highlighted in Section 9. We
conclude by describing the design of emerging streaming media content
delivery networks in Section 10. Further overview articles include [1,2,3,4,5].
2. OVERVIEW OF VIDEO STREAMING AND
COMMUNICATION APPLICATIONS
There exist a very diverse range of different video communication and
streaming applications, which have very different operating conditions or
properties. For example, video communication application may be for point-
to-point communication or for multicast or broadcast communication, and
video may be pre-encoded (stored) or may be encoded in real-time (e.g.
interactive videophone or video conferencing). The video channels for
communication may also be static or dynamic, packet-switched or circuit-
switched, may support a constant or variable bit rate transmission, and may
support some form of Quality of Service (QoS) or may only provide best effort
support. The specific properties of a video communication application
strongly influence the design of the system. Therefore, we continue by briefly
discussing some of these properties and their effects on video communication
system design.
Point-to-point, multicast, and broadcast communications
Probably the most popular form of video communication is one-to-many
(basically one-to-all) communication or broadcast communication, where the
most well known example is broadcast television. Broadcast is a very efficient
form of communication for popular content, as it can often efficiently deliver
popular content to all receivers at the same time. An important aspect of
broadcast communications is that the system must be designed to provide
every intended recipient with the required signal. This is an important issue,
since different recipients may experience different channel characteristics,
and as a result the system is often designed for the worst-case channel. An
example of this is digital television broadcast where the source coding and
channel coding were designed to provide adequate reception to receivers at
the fringe of the required reception area, thereby sacrificing some quality to
those receivers in areas with higher quality reception (e.g. in the center of
the city). An important characteristic of broadcast communication is that,
due to the large number of receivers involved, feedback from receiver to
sender is generally infeasible – limiting the system’s ability to adapt.
Another common form of communication is point-to-point or one-to-one
communication, e.g. videophone and unicast video streaming over the
Internet. In point-to-point communications, an important property is
whether or not there is a back channel between the receiver and sender. If a
back channel exists, the receiver can provide feedback to the sender which
the sender can then use to adapt its processing. On the other hand, without
a back channel the sender has limited knowledge about the channel.
Another form of communication with properties that lie between point-to-
point and broadcast is multicast. Multicast is a one-to-many
communication, but it is not one-to-all as in broadcast. An example of
multicast is IP-Multicast over the Internet. However, as discussed later, IP
Video Streaming: Concepts, Algorithms, and Systems
3
Multicast is currently not widely available in the Internet, and other
approaches are being developed to provide multicast capability, e.g.
application-layer multicast via overlay networks. To communicate to multiple
receivers, multicast is more efficient than multiple unicast connections (i.e.
one dedicated unicast connection to each client), and overall multicast
provides many of the same advantages and disadvantages as broadcast.
Real-time encoding versus pre-encoded (stored) video
Video may be captured and encoded for real-time communication, or it may
be pre-encoded and stored for later viewing. Interactive applications are one
example of applications which require real-time encoding, e.g. videophone,
video conferencing, or interactive games. However real-time encoding may
also be required in applications that are not interactive, e.g. the live
broadcast of a sporting event.
In many applications video content is pre-encoded and stored for later
viewing. The video may be stored locally or remotely. Examples of local
storage include DVD and Video CD, and examples of remote storage include
video-on-demand (VOD), and video streaming over the Internet (e.g. as
provided by RealNetworks and Microsoft). Pre-encoded video has the
advantage that it does not require a real-time encoding constraint. This can
enable more efficient encoding such as the multi-pass encoding that is
typically performed for DVD content. On the other hand, it provides limited
flexibility as, for example, the pre-encoded video can not be significantly
adapted to channels that support different bit rates or to clients that support
different display capabilities than that used in the original encoding.
Interactive versus Non-interactive Applications
Interactive applications such as videophone or interactive games have a real-
time constraint. Specifically the information has a time-bounded usefulness,
and if the information arrives, but is late, it is useless. This is equivalent to
a maximum acceptable end-to-end latency on the transmitted information,
where by end-to-end we mean: capture, encode, transmission, receive,
decode, display. The maximum acceptable latency depends on the
application, but often is on the order of 150 ms. Non-interactive applications
have looser latency constraints, for example many seconds or potentially
even minutes. Examples of non-interactive applications include multicast of
popular events or multicast of a lecture; these applications require timely
delivery, but have a much looser latency constraint. Note that interactive
applications require real-time encoding, and non-interactive applications
may also require real-time encoding, however the end-to-end latency for non-
interactive applications is much looser, and this has a dramatic effect on the
design of video communication systems.
Static versus Dynamic Channels
Video communication system design varies significantly if the characteristics
of the communication channel, such as bandwidth, delay, and loss, are
static or dynamic (time-varying). Examples of static channels include ISDN
(which provides a fixed bit rate and delay, and a very low loss rate) and video
storage on a DVD. Examples of dynamic channels include communication
over wireless channels or over the Internet. Video communication over a
dynamic channel is much more difficult than over a static channel.
Chapter 1
4
Furthermore, many of the challenges of video streaming, as are discussed
later in this article, relate to the dynamic attributes of the channels.
Constant-bit-rate (CBR) or Variable-bit-rate (VBR) Channel
Some channels support CBR, for example ISDN or DTV, and some channels
support VBR, for example DVD storage and communication over shared
packet networks. On the other hand, a video sequence typically has time-
varying complexity. Therefore coding a video to achieve a constant visual
quality requires a variable bit rate, and coding for a constant bit rate would
produce time-varying quality. Clearly, it is very important to match the video
bit rate to what the channel can support. To achieve this a buffer is typically
used to couple the video encoder to the channel, and a buffer control
mechanism provides feedback based on the buffer fullness to regulate the
coarseness/fineness of the quantization and thereby the video bit rate.
Packet-Switched or Circuit-Switched Network
A key network attribute that affects the design of media streaming systems is
whether they are packet-switched or circuit-switched. Packet-switched
networks, such as Ethernet LANs and the Internet, are shared networks
where the individual packets of data may exhibit variable delay, may arrive
out of order, or may be completely lost. Alternatively, circuit-switched
networks, such as the public switched telephone network (PSTN) or ISDN,
reserve resources and the data has a fixed delay, arrives in order, however
the data may still be corrupted by bit errors or burst errors.
Quality of Service (QoS) Support
An important area of network research over the past two decades has been
QoS support. QoS is a vague, and all-encompassing term, which is used to
convey that the network provides some type of preferential delivery service or
performance guarantees, e.g. guarantees on throughput, maximum loss
rates or delay. Network QoS support can greatly facilitate video
communication, as it can enable a number of capabilities including
provisioning for video data, prioritizing delay-sensitive video data relative to
other forms of data traffic, and also prioritize among the different forms of
video data that must be communicated. Unfortunately, QoS is currently not
widely supported in packet-switched networks such as the Internet.
However, circuit-switched networks such as the PSTN or ISDN do provide
various guarantees on delay, bandwidth, and loss rate. The current Internet
does not provide any QoS support, and it is often referred to as Best Effort
(BE), since the basic function is to provide simple network connectivity by
best effort (without any guarantees) packet delivery . Different forms of
network QoS that are under consideration for the Internet include
Differentiated Services (DiffServ) and Integrated Services (IntServ), and these
will be discussed further later in this writeup.
3. REVIEW OF VIDEO COMPRESSION
This section provides a very brief overview of video compression and video
compression standards. The limited space precludes a detailed discussion,
however we highlight some of the important principles and practices of
current and emerging video compression algorithms and standards that are
especially relevant for video communication and video streaming. An
important motivation for this discussion is that both the standards
(H.261/3/4, MPEG-1/2/4) and the most popular proprietary solutions (e.g.
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