| Abstract: This article first analyzes stream
media technology and content delivery network (CDN) technology,
the current two major technologies supporting broadband applications
and content, and then describes the 1+4 platform converging CDN
and stream media, which is built over Shanghai Telecom's IP MAN.
It also mentions the establishment of information sources in Shanghai.
Keywords: stream media, CDN, construction of broadband networks,
broadband information sources, broadband content
1. Development of Broadband Services and Content Platform Technology
With the development of broadband services and the increase
of competition, broadband operators, and major telecom carriers
in particular have gradually reached a common view that broadband
applications are the dominants in the market. The advent of stream
media technology and broadband content delivery network (CDN) technology
provides technical support for the establishment of applications
suitable for broadband networks.
The core of stream media technology is streaming technology and
high compression technology. These technologies enable multimedia,
video and audio information to be broadcasted while being downloaded,
thus completely replacing the inflexible way of presenting content
in the conventional Internet and enabling video programs with strong
visual impact to become one of the most important applications in
broadband networks. Through the development of stream media technology
and broadband networks, operators wish to reinforce the existing
Internet-based communication market and enter the newly emerging
entertainment service market. Broadband networks, stream media technology
and interactivity have brought broadband operators with opportunities
and bright promises to develop broadband interactive TV. It is a
common understanding that stream media will cause a change in the
broadband service industry.
In the meantime, the advent of CDN technology and its convergence
with stream media technology afford guarantee to the deployment
of broadband stream media applications in wide and metro areas.
The CDN is designed to deliver content, including stream media content
to the network edge nearest to the user by adding a layer in the
existing Internet so that the user can obtain the desired content
at the nearest place. By increasing the response speed of the content
accessed by the user, CDN improves the efficiency of information
flowing in the Internet, providing a complete technological solution
to congestion caused by excessive access amount and unevenly distribution
of websites, thus widening the range of the stream media content
accessed and increasing its response speed.
Video programs with strong visual impact can change the existing
situation where there is limited content provided and give free
play to bandwidth. The maturity of stream media and CDN technologies
will surely give a revolutionary impetus to broadband content and
even to the entire broadband industry. They will provide end users
with Internet-based information services such as multimedia news
release, on-line direct broadcast, network-based advertisement,
e-commerce, VOD, distance education, remote medicine, Internet-based
radio station and videoconferencing, and yield a comfortable profit
to the value chain of the broadband industry, including operators,
ICPs and ISPs.
2. Fundamentals of Stream Media Technology
Stream media refers to continuous time base media using streaming
transmission technology in the Internet/Intranet, for example, audio,
video or multimedia files. With stream media, files are not downloaded
wholly before being broadcasted, instead, only the beginning part
of the content is stored in the memory. Data streams of stream media
are transmitted and broadcasted at any time, with some delay only
in the beginning. The enabling technology of stream media is streaming
transmission.
Streaming transmission has a very broad definition. At present,
it is a general term for transmission of media such as video and
audio over the Internet. Its specific meaning is to transmit video
programs to PCs via the Internet. There are two types of streaming
transmission: realtime streaming and progressive streaming. Generally
speaking, if a video service is realtime broadcast or uses media
servers for streaming or realtime protocols like RTSP, it is realtime
streaming. If it uses HTTP servers, files are sent in sequential
streams. The choice of streaming modes depends on the user's need.
Of course, stream files also support downloading of the whole file
to the hard disk before it is broadcasted.
2.1 Principle of Stream Media Technology
Streaming transmission requires buffering. As the intermittent
asynchronous transmission of the Internet is based on packets, a
realtime A/V source or an A/V file stored is broken down into many
packets in the course of transmission. And as the network is dynamic,
each packet may be routed differently, resulting in different delay
at the user end. The packets transmitted first may arrive later.
Thus a buffer system is used to offset the impact caused by delays
and jitters and guarantee the correctness of the sequence of the
data packets so that the data can be output continuously without
any pause while being broadcasted, caused by temporary congestion.
Caching normally does not require large capacity. With the transmitted
content given up, streams can use caching space cleared to buffer
the subsequent content to be broadcasted.
Streaming transmission requires appropriate transfer protocols.
Needing more overheads, TCP is not suitable for realtime data transmission.
In streaming HTTP/TCP is normally used to transmit control information
while RTP/UDP is used to transmit realtime voice data.
The process of streaming is usually like this: After the user
selects a certain stream media, HTTP/TCP is used to exchange control
information between the Web browser and the Web server so as to
retrieve the realtime data to be transmitted from the original information.
Then the Web browser on the client starts A/VHelper and retrieve
the relevant parameters from the Web server using HTTP and initialize
Helper. These parameters may include directory information, the
code type of the A/V data or the address of the server involved
in A/V retrieval.
A/VHelper and the A/V server operate Realtime Streaming Protocol
(RTSP) to exchange the control information needed for A/V transmission.
Similar to the function provided by CD players or VCRs, RTSP provides
mechanisms for operate commands such as play, fast forward, fast
reverse and record. The A/V server transmits the A/V data to the
A/V client application using RTP/UDP (The client application is
generally considered to be equal to Helper). As soon as the A/V
data reach the client end, the A/V client application can broadcast
the output.
It should be noted that in streaming transmission, the reason
why to use both RTP/UDP and RTSP/TCP to link the A/V server is that
the output of the server can be redirected to a destination address
of a client other than those operating A/VHelper. Streaming transmission
usually requires dedicated servers and broadcasters. Its basic principle
is illustrated as follows.
Control information Location
Figure 1: Basic principle of streaming transmission
2.2 Mainstream Products and Options of Stream Media
2.2.1 Microsoft Windows Media Service
Microsoft Windows Media service is a delivery platform of stream
multimedia information, which is adaptive to various network broadband
conditions. It is a complete solution, including the production,
delivery, broadcast and management of stream media. It also provides
software development kit (SDK) for a second development task.
The core of Windows Media service is ASF (Advanced Stream Format).
ASF is a data format, through which multimedia information such
as audio, video, image and control command script is transmitted
in data packets and thus stream media content is delivered. The
content transmitted over the Internet is known as ASF stream. ASF
supports any compression /decompression coding and can use any bottom
layer transfer protocol, providing great flexibility. Windows Media
Service is composed of three parts: production, delivery and broadcast.
Windows Media Server provides the delivery of ASF stream media
over the Internet. It consists of two basic service modules: unicast
service and station service. The unicast service module provides
the customer with service based on point-to-point connectivity and
the station service module, with service based on broadcast. Unicast
service is in turn divided into on-demand unicast and broadcast
unicast, depending on the extent of participation by the user. In
the former, the user establishes point-to-point connection with
the delivery server, controlling the broadcast process, including
start, pause, fast forward, etc. In the latter, the user also establishes
point-to-point connection with the delivery server, however, he
or she can only watch the broadcast passively. Like broadcast service,
it is usually used in realtime information delivery.
2.2.2 RealNetworks and RealSystem
RealNetworks has formulated a specification for audio and video
compression, known as RealMedia. It is a multimedia application
standard for the cross-platform client/server structure, which prevails
over the Internet. It uses audio/video streams and synchronous playback
technologies to provide the best quality multimedia on a full broadband
basis, and stereophony and continuous video at the transmission
rate of 28.8 Kbps over the Internet. RealMedia includes three types
of files: RealAudio, RealVideo and RealFlash. RealAudio is used
to transmit audio data with the quality of CD; RealVideo is used
to transmit continuous video data; and RealFlash is a format of
motion picture with a high compression ratio recently developed
jointly by RealNetworks and Macromedia.
Its complete stream media solution RealSystem has client broadcast
software RealPlayer, which can run independently or in the browser
as a plug-in. Its production-end products are content production
software RealProducer and RealPresenter, which compress and convert
a file in the ordinary format into a file in the stream format.
Its server-end software RealServer is used to provide streaming
service.
2.2.3 Apple QuickTime
The Apple Computer Inc.'s QuickTime is a de facto industrial
standard in the digital media area, which can provide realtime playback
of digital information streams, work streams and files. It consists
of three parts: QuickTime Movie File Format, QuickTime Media Attract
Layer and QuickTime In-built Media Serve System. The QuickTime Movie
File Format defines the standard method of storing content of digital
media, enabling not only the storing of individual media content
(e.g., video frames or audio samples) but also saving the complete
description of the media work. The QuickTime Media Attract Layer
is a comprehensive media software structure, which defines how the
software tool and application access the QuickTime In-built Media
Serve System and how to upgrade the key performance of QuickTime
through hardware. The QuickTime In-built Media Serve System, helps
as the foundation of the software development tools, software developers
and users to make full use of the technical advantages of QuickTime.
2.3 Development of Stream Media Technology
In recent years, the wide deployment of broadband networks has
promoted the rapid progress of stream media technology. In particular,
two leading stream media technology providers Microsoft and RealNetworks
have introduced and strengthened serve functions in their respective
stream media systems. At the end of last year, RealNetworks launched
its newest RealONE system and Microsoft will soon roll out its newest
Media server Corona. From these servers, we can see some new trends
in stream media towards digital right technology, intelligent stream
technology and convergence of interactive functions.
2.3.1 DRM
Digital Rights Management protects audio and video against privacy
by asking the broadcaster to provide a consent key before broadcasting
content. The consent key can be obtained from the website of the
content owner or through a clearing exchange (e.g., Reciprocal).
It is untransferrable and can indicate a validity period. The integration
of distributed safety service and products like Microsoft website
server provides commercial service on a metering or duration rate
basis, and in-cut advertisements based on user parameters selected
or location.
2.3.2 Intelligent Stream Technology
Intelligent stream overstrides the limitation of broadband and
instantaneous transmission of video. It provides the encoder platform
to encode files with different bandwidths simultaneously and combine
them into one file. Intelligent stream also provides fault-tolerant
client/server transmission. When the bandwidth changes, audio and
video will find the best transmission combination from the multi-coded
file. With the change of the bandwidth, the server can modify the
number of messages to be transmitted. This can keep the RealPlayer
not re-executed. When the client is connected, the server will give
the client an appropriate code from the multi-coded file.
2.3.3 Convergence of Interactive Functions
In conjunction of media broadcast function and content subscription,
RealONE can broadcast archives such as MP3, MPEG, RA and RAM. Besides,
it has multi-layer screen function, that is, while a screen broadcasts
content from a video disk and CD, a side screen provides information
concerning the content or an advertisement. This function facilitates
business transactions, making the relationship between stream media
providers and businesses closer.
Corona, Microsoft's new generation server provides a server-end
broadcast list to allow content providers to update data streams
dynamically, for example, to input advertisements on a realtime
basis. It also provides developers with an additional means-- new
.NET plug-in model. With this model, digital media can be inserted
into applications. Microsoft hopes to be the de facto standard of
the business model of stream media.
3 Broadband Content Delivery Network Technology
The Composition of the Content Delivery Network (CDN)
A typical CDN is made up of the following five parts shown in
Figure 2.
Content buffer Content switch Content router CDN content management
Figure 2: Composition of CDN functions
The content functions of its component parts are
described in the following table.
Content buffer: It provides the user with the service
point of the real content, which is usually located at the concentration
point of user accesses or the POP of the backbone network. It can
buffer static Web content and stream media content.
Content Switch: Located at the concentration point
of user accesses or the POP, it provides buffer, load balancing
and access control for content. In the case of much equipment, it
and the content switch are located on the same item of equipment.
Content router: It selects the best access website
at the user's request using GSLB and proximity access technology.
Content delivery system: It is a content mirroring
system, which mirrors the content from all the delivery websites.
CDN management system: It provides centralized
CDN management because the CDN system provides multi-user sharing.
Working Principles of CDN Platform
When the user access has been put into the stream media content
of the CDN website, the platform first determines the best CDN node
closest to the user through DNS redirection technology and directs
the user request to that node. When the user request reaches the
defined node, the CDN server (the cache at the node) provides the
content requested to the user.
The following is the basic flow of user access:
a) The user enters the field name of the website to be accessed
into the browser;
b) The browser requests the local DNS to analyze the field name;
c) The local DNS sends the request to the host DNS of the website,
which in turn forwards the request for analysis to the redirected
DNS;
d) The redirected DSN determines the currently most suitable
CDN node according to a series of policies and sends the result
of the analysis (IP address) to the user;
e) The user requests the defined CDN node for the content of
the corresponding website;
f) The server at the CDN node responds to the user request and
provides the content desired.
CDN's service network within a metro area is shown in the following
figure. The user accesses various content at the best location through
the above access flow. Besides, the provision of CDN service needs
to rely on the support from various technologies such as load balancing,
dynamic content routing, caching, dynamic content delivery and reproduction
and safety service.
Edge node Edge node Edge node Edge node Core node
Figure 3: Structure of the CDN
1) Load balancing technology
Briefly speaking, load balancing technology distributes as evenly
as possible the network load to some servers or network nodes that
are able to perform the same functions so as to avoid overload of
certain network nodes, improving network performance and processing
efficiency.
In the CDN, load balancing is divided into server load balancing
(SLB) and global server load balancing (GSBL).
SLB
The increase of website content and functions leads to the increase
of the number of servers supporting the websites. SLB is a technology
that effectively distributes tasks dynamically according to the
processing capacity of each server so that the access speed of the
server can be accelerated. By using this technology, tasks can be
distributed to servers with different performances. This can ensure
that servers with poor performance will not become the bottleneck
of the system and that the resources of those with high performance
will be made full use of, thus accelerating the access speed of
the web servers.
GSLB
GSLB allows Web hosting providers, portals and businesses to
distribute content and service according to geographical areas.
Distributed content and service have many advantages. One of them
is to automatically direct the users to servers located in its geographical
area so as to reduce the response time and the time to use expensive
international data connections and guide the users to leave the
congested networks and servers. By using multi-website content and
service to improve fault tolerance and availability, it can also
prevent against faults resulting from interruption of local and
regional networks, outage and natural disasters.
GSLB of the server usually forwards the user request to the best
website according to certain standards so as to provide better service.
These standards can be the health status and distance of the website,
the response time needed by the retrieval of the defined content.
2) Dynamic content routing
When user access has been put into the content of the CDN website,
the request for field name analysis will be finally processed by
the redirected DNS. The DNS provides the user with the address of
the node closest to the user according to a set of pre-defined policies
such as the content type, the geographical area and the loading
status of the network, so that the user can be served soon. At the
same time, the DNS keeps communication with all the CDN control
nodes throughout the world to collect information about the health
status of each node so that the user request will not be distributed
to any useless node. It also has the capability of self-adaptive
rerouting in case of congestion and failure in the network.
3) Caching
Caching improves user response time in several ways, such as
proxy buffering, transparent proxy buffering and transparent proxy
buffering using redirection. Through caching, the user in accessing
content can minimize the flow over the WAN, thus reducing the pressure
on the backbone.
4) Dynamic content delivery and reproduction
The access response speed of the content websites depends on
many factors such as whether there exists a broadband bottleneck
in the network or congestion and delay on the route in transmission;
the processing capacity of the website server and access distance.
In most cases, the response speed of the website is closely related
with the distance between the user and the website server. Although
China Telecom has planned to increase the speed of its backbone
by eight folds and its bandwidth, network delay is unavoidable if
the distance between the user and the website server is too great.
An effective method is to use content delivery and reproduction,
delivering and reproducing most static web pages, images and stream
media data to the accelerated nodes.
The CDN can adopt intelligent routing and stream management technologies
to find in time the closest accelerated node to the visiting user
and forward the user request to the accelerated node, which then
provides content service. In using content delivery and reproduction,
the hosing user does not need to modify his or her original network
structure. He or she can accelerate the response speed of the network
simply by slightly modifying the configuration of the DNS.
4 Structure of Shanghai Telecom's Broadband Information Source
Platform
4.1 Service Architecture of the Information Source Platform
As the extension of the information service provided by Shanghai
Telecom, its broadband information source is characterized by its
high interactivity and multimedia combining audio and video. Shanghai
Telecom's information source platform integrates currently most
advanced stream media and content delivery technologies, taking
into account the establishment of a good management and cooperation
mode between broadband access, broadband information source platform
(stream service platform, mass storage and content delivery) and
broadband content providers.
The architecture of the information source platform is shown
in Figure 4. The entire platform is a distributed architecture built
on the basic network (ATM, hotline backbone and broadband backbone)
of Shanghai Telecom, providing a strong capability to distribute
network accesses. The information source services provided are based
on high and lower middle speed stream technological platforms (the
combination of MMS and REAL). A reliable and stable physical platform
is provided to content service by backward mass storage and broadband
CDN.
Interactive media on demand Multimedia gaming On-line multimedia
stock info On-line multimedia education Multimedia messaging center
Interactive video applications
Central validation, management and charging system for applications
Stream media service platform Stream platform management
Multimedia mass storage platform Storage, backup
Broadband CDN Application network management
Telecom broadband basic network (ATM, IP, ADSL/LAN/HOMEPNA)
Figure 4: Architecture of the broadband information
source
4.2 Structure of Information Source Platform
The information source platform network has a 1+4 structure,
which consists of a core node and four subnodes, as is shown in
Figure 4. The core node is the management and control center of
the CDN system, delivering the content (MMS, REAL, etc.) of the
central node to different sub-central nodes by batch, periodically
and as scheduled. The four switches of the central nodes and the
sub-central nodes constitutes a GSBL network. When a user accesses
a given program source via the local broadband access server (BAS),
user access will be redirected to the most rational access point
according to the physical distance between the user and different
central nodes and the current loading status of the equipment.
Central node content, program Sub-central node Sub-central node
Sub-central node Sub-central node IP Backbone User
Figure 5: Structure of the information source platform
The core layer is connected to the central content delivery server
and all the application servers, including the stream media platform
via the switches at the four layers. It delivers the content provided
by the central stream media service platform to the content delivery
server in the regional node periodically and by type. The switches
at the four layers of the core layer performs redirection of user
access in conjunction with the regional switches. In view of the
great amount of storage and the great variety of the broadband applications,
all the application data are written and read by the central storage
system so as to facilitate centralized management and the rational
use of the resources.
The four regional nodes include the caching node of the stream
media platform, which provides caching for regional content; the
delivery node for broadcast service, which provides secondary broadcast
service; and some front-end delivery nodes. Each node overlaps with
the nodes of the broadband IP backbone, and is connected to the
routers of the broadband IP backbone via a gigabit port and to the
core node of the broadband information source, making use of the
topology of the broadband IP backbone.
4.3 Current CDN Functions
The CDN-based broadband content service platform implements
three functions: user proximity access, authorized content control
and GSLB. Service management based on application validation and
charging and the service platform implemented in cooperation with
the content provider perform user validation, charging and accounting
functions in the operation of broadband content and in content cooperation.
Proximity access
Proximity access enables broadband users to access a nearby cache
when they want to access the cache content of a specific stream
media service or that deployed by the CDN to a remote node. That
is to say, it directs user access to a relatively near cache that
operates normally so that the user can be served directly by an
edge cache. This allows the saving of bandwidth and alleviation
of pressure on the core server as well as the acceleration of the
access to medium and high speed stream media.
Authorized access
Only after the user is validated and authorized can the content
stored in a cache (including that deployed from the core node to
an edge node and stream media content buffered like REAL, Windows
MMS, etc.) be provided directly from the cache at the edge node
with the access time, number of accesses and data volume obtained
recorded.
Management of content delivery
It can implement a content deployment and delivery system that
can be controlled, managed and customized. It can deploy the content
which a contracted customer requests to the cache at an edge node
according to certain contracted arrangements, for example, what
content, where, when and how to deploy, when to start, who will
use the content, etc.
Besides, the CDN not only expands the service scope of the stream
media platform but also its management scope. When the subnodes
of the CDN responds to the access request, all the users' request
commands will be forwarded to the stream media platform at the central
node and let the servers judge whether the users concerned are entitled
to viewing the films. It can also prevent the users from using the
content of a CDN node concerned by bypassing the validation system.
Data content of stream media is provided by caches.
User REAL or MMS stream service platform Content Content
Figure 6: Convergence of the stream media service
system and the CDN
4.4 Summary
Shanghai Telecom's broadband information source platform started
construction in August, 2001 and was completed and put into pilot
operation in March, 2002. The platform enables the company's broadband
(ADSL, LAN, etc.) users to access stream media content in the metro
area via the best route, at the highest speed and in the most stable
manner. Meanwhile, Shanghai Telecom has enriched the content through
the modification of Shanghai Hot Line II and the introduction of
charging based on content cooperation. This provides the company
with strong competitive advantages in the development of broadband
services.
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