You can deliver digital media audio and video files and live streams either from
a
Windows Media server (a server that is running Windows Media Services) or from
a
Web server (a server that is running Internet Information Services (IIS) 7).
Clients use different methods to receive digital media content from Windows Media
servers and Web servers: they stream digital media from a Windows Media server and
download digital media from a Web server.
With the broader availability of high-bandwidth networks and new media delivery
features for IIS 7, the differences that previously favored the use of a Windows
Media server over a Web server for delivering digital media content have blurred.
In non-multicast streaming scenarios, depending upon your business need, a Windows
Media server or a Web server can both be viable options for digital media content
delivery today.
This article introduces the delivery methods and bandwidth-management capabilities
of Windows Media servers and Web servers, two important aspects of media delivery
to consider when you are trying to decide which server type to deploy for your delivery
scenario. It includes the following topics:
Introducing Windows
Media servers and Web servers
This section introduces the two primary Windows Server roles
that are used to deliver digital media to clients:
Windows Media servers
A Windows Media server is designed specifically for streaming
on-demand and live digital media to clients. It provides high-quality streaming
over a wide range of bandwidths to
Windows Media Player and to Web browsers that use the Windows Media Player 9
Series ActiveX control or the
Microsoft Silverlight browser plug-in. It is especially useful for streaming
large amounts of data over busy, congested networks and low-bandwidth connections.
Streaming uses bandwidth more efficiently than downloading because it sends data
over the network only at the speed that is necessary for the client to render it
correctly. This helps prevent the network from becoming overloaded and helps maintain
system reliability.
Users that connect to a stream can start to play it almost immediately.
If streaming on-demand content was indexed during encoding, they can use Seek functions
(Fast-Forward and Rewind) on their player to jump to any part of the stream without
waiting for a download to finish. Streaming prevents users from downloading files
directly to their hard disk because the streaming data packets are delivered directly
to the client application and discarded by the client application after they are
rendered.
A Windows Media server supports delivery of a limited number
of popular digital media file formats, including Windows Media Audio (WMA), Windows
Media Video (WMV), and MP3. It also supports the use of JPEG files as media elements
in server-side playlists or as banner graphics to be displayed in Windows Media
Player.
Web servers
A Web server is designed to host Web sites and serve the Web
site's pages and files to clients upon request. When a client requests a file from
a standard Web server, all available network bandwidth is used to download the data
to the client as quickly as possible. Web server deployments that serve small files
to a limited number of users simultaneously require little bandwidth. But as delivery
of audio and video files (which are frequently large) has increased, and as sites
must serve greater numbers of simultaneous client requests, more bandwidth is required
to fulfill the client requests in a timely manner.
A Web server supports delivery of an unlimited number of popular
digital media file formats using standard file-download. If the IIS Media Services
Smooth Streaming extensions are installed on a Web server running IIS 7, delivery
of on-demand and live streams in the MP4 container format is also supported.
Streaming vs.
Downloading
This section compares the content delivery methods for Windows
Media servers and Web servers:
Streaming
User Datagram Protocol (UDP) has long been the preferred transport
protocol for traditional streaming media. It is a fast, lightweight protocol without
any retransmission or data-rate management functionality, making it an ideal protocol
for transmitting real-time audio and video data, which can tolerate some lost packets.
UDP also has higher priority than Transmission Control Protocol (TCP)-based HTTP
for Internet traffic, giving streaming audio and video data higher priority over
file and Web page transfers and increasing the likelihood of uninterrupted viewing
on congested networks. A Windows Media server also uses UDP Resend, an intelligent
UDP-retransmission scheme that ensures that it only retransmits lost packets that
can be sent to a player in time to be played, instead of the blind retransmission
scheme employed by TCP. This smart-resend feature conserves additional bandwidth
on congested networks.
A Windows Media server may stream digital media by using the
Real Time Streaming Protocol (RTSP) with UDP-based transport (RTSPT) to take advantage
of UDP-based streaming efficiencies. On networks that don’t support UDP, the server
is normally configured to use a process called
protocol rollover to try TCP-based streaming, first by using RTSP, and if that
doesn’t work, it uses its own version of HTTP for firewalls that allow Web traffic
through port 80. This enables corporate users to view Internet content without compromising
firewall security and ensures that all users on all networks can access all streaming
media content. For more information about the streaming protocols that are used
by Windows Media Services, see
Comparing HTTP Streaming Protocol with RTSP.
As network bandwidths increase, the use of TCP is becoming more
common. Because TCP guarantees delivery of every packet, it is preferred for
video-on-demand delivery, especially if end users are paying for the content through
a pay-per-view or subscription billing model.
Downloading
The bandwidth-management capabilities that are present in a Windows
Media server are lacking in a standard Web server. When a client requests digital
media from a Web server, the Web server downloads the content to the client as fast
as the network will allow without monitoring the quality of the delivery and adjusting
the bit rate for the client in the way that a Windows Media server does. A client
can start to play the content as soon as enough data is downloaded to its Internet
cache (this is referred to as progressive downloading); however, in bandwidth-constrained
systems, simultaneous requests from multiple clients can quickly saturate the available
network bandwidth and clients must buffer more data to the cache before starting
or resuming playback.
Downloading also uses the available bandwidth less efficiently
than streaming. Web server content delivery uses HTTP, the standard Web protocol
that is used by all Web servers and Web browsers for communication between the server
and the client. HTTP operates on top of the TCP transport protocol, which handles
all data transfers. TCP is optimized for non-real-time applications such as file
transfer and remote log-in; therefore, it maximizes data transfer rates while ensuring
overall stability and high throughput for the whole network. TCP achieves reliable
data transfer by re-transmitting all lost packets, but it can’t ensure that all
resent packets will arrive at the client in time to be played, and so sometimes
wastes bandwidth.
With IIS, you have a choice. You can use standard progressive
downloads, with the limitations mentioned above, or you can use a new IIS feature
called
Bit Rate Throttling, described below, which provides some of the benefits of
traditional streaming to almost any type of media file.
Windows Media Services
With its built-in bandwidth-management capabilities, a Windows
Media server is an ideal way to deliver digital media content to large numbers of
concurrent users using traditional streaming.
A Windows Media server sends data at the same bit rate as the
content, leaving more bandwidth available for servicing concurrent client requests
for content and resulting in better quality audio and video for connected clients.
There is typically a delay between the time the stream is received by a player and
the point at which it starts to play because the player must first buffer some data
in case there are delays or gaps in the stream. This buffer allows the media to
continue playing uninterrupted, even during periods of high network congestion.
Because data streaming and rendering occurs almost simultaneously, streaming also
enables you to deliver live content. Windows Media Services contains many additional
features that are used to optimize network throughput. This section describes two
of the most important: Intelligent streaming and Fast Streaming.
Intelligent
streaming
The most difficult task of streaming audio and video over a network
is maintaining a continuous presentation to the user in a highly changeable environment.
Buffering is the biggest problem of streaming digital media. It is caused when the
client runs out of data in memory (the buffer) and must wait for more to arrive.
The client will always run out of data if the bit rate of the incoming stream exceeds
the current available bandwidth.
Unpredictability of bandwidth is taken for granted on the Internet.
The actual bandwidth at any given point is determined by network conditions and
traffic, which are constantly fluctuating. To ensure a continuous presentation during
traditional streaming delivery, Windows Media technologies use
intelligent streaming, which adjusts the bit rate of the content stream to counteract
changes in available bandwidth. When a client first connects to a stream, intelligent
streaming ensures that the client receives content at the bit rate that is most
appropriate for the current bandwidth. As the stream plays, intelligent streaming
is designed to dynamically and seamlessly adjust the bit rate of the streaming content
as the available bandwidth changes.
Intelligent streaming progresses through a series of strategies
to modify the bit rate of the stream so that it plays continuously on the client
regardless of the current bandwidth. As bandwidth fluctuates between server and
client, the server detects the changes and adopts the best strategy. The strategies
are as follows:
1. The server and client automatically determine the current
available bandwidth, and then the server selects and delivers the stream with the
most appropriate bit rate. This requires that the content be encoded as a multiple-bit-rate
(MBR) stream.
2. During transmission, if the bandwidth decreases, the
server automatically detects the change and switches to a stream with a lower bit
rate. If bandwidth improves, the server switches to a stream with a higher bit rate,
but never higher than the original bit rate. The process of selecting the appropriate
stream is transparent to the user.
3. If the bandwidth can no longer support streaming video,
the client and server intelligently degrade image quality to avoid buffering. This
method of decreasing the bit rate to accommodate the current bandwidth is called
stream thinning. The server decreases the video frame rate first. If the bit rate
is still too high, the server stops sending video frames.
4. After the server stops sending video frames, it uses intelligent
streaming to try to maintain a continuous audio stream. If audio quality starts
to degrade, the client reconstructs parts of the stream to preserve quality.
When bandwidth is at its best, the server uses the first strategy.
As conditions worsen, the server tries each strategy in the previous list of options
in succession until the bit rate is optimized for the current bandwidth.
A newer and improved form of intelligent streaming for delivery
of media from Web servers is also available using IIS. See the
Smooth Streaming section of this article for more information.
Fast Streaming
Fast Streaming refers to a group of features in Windows Media Services that
improve the quality of streaming media content for the user. These features allow
for a combination of streaming, downloading, and caching to provide the best user
experience:
-
Fast Cache. Enables Windows Media Services to use any additional
bandwidth to send additional data to the player buffer, allowing the player
to better withstand network bandwidth fluctuations.
-
Fast Start. Enables the beginning part of the content to be quickly
downloaded to a player at the maximum available bandwidth, reducing the
time required to fill the player’s buffer requirement and the amount of
time a user has to wait (also known as latency) before starting to
receive the stream.
-
Fast Recovery. Enables the server to provide an uninterrupted viewing
experience to users receiving content over high latency network connections,
such as wireless and satellite networks, by using forward error correction
to greatly reduce packet corruption and interruption.
-
Fast Reconnect. Enables the server to automatically restore client
connections—including encoders, distribution servers, and players—that are
lost during a broadcast because of network conditions.
-
Advanced Fast Start. Adds to Fast Start capabilities by allowing
the player to start to play content as soon as its buffer receives a minimum
amount of data, further reducing the amount of latency before the user starts
to receive the stream.
IIS Media Services
IIS Media Services extensions are designed to add media delivery
capabilities to a Web server that is running
Internet Information Services (IIS) 7. The IIS Media Services extensions take
advantage of the broader market availability of high-bandwidth networks, which make
a Web server an increasingly practical option for digital media content delivery.
This section describes the following streaming and bandwidth management extensions
that are available in IIS Media Services:
IIS Media Services includes additional extensions that add Windows
Media Services-like capabilitres, such as server-side playlists and rich client
logging, to Web servers running IIS 7. For more information about these extensions,
and additional extensions that may be developed to enhance delivery of digital media
content from Web servers, see the
IIS Media
Services Web site.
Smooth Streaming
Smooth Streaming is the Microsoft implementation of adaptive
streaming technology, which is a form of Web-based media content delivery that uses
standard HTTP. Instead of delivering media as full-file downloads, or as persistent
(and thus stateful) streams, the content is delivered to clients as a series of
MPEG-4 (MP4) fragments that can be cached at edge servers. Smooth Streaming-compatible
clients use special heuristics to dynamically monitor current network and local
PC conditions and seamlessly switch the video quality of the Smooth Streaming presentation
that they receive. As clients play the fragments, network conditions may change
(for example, bandwidth may decrease) or video processing may be impacted by other
applications that are running. Clients can immediately request that the next fragment
come from a stream that is encoded at a different bit rate to accommodate the changing
conditions. This enables clients to play the media without stuttering, buffering,
or freezing. As a result, users experience the highest-quality playback available,
with no interruptions in the stream.
Smooth Streaming provides media companies with a better way to
make full-HD (1080p) streaming on the Web a reality. It offers code-free deployment
and simplified content management for content creators and content delivery networks.
For users, the improved viewing experience brings the reliability and quality of
television to their favorite video Web sites.
As a content producer, you can encode live and on-demand Smooth
Streaming video using
Microsoft® Expression® Encoder 4. As a content provider, you can use IIS Media
Services to serve the encoded Smooth Streams. And as a content consumer, you can
play the Smooth Streams using a Smooth Streaming-compatible client, such as Microsoft®
Silverlight™.
For more information
-
-
-
Experience Smooth Streaming. Go to this Web page to use a Silverlight
browser plug-in to simulate different bandwidths and see Smooth Streaming
in action.
Bit Rate Throttling
If you are concerned about the overall bandwidth limitations
of your Web server system, instead of switching your network computers to a new
streaming distribution system, you may be able to use the Bit Rate Throttling extension,
which offers similar bandwidth-management and client-connection capabilities to
those in Windows Media Services. Unlike Windows Media Services, you can use Bit
Rate Throttling to intelligently deliver any media file type, not just Windows Media
files. (Even better, Bit Rate Throttling can be used to deliver any file
type in a controlled manner, not just media files.)
You can configure Bit Rate Throttling to send an initial burst
of data to a client’s cache (similar to the Fast Start capability of Windows Media
Services that reduces latency) so that the client application can start to play
the file by using progressive download, and then configure the rate at which the
remaining data is downloaded to the client. Typically, this throttle rate is slightly
greater than the encoded bit rate of the file, so that the client renders the throttled
data as it is downloaded (similar to a streaming experience). When multiple clients
request data from the Web server, they consume the data at the throttled rate, instead
of all at once. This frees additional bandwidth for servicing additional client
requests.
On standard Web server networks that have lots of available bandwidth,
a complete file can be downloaded to a user’s Internet cache in just a few seconds,
provided it isn’t exceptionally large, and content providers can incur costs related
to unused bandwidth if users stop playing the downloaded file after a short time.
By throttling the content download rate so that clients receive just enough content
to render it, content providers don’t incur costs related to the bandwidth that
is wasted in the unviewed parts of downloaded files.
The Bit Rate Throttling extension offers advanced settings that
allow you more control of the cumulative network throughput from your Web server.
For example, you can specify global default and maximum throttle rates for all digital
media files (regardless of their encoded bit rates) and specify a total bit rate
throughput limit for a site so that the available bandwidth is dynamically allocated
to connected clients. The Bit Rate Throttling extension also offers a useful secondary
feature that throttles data file types at specified bit rates.
For more information, see
Bit Rate Throttling.
Feature checklist
A Windows Media server is designed to handle the special requirements
of streaming and administering Windows Media-based content. However, a Web server
can be a practical option for delivering digital media content, especially by installing
IIS Media Services extensions for IIS 7 that bring Windows Media server capabilities
to the Web server.
Windows Media Services 2008 and IIS 7 have additional features
that are not described in this article that might affect your choice for delivering
digital media. For example, Windows Media Services is the only server role that
can be used for multicast delivery of content on enterprise networks. Before you
make a final decision about which server role to deploy to facilitate your business
model, review the features for each server role in the following table.
= This feature
or function is supported.
= This feature
or function is supported on a Web Server running IIS 7 if the referenced extension
is installed. The implementation on the Web server may differ from the Windows Media
server.
=
This feature or function is not supported.
|
Windows Server 2008 Feature
|
Windows Media Services 2008
|
Internet Information Services 7
|
|
Optimized for streaming
Windows Media-based content
|
|
1
|
|
Delivers content through most
firewalls
|
|
|
|
Streams without downloading
|
|
|
|
Broadcasts live content
|
|
2
|
|
Delivers content using
multicast streaming
|
|
3
|
|
Supports trick mode playback of on-demand files
|
|
4
|
|
Delivers content protected with
Digital Rights Management
|
|
|
|
Delivers content using Fast Streaming
|
|
1, 2, 4
|
|
Delivers content using Intelligent Streaming
|
|
2, 4
|
|
Supports cache/proxy delivery scenarios
|
|
5
|
|
Supports rich client logging
|
|
6
|
|
Delivers content to cross-platform
Silverlight
clients
|
|
|
|
Delivers content to iOS devices (iPhone, iPad)
|
|
|
|
Delivers content referenced by server-controlled playlists
|
|
7
|
|
Provides advanced performance boundary limits properties
|
|
1
|
|
Supported on
Server Core installations
|
|
|
|
Available for
Windows Web Server 2008
|
|
|
1
IIS Bit Rate Throttling
2
IIS Live Smooth Streaming
3
Coming soon in
IIS Smooth Multicast
4
IIS Smooth Streaming
5
IIS Application Request Routing
6
IIS Advanced Logging
7
IIS Web Playlists
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