Media Streaming Server Features

Abstract
There are two major methods of delivering streaming audio and video content over the Web. The first method uses a standard Web server to deliver the audio and video data to a media player. The second method uses a separate streaming media server specialized to the audio/video streaming task. This paper shows that, while Web server streaming can be an effective interim solution, a streaming server is more efficient and flexible and provides a better user experience.

Introduction
Until recently, audio and video on the Web was primarily a download-and-play technology. You had to first download an entire media file before it could play. It was like pouring milk into a glass and then drinking it. But because media files are usually very large and take a long time to download, the only content found on the Web was short 30-second clips—often even shorter. Even these files could take 20 minutes or longer to download. In other words, it took a long time to pour the milk, and then it would barely quench your thirst.

Watching audio and video files that stream is more like drinking straight from the carton; streaming media files begin playing almost immediately, while the data is being sent, without having to wait for the whole file to download. Other than a few seconds of delay before the file starts to play, you don't have to wait to start watching, no matter if the file lasts 30 seconds or 30 minutes.

As audio and video streaming over the Internet has become more popular, two primary methods for streaming content have emerged. The first method is the Web server approach, in which a standard Web server is used to supply data to the client. The second method is the streaming media server approach, in which a specialized streaming server delivers the data to the client. Both methods have advantages that we will discuss, but first let's take a look at the way each process works.

How the Two Methods Work

Streaming with a Web Server

Posting and Hosting
Deploying streaming media content with the Web server approach is actually only a small evolutionary step away from the download-and-play model. Uncompressed audio and video is first compressed into a single "media file" for delivery over a specific network bandwidth such as a 28.8 kilobits per second (Kbps) modem. This media file is then placed on a standard Web server. Next, a Web page containing the media file's URL is created and placed on the same Web server. This Web page, when activated, launches the client-side player and downloads the media file. So far, the actions are identical to those in the download-and-play case. The difference lies in how the client functions.

Data Delivery
Unlike the download-and-play client, the streaming client starts playing the audio or video while it is downloading, after only a few seconds wait for buffering, the process of collecting the first part of a media file before playing. This small backlog of information, or buffer, allows the media to continue playing uninterrupted even during periods of high network congestion. With this delivery method, the client retrieves data as fast as the Web server, network and client will allow without regard to the bit-rate parameter of the compressed stream. Only certain media file formats support this type of "progressive playback". Microsoft's Advanced Streaming Format (ASF) is one of the most popular.

Web server streaming uses the Hyper Text Transport Protocol (HTTP), the standard Web protocol used by all Web servers (such as Microsoft^® Internet Information Server) and Web browsers (such as Microsoft Internet Explorer) for communication between the server and the client. HTTP operates on top of the Transmission Control Protocol (TCP), which handles all the data transfers. Optimized for non-real-time applications such as file transfer and remote log-in, TCP's goal is to maximize the data transfer rate while ensuring overall stability and high throughput of the entire network. To achieve this, using an algorithm called slow start, TCP first sends data at a low data rate, and then gradually increases the rate until the destination reports packet loss. TCP then assumes it has hit the bandwidth limit or network congestion, and returns to sending data at a low data rate, then gradually increases, repeating the process. TCP achieves reliable data transfer by re-transmitting lost packets. However, it cannot ensure that all resent packets will arrive at the client in time to be played in the media stream.

Streaming with a Streaming Media Server

Posting and Hosting
In the streaming media server approach, the initial steps are similar to the Web server approach, except that the compressed media file is produced and copied to a specialized streaming media server (such as Microsoft Windows Media Services) instead of a Web server. Then a Web page with a reference to the media file is placed on a Web server. Windows Media Services and the Web server may run on the same computer.

Data Delivery
The rest of the streaming media server delivery process differs significantly from the Web server approach. In contrast to the passive burst methodology employed in Web server streaming, the data is actively and intelligently sent to the client, meaning that it delivers the content at the exact data rate associated with the compressed audio and video streams. The server and the client stay in close touch during the delivery process, and the streaming media server can respond to any feedback from the client.

While streaming media servers can use the HTTP/TCP protocols used by Web servers, they can also use specialized protocols such as the User Datagram Protocol (UDP) to greatly improve the streaming experience. Unlike TCP, UDP is a fast, lightweight protocol without any re-transmission or data-rate management functionality. This makes UDP an ideal protocol for transmitting real-time audio and video data, which can tolerate some lost packets. As a bonus, because of the back-off policies implicit in the TCP protocol, UDP traffic gets higher priority than the TCP traffic on the Internet. And instead of the blind retransmission scheme employed by TCP, streaming media servers such as Microsoft's Windows Media Services use an intelligent retransmission scheme on top of UDP. Windows Media Services' UDP Resend feature ensures that the server only retransmits lost packets that can be sent to the client in time to get played.

Comparative Analysis

The differences between the Web server and streaming media server solutions translate into clear distinctions in both ease of implementation, ease of management, and quality of user experience. For the remainder of this white paper, the comparison will be between a generic Web server and Microsoft's streaming media server, Windows NT Server Windows Media Services (hereafter referred to as a Windows Media server).

Streaming with a Web Server: the Advantages
There is really only one major advantage to streaming with a Web server rather than with a streaming media server—utilizing existing infrastructure. Because the Web server approach uses only the standard Web server--that presumably already exists in the organization—no new software infrastructure need be installed or managed. The Windows Media server approach, on the other hand, requires the content producer and/or the systems administration staff to install and manage additional server software. This can result in incremental training and staffing costs to learn and manage the more complex, but also more powerful, Windows Media server environment.

It is important to note that the increased load that Web server-based streaming puts on existing Web server infrastructure often results in the need for additional Web server hardware to service the client requests. Choosing Web server streaming over a dedicated streaming media server based on hardware cost alone usually does not result in any financial savings.

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Streaming with a Windows Media Server: the Advantages

1. More Efficient Network Throughput. We've already mentioned one of the main advantages of Windows Media server streaming—the ability to use UDP, the specialized protocol optimized for live and on-demand streaming. The TCP-based transfer used in Web server streaming is designed to repeatedly drive the slowest network link (likely the 28.8 Kbps modem link) to packet loss. This wastes bandwidth by: (i) re-transmitting data to replace the lost packets; and (ii) under-utilizing the network link while re-estimating the throughput that can be supported by the network connection.
   
   The UDP protocol allows higher bandwidth to be delivered to the client (resulting in better video quality), even when assuming the same network connectivity between server and client and the same level of congestion on the Internet. By having a specialized streaming media server, we know what rate the data is going to be consumed, based on headers of the compressed media file. The Windows Media server sends data to the client (Windows^® Media Player) only at this required bit-rate, and it doesn't drive the bottleneck network link to loss. Thus the network throughput is better, resulting in better quality audio and video for the client.
   
   
2. Better Audio and Video Quality to the User. Better network throughput is only one of several ways that a Windows Media server delivers superior audio and video quality for users. Here are two more examples:
   • Because the Windows Media server and the Windows Media Player remain in contact throughout the play interval, the server can dynamically respond to client feedback. If network congestion is allowing only 22 Kbps of data to reach the client (instead of 28.8 Kbps), the server can decide to retain the audio quality but slightly lower the frame rate of the video stream so that it doesn't overshoot the available 22 Kbps. This ability is not possible with the Web server approach. In a Web server scenario, with no feedback from the client and no ability to dynamically prioritize audio over video, the audio/video delivered by a Web server would be stop-and-go at the client, causing the insidious "rebuffering" delays common to early implementations of streaming media. In contrast, the Windows Media server provides a continuous, smooth stream with barely perceptible changes in video frame rate during periods of network congestion.
   • Streaming with a Windows Media server takes advantage of UDP's inherent higher priority over HTTP traffic to give the streaming audio and video data higher priority than file and Web page transfers. This increases the likelihood of uninterrupted viewing.
   
   
3. Support for Advanced Features. The Windows Media server approach supports such advanced features as detailed reporting of streams played, VCR controls (seek, fast-forward, rewind), live video delivery, and delivery of multiple streams to the client. With Web server streaming such features, if they are even possible, are difficult to implement and inefficient to support.
    
   
4. Cost Effective Scalability to Large Number of Users. In the early days of streaming media, deployments often needed to serve only a small number of users simultaneously, making Web server streaming an adequate solution. But as delivery of audio and video has increased, sites often serve hundreds or thousands of simultaneous users. In these situations, two key capabilities of the Windows Media server provide increasing advantages over a Web server:
   • Specialization. In the Web server approach, the Web server is used to deliver the media files to the client. Web servers, however, are optimized for delivering lots of small HTML files, not large media files. With high volumes of file requests, a Windows Media server greatly improves performance by optimizing how media files are read from the disk, buffered in main memory, and streamed onto the network. A Windows Media server can easily improve scalability by a factor of 2 to 3 over a Web server.
   • Multicast Support. One way to get live or stored audio and video to large audiences with minimum network congestion is to use multicast networking technology. Multicast allows a single media stream to be played simultaneously by multiple clients, drastically reducing bandwidth use. Only a specially designed streaming media server, such as a Windows Media server, has this capability.
   
   
5. Protection of Content Copyright. Because Web server streaming creates a local cached copy of every media file played, there is no way to prevent end users from copying the files to a personal directory for later viewing. This hurts content providers who have a pay-per-view business model, or who have an advertisement-based revenue model, as the end users need not visit their site repeatedly. With a Windows Media server, users can only stream data and are prevented from downloading the file directly to their hard disk. As data packets are received over the network, they are delivered directly to the client application with no easy way for the end user to intervene and make a copy.
    
   
6. Multiple Delivery Options. With a Windows Media server, the media may be streamed with the optimal UDP or Multicast protocols when possible, and streamed with the TCP protocol when necessary. 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. The Windows Media server implements its own version of the HTTP protocol to enable streaming through a firewall or proxy server while still retaining most of the advantages a Windows Media server.

   Windows Media Services support four different protocol configurations, each offering specific benefits.

   • UDP – As detailed in the Windows Media server section, UDP provides the most efficient network throughput and can have a very positive impact on the user (player) experience. The only downside to UDP is that many network administrators close their firewalls to UDP traffic, limiting the potential audience of UDP-based streams
   • TCP – As detailed in the Web server section, TCP provides an adequate, though not necessarily efficient, protocol for delivering streaming media content from a server to a client. As customers often open the TCP ports in their firewalls, Windows Media Services can use the TCP protocol enable streaming media to flow through these firewalls, which often block UDP traffic.
   • HTTP + TCP – The Windows Media server can also support HTTP-based control commands along with TCP-based data delivery. This combination has the benefit of working with all firewalls that let Web traffic through (port 80) and provides much more control (fast forward, rewind, etc) than a standard Web server, but also adds some overhead to the raw TCP stream that decreases scalability.
   • Multicast – The Windows Media server can also support IP Multicast protocols to allow very efficient delivery of streaming content to large numbers of users. Multicast enables hundreds or thousands of users to play a single stream, but will only work on networks with Multicast-enabled routers. Multicast is becoming prevalent on corporate networks, but is still very rare on the Internet.
   
   
   The Windows Media server will automatically switch to the appropriate protocol so no client-side configuration is necessary. The server will initially attempt to transmit files using the optimal UDP or Multicast protocols. If unable, the server will then attempt to send first via the raw TCP protocol, then via TCP with HTTP-based control.
   
   

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Conclusion

This paper has evaluated the two primary methods for streaming media content to users. The first, the Web server approach, uses a standard Web server and the associated HTTP and TCP protocols to request and deliver the content for the client. The second approach uses a streaming media server specialized to the audio/video-streaming task. The specialization takes many forms, including optimized routines for reading the huge media files from disk, the flexibility to choose any of UDP/TCP/Multicast protocols to deliver data, and the option to exploit continuous contact between client and server to dynamically improve content delivery to the client.

The primary advantage of the Web server approach is that it requires one less software component (the streaming media server) to learn and manage. This method can be an effective first step in developing a streaming solution.

The streaming media server approach, using Microsoft Windows NT Server Windows Media Services, has these advantages:

• More efficient use of the network bandwidth.
• Better audio and video quality to the user.
• Advanced features like detailed reporting and multi-stream multimedia content.
• Supports large number of users.
• Multiple delivery options.
• Content copyright protection.

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