Streaming Methods: Web Server vs. Streaming Media Server

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.

Streaming with a Windows Media Server: the Advantages
Designed specifically for the task of delivering live or on-demand streaming media rather than many small HTML and image files, a Windows Media server offers many advantages over standard Web servers.