Networking

Streaming Video Over the Internet

David Ackerman and Jimmy Kyriannis

The evolution of information services on the Internet is proceeding rapidly. For many years, the typical transmission, whether via e-mail, ftp or gopher, was in plain text. Of course it was also possible to send files that weren't text, like binary programs or graphic images, but they had to be used or viewed as a second step, after having first downloaded them.

The graphical web browser changed this. Documents consisting of formatted text with images could be easily and immediately delivered by simply clicking on a page. It was also possible, but not very convenient, to view video files by clicking on them, waiting for them to download to your local computer, then viewing them with appropriate software.

In 1995, RealAudio 1.0 was released. This was the introduction of a new technology called streaming media. When you click on a streaming media item such as an audio file, a download of the file begins. But instead of having to retrieve the entire file before viewing it, playback starts when enough of the content (commonly referred to as a buffer) has arrived to ensure a smooth and continuous broadcast of the file. So, you click, wait a few seconds for the audio to be buffered, and then start listening.

RealNetworks then came out with RealMedia, which added video. This enabled true broadcasting over the Internet, with webcasts either live or on demand. In other words, one could tune in to a live broadcast at the time the event was actually taking place, or see a rebroadcast at any later time.

How Does it Work?

Either method is highly computation-intensive, since the video and audio signals must be processed quickly enough to keep up with the constant incoming video and audio feeds. Typical single-processor Pentium systems simply aren't powerful enough to do the job, so ACF uses a dual processor 400MHz Pentium II system, with a PCI bus for best possible I/O (Input/Output) performance from the video and audio acquisition boards. It's also portable, for easy transport to the location of an event.

The RealEncoder software allows us to select how lossy the compression of the audio/video sessions are. Lossy refers to the degree of information discarded to reduce the size of the file. The more lossy, the less information stored per time unit and the less bandwidth required to transmit the data to the audience. With the current technology, full-motion video compressed down to meet the transmission speed of a 28.8 Kbps modem link lose a significant amount of its video information, resulting in poor broadcast quality.

We've found that encoding at 110Kbps (compared to an ethernet LAN connection that operates at 10Mbps) offers fairly good video and excellent audio quality, without impacting network performance to a significant degree. The next-generation RealSystem G2 software from RealNetworks promises to address video demands over a modem link by significant enhancements to the distribution and transmission.

RealServer, the current distribution server, transmits the encoded multimedia session to the RealPlayer client at a fairly constant rate. This is called streaming. The streamed data is transferred through the RealEncoder either as a saved file or live as the event is being digitized.

Due to the potential for many RealPlayer viewers to connect to the RealServer at any one time, we have chosen a very powerful dual processor Sun SPARC system with a 100Mbps connection to NYU-NET as our server. The centralized placement of the server on the network offers the best possible performance to viewers within NYU, as well as to others on the Internet who wish to view NYU events.

Traditionally, RealServers send out one stream of multimedia data per RealPlayer viewer. This poses network scalability problems since it's highly inefficient for a server to simultaneously transmit the same stream many times. Individual RealMedia streams are sent to the RealPlayer in unicasted IP packets. Unicasted packets are sent directly from one machine to another, just as they are for telnet, ftp or HTTP sessions. Using this scheme, 100 players would require 100 unicasted packet streams on the network. At 110Kbps, this would saturate a 10Mbps Ethernet.

For live webcasts and scheduled broadcasts of a recorded session, there's an alternative. One single data stream transmitted with multicasted IP packets can support an indefinite number of RealPlayer clients. The RealPlayers are instructed to tune in to this single data stream, similar to tuning in to a TV broadcast. IP multicasting requires highly specific support from the network router infrastructure. Though NYU-NET has been engineered to support it, the commodity Internet still does not. Experimental and future networks, such as the mBONE and Internet2, will pave the way for the future use of multicasting on a worldwide level.

The RealServer is responsible for transmitting the webcast to the viewer. Available webcast sessions are listed on a web page. When a viewer clicks on a RealMedia URL, the server tells the browser to play the session with RealPlayer. This provides a quick and efficient means of integrating webcasts with descriptive text and graphics on a single web page. It also allows for the creation of an online multimedia archive for video-on-demand applications.

The Bloomsday Webcast

Thanks to the use of webcasting, Bloomsday has now gone global. This year, readings were broadcast over the Internet in RealAudio, allowing people worldwide to listen to readings being held sequentially in 18 cities across the globe. Melbourne, Dublin and New York also broadcast RealVideo coverage of their Bloomsday events.

ACF broadcast the New York reading live from NYU's Glucksman Ireland House. The readers were Frank McCourt, Fintan O'Toole, Brian F. O'Byrne, Tim O'Connor and Nicholas Joyce. The global event was broadcast at the Irish Times website (previously at http://www.irish-times.com/globalreading/). The NYU event was previously at http://www.nyu.edu/acf/live/.

Information about Bloomsday was provided by Lorna Hughes, ACF's Assistant Director for Humanities Computing.

Posted October 5,1998