Even while the protocols for transporting video and audio data over the Internet continue to evolve, there are a number of efforts that use the technology already in place. KJHK at the University of Kansas is one, as mentioned in Rob Burcham's article.
Carl Malamud's Internet Multicasting Service (IMS) is a high-profile operation that has been broadcasting on the Internet, via MBone, for more than two years. IMS now offers a range of radio programming, on a number of ``channels.'' These programs are also available via ftp from town.hall.org, and via audio cassette from O'Reilly & Associates. IMS' Town Hall channel covers public-affairs topics, including broadcasts of speakers at the National Press Club.
Perhaps the largest audience for an MBone broadcast occurred on November 18, 1994, when the Rolling Stones broadcast portions of their Dallas concert. How do you access MBone? The Rolling Stones' Web site has some simple instructions: ``Here's what you do: get access to a Sun SPARCstation (or other suitable computer) and a fat connection to the Internet and MBONE, install sd, vat and nv (aka nvbin). Tune in and invite other people to watch too.''
As you can imagine, MBone is not for those of us with SLIP/PPP connections over a skimpy 14.4-Kbps link. MBone is a virtual network layered over the existing Internet, via the technique known as ``tunneling,'' which encapsulates packets of the new protocol within packets of the existing IP protocol. This requires a network of routers running mrouted, the multicast IP routing software. Eventually, multicasting will be built into commercial routers, but today the MBone is built on workstation-class machines (SGI boxes, SPARCstations, DECstations) running the routing software, connected by T1 or higher bandwidth links. To run mrouted, certain patches are needed to the operating system kernel. The number of participating networks with Mbone traffic has been estimated at 1400, a fraction of the current Internet.
On the client side, the workstations on an MBone subnet need modifications to the TCP/IP stack. At the moment, Apple Computer is the only PC vendor providing support for IP multicast on client workstations, in the successor to MacTCP (known as ``OpenTransport/TCP'') that has just emerged from beta and will ship on the new Mac PCI models. There will be an upgrade for existing Mac users. For more information, check out ftp://seeding.apple.com.
A comprehensive source of info about MBone is the Web site at www.eit.com.
For those of us left out in the cold with 14.4 SLIP/PPP links, there is one commercial technology that works over lower bandwidth connections. RealAudio, from Progressive Networks of Seattle, makes several products that let you publish sounds on your Web pages, and access this data over low-speed links using a proprietary compression format.
How is RealAudio different from existing Web technology? WWW users have always been able to access Web pages that have audio and video data. But this data can't be heard or seen in real time. When you view a Web page with an interlaced GIF image, it shows up in real time along with the text. By contrast, when your Web browser encounters audio or video data, you have to wait until the data file is downloaded.
The RealAudio system consists of several components. On the client side, the RealAudio Player (a ``helper application'' for your Web browser) runs on Windows PCs with sound cards or on Macintoshes. Progressive Networks makes server software that transfers data over the internet to the player. The company also makes space available on their Web site for your audio files (for free, at the moment). Your pages are located on the server of your existing provider, but can contain links to audio data on the server at Progressive Networks. However, as you might expect, there currently are severe capacity problems on this server.
The final component in RealAudio sound publishing is creating audio files. This is done via an encoder/converter known as ``RealAudio Studio,'' which can convert WAV, AU, and AIFF files to the proprietary RAM format. Currently RealAudio Player, Studio, and Server are in beta, and available free of charge from http://www.realaudio.com.
One broadcaster using RealAudio technology is Radio HK, which bills itself as ``the first 24-hour, Internet-only radio station.'' This station is a project of Hajjar/Kaufman Advertising (http://www.hkweb.com/radio). Radio HK initially was on the MBone, but switched to RealAudio to gain the (potentially) wider audience.
Ray Valdes is senior technical editor at Dr. Dobb' Journal and can be contacted at ray@valdes.com.