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To access the contents, click the chapter and section titles.
Platinum Edition Using HTML 4, XML, and Java 1.2
ChannelsTo overcome some of the limitations associated with subscriptions, Microsoft has developed the notion of a Webcast channel that starts with the content provider rather than with the user. Channels enable content providers to better manage what they put out on the Web, as well as when they put it out, just as a television station needs to manage its programming and schedule. Users tune in to Webcast channels using Internet Explorer the same as they would use their television sets to tune into a TV broadcast channel. Figure 17.1 shows the Warner Brothers Studio Store Webcast channel.
Channel authoring begins with Microsoft’s Channel Definition Format (CDF), a markup language based on the eXtensible Markup Language (XML) standard. A CDF file defines a channel by specifying the following:
This information is stored in a file ending with the .cdf extension. When a user tunes in to a channel and subscribes to it, Internet Explorer accesses the CDF file and notes the pages and update schedule specified there. Users don’t need to know exactly which pages they’re subscribing to and don’t have to guess at what an appropriate update schedule is because all that is now supplied by the content provider (who would know best about what pages are essential to a channel and when those pages might change). Channels allow for another important benefit, as well. Because CDF files can be generated on-the-fly, it’s possible for channels to be customized to a user’s preferences. A user can specify these preferences when subscribing to the channel, and the Web server stores the preferences in a cookie file on the user’s machine. Later, when the channel is accessed again, the cookie file is retrieved and the information is used to display channel content according to the user’s interests.
Push TechnologyAlthough channels represent a more advanced form of Webcasting than subscriptions, they are also examples of “intelligent pull” rather than “push” technology because the browser is still initiating the update step (with channels, however, the intelligence is based on information from the content provider rather than the user). True push technology in the Microsoft Webcasting scheme involves the use of multicast protocols, which is one server broadcasting to many clients. These special protocols make efficient use of available bandwidth to distribute content across a network. Note that this is different from subscriptions or channels because they only involve an interaction between one server and one browser. Internet Explorer provides an open architecture for implementation of push technology client software. Although many such client programs are provided by third-party vendors, you can also use Microsoft’s Windows Media Player to tune into a true push broadcast. On the server side, push delivery cannot be provided by your common, everyday HTTP server alone. Rather, you have to use some kind of streaming media server that knows the Multicast File Transfer Protocol (MFTP), and Microsoft’s NetShow Server is one such server product.
Microsoft’s Channel Definition Format (CDF)If you’ve subscribed to some of Microsoft’s premium channels, you may have seen some filenames that ended with the extension .cdf. These files are Channel Definition Format (CDF) files and are at the heart of Internet Explorer Webcasting. Microsoft developed the CDF standard so that content providers can quickly and easily author Webcast channels. This section introduces you to CDF and shows you how you can use it to create a channel of your own.
Recall from the last section that three levels exist in Internet Explorer Webcasting. These are
CDF comes in at the channel Webcasting level. By enabling the content developer to decide which content to deliver and when, the situation becomes more akin to a television station making decisions about a broadcast schedule. Accordingly, sites whose content is managed in this way are referred to as channels. Site administrators create channels out of their sites by authoring a CDF file to support the channel. No change is necessary to the HTML files that compose the site. CDF is a markup language based on the XML and is intended to be an open, scalable solution for creating managed content channels. A CDF file provides a map of the information on a site, grouped into logical categories. Along with the information map, the CDF file also specifies which information should be Webcast and when, giving a site administrator complete control over what content becomes part of the channel. The basic elements of CDF are discussed over the next several sections. To use a CDF file you’ve created, place it in the root directory of your Web server and set up hyperlinks on your Web page that point to it. CDF-compliant browsers such as Internet Explorer 4 will then parse the file and set up the channel.
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