Characteristics of a Well-formed Enhancement (ATVEF)

Whatever type of authoring tool you create, the result should be a well-formed enhancement which is broadcast to clients efficiently and robustly. This section describes guidelines that your authoring tool can follow to create well-formed enhancements.

One of the greatest challenges for any enhancement is to handle the case where a viewer tunes into a show in the middle of a broadcast. Because viewers often switch channels or "channel-surf," you cannot assume that broadcast clients receive an entire enhancement. To handle this, a well-formed enhancement repeats events. For example, if a client misses the first occurrence of an announcement, file download, or trigger, it can receive the second, third, or fourth occurrence.

A related concern is ensuring that the client receives enhancement files with all their dependency files. For example, if an enhancement displays three images, the client must receive all three images or the enhancement file appears broken when it is displayed.

To handle these and other challenges, a well-formed enhancement has the following attributes:

• A single announcement that is repeated as often as possible.
• The package containing the base page (also known as the top-level page) is transmitted as often as possible. The base page contains any FRAMEs or DIVs used to display other HTML pages. This page also contains any script functions called by triggers.
• Subsequent packages are wrapped by triggers that signal the beginning and end of package reception. This information can be used by script on the base page to check that the entire package was received before it displays the package.
• The enhancement components are transmitted in proper order: announcements before file downloads, file downloads before display triggers.
• Sufficient time is allowed between each component to allow the client to react. For example, there should be a pause between the time an announcement is sent and the time the package is sent. This enables the client to configure itself to receive the package. The amount of time required depends on the speed of the client.

For more information about how to create a well-formed enhancement, see Outline of a Typical ATVEF Enhancement.