The key property of a link is its QoS. Definitions of link QoS for multimedia applications cite several contributing factors . For the Web user, however, the key QoS criteria is time. In simple terms, what the user wants to know before initiating a download is the delay before the download is completed. For the link, this can be defined as a function of site and channel latencies and channel bandwidth. Latencies reflect server and network congestion causing competition for these resources.
A full link time affordance, however, must also provide the user with information about the task in progress. The concept of time affordance is informed by users' subjective desire for closure, the knowledge that a system task has been successfully completed. Until this is evident, users will tend to monitor the task, assessing its progress and preparing contingency plans should they determine that it has failed. Lack of adequate time affordances may mislead users, for example, causing them to terminate the task prematurely.
According to Conn, a complete time affordance should meet eight requirements, representing information about distinct task properties . We confine ourselves here to those where the link user interface is particularly deficient, and discuss ways in which an enhanced link might satisfy them. Some of the solutions we propose raise functional requirements which are addressed later.
Improved satisfying of acceptance requirements could be achieved by server and document status information being available when the user is in the act of specifying the download task. This is the most timely moment to provide this information and resolves the inconsistency between visibility and existence.
Download scope is a function of the referenced document size and the link QoS. Satisfying scope requirements implies that these factors can be determined in advance of the download being initiated, In practice, however, only the document size can be accurately determined in this way -- given suitable functional extensions.
Link QoS figures should take into account the effects of caching. Caches were introduced to exploit locality in users' document downloading behaviour. For example, there may be a cache on the user's (client) machine and a cache on a local proxy serving a number of users. If a document has been previously retrieved it could be available on these caches. A cached document will almost always download more quickly than the copy on its server.
An example of an improved link time affordance is shown in Figure 6. The graph shows site QoS information and the predicted download scope. Download progress is indicated by animation of the document media content display in Figure 5. There is also a map display (accessible via the Location tab) which shows the channel as a physical connection between the user's client and the document server. Its colour is used to represent predicted channel QoS (ranging from blue for poor to red for good).