Research on mobile computing has made significant progress in adapting applications for viewing multimedia content on mobile devices [5,8,21]. Multimedia authoring and collaborative work on these platforms remain, however, open problems.
We identify three factors that hinder multimedia authoring and collaborative work over bandwidth-limited links:
This paper introduces adaptation-aware editing and progressive update propagation, two novel mechanisms that enable document authoring and collaborative work over bandwidth-limited links. These mechanisms extend traditional replication models to account for the fidelity level of replicated content. Both mechanisms decompose multimedia documents into their component structure (e.g., pages, images, sounds, video), and keep track of consistency and fidelity at a component granularity. Adaptation-aware editing enables editing adapted documents by differentiating between modifications made by the user and those that result from adaptation. Progressive update propagation reduces the time and the resources required to propagate components created or modified at the bandwidth-limited device by transmitting subsets of the modified components or transcoded versions of those modifications. Adaptation-aware editing and progressive update propagation also reduce the likelihood of update conflicts in two ways. First, by working at the component level rather than the whole-document level, they reduce the sharing granularity. Second, because both mechanisms lower the cost to download and upload component data, they encourage more frequent communication, hence increasing the awareness that users have of their collaborators' activities [3].
By reducing the cost of propagating multimedia content, adaptation-aware editing and progressive update propagation enable new types of applications and extend the reach of existing applications into the mobile realm. The following two examples illustrate the use of both mechanisms:
The previous scenarios cannot be handled by current adaptation systems that only handle adaptation of read-only content. They also cannot be supported by current replication systems. Propagating transcoded versions of components as described in the above examples, requires the replication model to account for the fidelity level of replicated content. Upgrading the fidelity of an image in a particular version of a document is different from creating a new version with (user) modifications to the document.
This paper shows that fidelity can be added to a replication protocol independently of the mechanisms used for concurrency control and consistency maintenance. Replication models are typically represented by state diagrams, and we follow this general paradigm. We present state diagrams that incorporate the presence of transcoded versions of components, for use with both optimistic and pessimistic replication. The introduction of transcoded component versions is orthogonal to the maintenance of consistency between replicas. More specifically, new states are added to represent transcoded versions, but the semantics of the existing states and the transitions between them remain unchanged. Therefore, fidelity can be added easily to any replication protocol, whether optimistic or pessimistic.
There are several possible implementations of adaptation-aware editing and progressive update propagation. We present a prototype implementation of these mechanisms that takes advantage of existing run-time APIs and structured document formats [5]. This implementation allows us to adapt applications for multimedia authoring and collaboration without changing their source code.
We demonstrate our implementation by experimenting with the Outlook email browser and the PowerPoint presentation software. Both applications see large reductions in user-perceived latencies. For Outlook, progressive update propagation reduces the time a wireless author has to stay connected to propagate emails with multimedia attachments. For PowerPoint, adaptation-aware editing and progressive update propagation reduce the time that wireless collaborators need to wait to view changes made to the presentation by their colleagues.
The rest of this paper is organized as follows. Section 2 introduces adaptation-aware editing and progressive update propagation and explores the implications of extending pessimistic and optimistic replication models to support these mechanisms. Sections 3 and 4 present the design and evaluation of our prototype implementation of adaptation-aware editing and progressive update propagation. Finally, Sections 5 and 6 discuss related work and conclude the paper.