Contemporary systems use centralized as well as peer-to-peer mechanisms for the large scale distribution of media objects. In this work, we investigate a serendipitous mechanism for directly sharing media objects among a local community of wireless users. This localized sharing is attractive when wide area network connectivity is undesirable, expensive or unavailable; especially when the shared media objects are large. With some restrictions, such localized sharing of media objects is also acceptable to content owners. However, localized sharing has to contend with far fewer media providers who may also not offer the variety of objects available from wide-area services. We collected empirical data from the widely deployed Apple iTunes application for our analysis. We showed that users are already making a significant amount of media objects available for serendipitous sharing. Our analysis showed that the shared object annotations exhibited a Zipfian long tail distribution. The availability patterns of wireless iTunes users and the object annotations makes serendipitous sharing inappropriate for scenarios that require access to a specific object. Instead, mechanisms that allow the user to specify classes of interesting objects are better suited for such users. Also, given the smaller scale of these systems, serendipitous sharing can benefit from approaches that allow users to disseminate a compact representation of their shared objects. Though the wireless user availability rates was not as high as what was observed in a corporate desktop setting, a large fraction of the users showed high temporal consistency. This allows for high availability with reasonable replication during weekday daytime hours. We answer important questions regarding the viability of a campus-wide media sharing system.

Asynchronous group communication systems propagate updates from each group member to every other member. Contemporary users are wireless and operate from a variety of locations. Hence, we investigate the propagation performance for these users. An analysis of wireless users in academia, corporation and in a city-wide hotspot federation shows that the availability durations and the duration between sessions depends on the locale. A longer term analysis shows that the user session lengths are becoming smaller while the duration between sessions are becoming larger with significant user churn. We show that the propagation performance depends on the locale. New users incur a heavy cost in receiving updates from prior users; applications need to develop expiration mechanisms to reduce this overhead. Also, since users regularly leave the system, practical communication mechanisms need to choose their propagation periodicity in order to respond to scenarios when the user abruptly leaves the system without propagating their updates to other users.

We evaluate an asynchronous gossiping middleware for wireless users that propagates messages from any group member to all the other group members. This propagation can either be implemented through distributed mechanisms or can be mediated through servers. Our analysis of asynchronous mechanisms using wireless user availability traces from an university, corporation and a hot spot federation shows that the fundamental impediment to the system performance is the wireless user availability patterns. We then investigate the relative performance for several distributed as well as server mediated approaches. We show that pull mechanisms effectively randomizes the times when messages are propagated and thus achieves better performance than push based mechanisms. We then develop an adaptive approach that customizes the propagation frequency using the last session duration and show that this mechanism exhibits good performance when the required propagation intervals are large. We also show that for a given number of gossips, it is preferable to propagate messages to all available nodes rather than increasing the frequency while correspondingly reducing the number of nodes to propagate messages. Our results allow middleware developers to choose the appropriate propagation model to satisfy their application constraints.

The ubiquitous deployment of wireless LAN networks are allowing students to embrace laptops as their preferred computing platform. We investigated the viability of building collaborative applications to share contents amongst student groups. In our application scenario, the university will provide wireless infrastructure throughout the campus but not the storage infrastructure required to store the shared contents. Laptops will likely exhibit weak availability. Hence, these collaborative applications need to tolerate long delays in propagating updates amongst the participants. In this paper, we presented a preliminary analysis of message forwarding behavior under realistically resource constrained node scenarios. Our experiments were based on the observed wireless user behavior at the University of Notre Dame. Our experiments showed the inherent limits of epidemic propagation in real campus wireless network scenarios.

This work focuses on scenarios that require the storage of large amounts of data. Such systems require the ability to either continuously increase the storage space or reclaim space by deleting contents. Traditionally, storage systems relegated object reclamation to applications. In this work, content creators explicitly annotate the object using a temporal importance function. The storage system uses this information to evict less important objects. The challenge is to design importance functions that are simple and expressive. We describe a two step temporal importance function. We introduce the notion of storage importance density to quantify the importance levels for which the storage is full. Using extensive simulations and observations of a university wide lecture video capture and storage application, we show that our abstraction allows the users to express the amount of persistence for each individual object.