In this chapter we propose a platform-independent concept for the transparent replication of digital assets in hybrid wireless networks. The concept allows a seamless integration with existing standards and technologies. It tackles and overcomes typical problems of common file sharing applications, such as the mixing of the logical property of a file being shared with its physical location. The identification mechanism introduced allows sharing on a per file basis, completely independent of their physical location, even tolerating subsequent relocation. RSS feeds are used as a basis to disseminate the files together with attached meta-information, such as tags, in a platform-independent manner. To optimize the communication flow among the mobile devices, a clustering algorithm for mobile networks is employed. The current prototype acts as proof-of-concept for the proposed concept.
Nowadays, more and more people are using mobile devices, such as laptops, tablet PCs, smartphones, and PDAs. Typically, these devices are able to communicate with each other via Bluetooth or Wi-Fi in ad hoc mode. Spontaneous groups are formed and maintained in a self-organizing fashion, enabling users to exchange and share data. However, pure mobile ad hoc networks (MANETs) limit the accessible data to information available in the local environment. Wireless networking technologies, such as GPRS, UMTS, or Wi-Fi, are used to establish communication links to a backbone network—or more generally to the Internet. Devices capable to establish such uplinks can share their connection with other devices (Papadopouli & Schulzrinne, 1999). In this way devices currently unable to connect to the Internet directly are enabled to stay connected. Through such uplinks it is also possible to interconnect different ad hoc partitions (Brust, Rothkugel, & Ribeiro, 2006). The augmentation of mobile ad hoc network partitions with links to infrastructure networks is called hybrid wireless network throughout this chapter. Figure 1 illustrates such a scenario where two ad hoc network partitions are connected to the Internet.
Sharing digital assets such as documents or pictures is an essential daily task. This includes for instance the sharing of podcasts, pictures, learning materials, and so forth. However, typical peer-to-peer file sharing applications, such as BearShare (2007) or LimeWire (2007), are not suitable for hybrid wireless networks. The biggest problem is the large amount of network traffic induced by the Gnutella protocol (2007), which causes problems to mobile devices (Hu, Thai, & Seneviratne, 2003), as links can break down suddenly.
In this chapter a file sharing application for hybrid wireless networks based on the concept of Spaces (Wehling & Rothkugel, 2007) is proposed. Spaces is a replication scheme for sharing digital assets in hybrid wireless networks. It enables users to share digital assets in a transparent manner on a per-file basis without imposing any folder structure for the local organization of the files. It is based on a publish/subscribe paradigm to reduce the amount of information presented to the user. In addition to that, tags are employed enabling users to retrieve and group files. The proposed concept allows a seamless integration with existing technologies and operating systems. As a proof-of-concept the proposed concept is prototypically implemented. The prototype consists of two modules, one platform-specific module that is responsible for the local file organization and another one that is in charge of the communication.
The remainder of this chapter is organized as follows. In the next section we discuss some problems that have to be tackled when designing a file sharing application, followed by a brief overview of the concept of Spaces. Then a concrete application scenario is given, which further motivates this work. The next section covers related work. Then an insight in the local organization of files is given and the dissemination of the information is discussed. Before concluding this chapter, an outlook on future trends is given.Top
Problems To Be Tackled
Besides the technical issues concerning the amount of network traffic induced, some other major problems exist. The following gives an overview of the problems that have to be tackled when designing a file sharing application for hybrid wireless networks.
Key Terms in this Chapter
File Sharing: The sharing of files with other users, like it is done in common file sharing applications such as BearShare or LimeWire. The files are shared and stored by the mobile devices of the users, where every user can provide and receive files.
RSS: RSS (Really Simple Syndication or Rich/RDF Site Summary) is a format for delivering changing Web content. The content is published in RSS feeds, which enables people to stay informed about information they are interested in.
Space: One Space is a collection of replicated files belonging to one special topic. Every logical Space consists of several physical Spaces distributed across different devices like laptops, PDAs, Smartphones, and servers. Locally, a Space is a virtual folder where files are only logically grouped together.
Hybrid Wireless Network: A hybrid wireless network is the combination of a mobile ad hoc network (MANET) and an infrastructured network. Every local ad hoc network partition can be augmented with uplinks to a backbone network or more general the Internet, via technologies like GPRS, UMTS, or Wi-Fi. Such uplinks can be shared among the members in the local ad hoc network partitions.
Clustering Algorithm for Mobile Networks: A clustering algorithm for a mobile network creates hierarchical structures, called clusters, on an otherwise unordered and flat network.
Mobile Device: A mobile device is a small computer that can be carried by the owner. Typical examples are laptops, tablet PCs, Smartphones, and PDAs.
Spaces: Spaces is a replication scheme for sharing files among a group of users in a transparent fashion.
URI Scheme: A URI scheme defines the syntax for a class of URIs. Every URI belongs to a special scheme and must follow the defined syntax in order to be valid. Common URI schemes are for instance the http and the ftp URI scheme.
Tag: A tag is a textual label assigned to a resource, which can contain additional information about the content of that resource. Tagging describes the process of assigning tags to resources. In this context the resources are files.