Mobile devices such as handheld computers, smart phones, and personal digital assistants (PDAs) enable users to have access to and exchange a variety of contents anytime and anywhere. Although mobile devices offer rich multimedia communication and applications, such as audio, video, data and image transmission, they present critical challenges in that they need to provide delivery quality adaptable to heterogeneous devices of users.
For effective content delivery in mobile environment, it is desirable to identify the crucial factors and characterize the features related to users' context, such as locality, locale, or temporal aspects (e.g., sound mode, time of data access, number format, daylight saving time, etc.) [1,2]. For example, a user can request traffic service with his or her smart phone from a navigation system. In response to the request, the navigation system provides the user with the information by means of a voice system embedded within the smart phone.
Multimodal contents can be considered to be a set of multiple components which are dynamically associating, correlating, and combining in appropriate manners. Furthermore, since the circumstances surrounding mobile devices change dynamically, special care needs to be taken for adaptive content delivery to provide context-aware information [3,4].
To adapt multimodal contents to heterogeneous performance of devices and different specifications of transmission, multimodal contents, such as video and audio data, are often transcoded. The adaptation that is referred to as transcoding is considered from two perspectives: application-adaptive and environment-adaptive. Application-adaptive refers to the agility and capability of a mobile device to respond to a user's request, whereas environment-adaptive is closely related with modality and the form of response with an effective presentation adjustment. With regards to content delivery, the adaptability of contents is crucial for increasing the quality of service delivered to users.
For example, weather or traffic contents can be transmitted to users in the form of combining video, audio, and text, through mobile devices such as a PDA or a cell phone. In this case, the locality of the users or temporal issues should be considered to provide quality of service in consideration of a user's environment. More specifically, the information which is provided to users needs to be up-to-date in terms of the time of delivery or well-suited to the current location of the user if it is to be useful. The capability or requirements of devices that the user carries should also be considered in delivery of the information.
Much research on content delivery in terms of multimedia has been conducted on technological aspects and some of the work have shown potential success [5,6]. However, content delivery of multimodal contents has not yet received much attention. Furthermore, most studies in the content delivery have not taken much consideration into analytical models to assist in evaluating the performance of an individual device or in the efficiency of an overall mobile computing environment.