Metadata for Client-side Content Adaptation
Nokia Research Center
Disclaimer: Opinions and views expressed in this paper belong to the author and do not necessarily represent the views of Nokia.
Since mobile phones have a very heterogeneous set of properties and capabilities, content adaptation and device independency are interesting to us. We have been looking at different aspects of content adaptation since the year 2000, and collected considerable experience in the area of server-side content adaptation . We have examined server-side adaptation in different forms: content selection based on metadata, content transcoding based on media properties, and content transformation based on device independent content. As an example of the latter, we participated in the European Union 5th framework programme research project Consensus , researching a device independent content format for mobile corporate applications. The learnings from the Consensus project have been described earlier , but one of them is worth repeating in this context: Content selection is a necessary ingredient for device-independent authoring. We should perhaps also add a corollary: Metadata is required for content selection, and therefore metadata is also a necessary ingredient for device-independent authoring.
Currently adaptation of content for mobile phones is performed mostly on server side (small-screen browsing modes in latest browsers are a notable exception). This has been a natural choice since the capabilities of mobile phones have until only very recently been insufficient for client-side content adaptation. However, the need for the server-side adaptation infrastructure stifles the development of mobile content. An additional problem is the maintance overhead of such infrastructure: the terminal profile databases must be kept up to date. So far attempts to get rid of profile database maintenance, e.g. by including a profile URL in request headers (static UAProf ), have not been very successful.
Another problem related to server-side adaptation is that sometimes the user would like to store or forward the content for viewing later on another device. Since the content has already been adapted for the user’s current device, it will not be suitable for viewing on other devices.
Client-side adaptation avoids these problems by not requiring the server to include adaptation functionality. As a result, there is no need to transmit a description of client properties to the server, and the server performs no content modifications. Another advantage of client-side adaptation is that the client knows its own properties intimately. It can therefore better guide adaptation than a server-side system which has to rely on standardized profiles that may be erroneous or out-of-date. A client-side adaptation system can also more easily forward decision-making to the user when the right decision is not obvious – for example, it can ask the user whether he wants a shortened, high-usability or a full, low-usability version of the content.
As mobile phones are becoming more capable, performing the content adaptation on the mobile phone itself is rapidly becoming feasible. If there were a standard way to describe different available versions of content, a phone could use such metadata to decide which variant of the content to get from the server. XHTML 2.0 already has a basic content selection mechanism in place, in the form of nested object elements. Currently the object element only supports selection by media type, but it would be easy to extend it towards arbitrary selection conditions. Another option would be a new XML specification meant especially for content selection, such as the Content Selection for Device Independence Working Draft recently published by the W3C Device Independence working group .
The two problems where the client-side content selection approach seems most promising are size and format. Even with the deployment of 3rd generation mobile networks, the size of media content such as video clips remains a problem in the terms of download speed, even though storage is becoming a non-issue. Similarly, even though the capabilities of mobile devices are rapidly increasing, the number of possible content formats is huge and keeps growing. Since many formats are proprietary and support for them depends on third parties, we must assume that format support will be a problem for mobile phones for a long time to come.
Presentation is also a problem, although perhaps not as serious – a presentation meant for a different environment usually does not make the content incompatible with the terminal, merely hard to use (assuming that all terminals understand the presentation markup). While media content has issues with size and format, presentation of content is by its nature geared towards a certain type of terminal. Therefore there should to be alternative presentations to support different terminals. This could be e.g. in the form of alternative CSS files, or one CSS file which contains alternative settings.
There are some important requirements for the metadata if it is to enable the client-side content adaptation we envisage. Firstly, it must be simple, so that use of the metadata can become widespread. History shows that simplicity is more important that expressiveness – authors have to be able to easily adopt and understand the use of metadata. Secondly, it must be compact so that even massive amounts of metadata will cause only neglible overhead. Thirdly, it must be sufficient for common applications, and extendable – in a well-defined manner – by special applications.
And finally, it must be efficient – if it is slower or more difficult to interpret the metadata than it would be to analyze the media content itself, there is no point in it.
 Coulombe S, Koskimies O., Grassel G.: Content Adaptation for the Mobile Internet, chapter 7 in the book ”Content Networking in the Mobile Internet”, Wiley-Interscience, ISBN 0471466182, September 2004.
 Consensus project, EU 5th framework programme, http://www.consensus-online.org/.
 Koskimies O., Wasmund M., Wolkerstorfer P. and Ziegert T.: Practical experiences with device independent authoring concepts. Advanced Visual Interfaces 2004 Conference, Workshop on XML-based High-Level User Interface Descriptions, May 25th 2004.
 Open Mobile Alliance: OMA UAProf 2.0 Specification, http://www.openmobilealliance.org/release_program/enabler_releases.html#UAProf, May 20th 2003.
 W3C Device Independence Working Group: Content Selection for Device Independence, W3C Working Draft, http://w3.org/TR/cselection/, June 11th 2004.