Information Technology — MPEG-21 Multimedia Framework
MPEG-21 Use Case
To illustrate the innovative uses MPEG-21 enables, an example use case is described here. For the sake of clarity, it is not intended to cover all MPEG-21 parts and concepts.
Consider a distributed multimedia system comprising a certain number of Users exchanging Digital Items across a wide range of networks such as the Internet, mobile phone connections, etc. to a variety of terminals (Figure 1 ).
Figure 1 — Example Distributed Multimedia System.
Rights holders and their authorised representatives will want to set the usage conditions for the different DIs. These can be simple — the Resources of this DI can be used – or it may contain complex conditions — the resources of this DI can be used provided that certain conditions are met, which may, for instance, include temporal, spatial or group membership provisions. Complex Rights Expressions of this nature will enable rights holders to develop a multiplicity of business models.
Additionally, in order to guarantee a smooth delivery of such Digital Items over heterogeneous networks “adaptations” of Resources may be required, for example to overcome network congestion or to allow DIs to be routed at the same time to Users connected via mobile devices and to Users on fixed lines. A large number of adaptation possibilities exist, e.g., videos can be adapted by simple frame dropping or by modifying the quantization coefficients. Such adaptations can happen anywhere in the delivery chain from the DI provider to the DI consumer and can be governed by Rights Expressions.
Assume that a mobile User requests a Digital Item containing the newest movie trailers that are available without payment from another User providing a repository of Digital Items. In addition, the mobile User only accepts videos with a bitrate lower than 500 kbps. On its way to the Digital Item repository, the request for the trailer passes another User that has the functionality of a proxy cache and, fortunately, the requested resource is cached there. Assume, however, that the bandwidth requirement of the cached version exceeds the resource constraints.
Based on watching the movie trailers, the User decides to watch a movie at high resolution in a stationary environment. This high-resolution version is not free and cannot be redistributed to other Users.
Digital Item Declaration
Part 2 of MPEG-21 (ISO/IEC 21000-2) defines the structure of the Digital Item. These Digital Items comprise:
The DID is an XML file describing the Digital Item whereas the Resources are the individually identifiable multimedia Assets of the Digital Item (DI). The DID file may include information such as unique identifiers for the complete DI as well as for Resources, expressions on rights and permissions pertaining to the DI (or parts thereof) and generic metadata describing the Digital Item and its Resources. Finally, the DID contains references to the Resources). Typical examples of Resources include AAC audio files, MPEG-2 video clips, JPEG images, MPEG-4 presentations, HTML pages – but also e.g., video clips or text in proprietary formats.
Rights Expression Language/Rights Data Dictionary
MPEG-21 Part 5 provides a machine-readable Rights Expression Language (REL) that can declare rights and permissions using the terms as defined in the Rights Data Dictionary (RDD, MPEG-21 Part 6). The provider of the movie will use the REL to express under what conditions which User shall be able to access the item.
After paying for the high-resolution movie, the User will obtain an MPEG-21 REL License (e.g., in a second DI) containing the conditions under which the User can view the movie. This License contains a grant with four parts:
The task of enforcing these rights is the job of the IPMP system (MPEG-21 Part 4).
MPEG-21 Part 7 provides the means to steer Resource adaptation, e.g., in the proxy cache, and to carry it out in a more efficient way. The first tool to this end is the DIA AdaptationQoS Descriptor, which specifies the relationship between resource constraints, feasible operations on the resource and associated resulting qualities. The second descriptor is the DIA Bitstream Syntax Description (BSD) which describes the high-level structure of the media bit stream and which is appropriate for the adaptation of media resources on devices with constrained computational or memory resources. The advantage of using a BSD for adaptation is that it has a considerably smaller size than the media described and that adaptation engines need not be aware of the specific coding format of the movie. Rather, an adaptation module may rely on the BSD to modify the description/structure of the bit stream (by “editing” the BSD) and to further generate the adapted media data from the adapted BSD. This results in a more lightweight, coding format-independent way of adapting media streams, as compared e.g., to a sophisticated transcoding technique. Additional MPEG-21 tools provide so-called Usage Environment Descriptors which specify terminal and network characteristics and User preferences, among others.
Let us assume that the Digital Item cached in the proxy cache has, beside the resource, MPEG-7 descriptors available on the media format and media profile, also the AdaptationQoS and a BSD available. The proxy cache has to determine the bandwidth requirements of the version cached. This may be obtained from MPEG-7 descriptors in the Digital Item. The User constraint is then matched against the bandwidth requirement. If an adaptation is required, the AdaptationQoS associated to this video is consulted and the appropriate adaptation operation is extracted, together with a link to the adaptation process, which acts on the BSD description available for the video.