3.1 MPEG-21 Concepts and Structure

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3.1.1 MPEG-21 Concepts

MPEG-21 is based on two essential concepts: the definition of a fundamental unit of distribution and transaction (the digital item) and the concept of users interacting with DIs. The DIs can be considered the "what" of the multimedia framework (e.g., a video collection, a music album), and the users can be considered the "who."

3.1.1.1 User Model.

A user is any entity that interacts in the MPEG-21 environment or that makes use of a DI. Such users include individuals, consumers, communities, organizations, corporations, consortia, governments, and other standards bodies and initiatives around the world. Users are identified specifically by their relationship to another user for a certain interaction. A single entity may use content in many ways (publish, deliver, consume, and so on), and users may assume specific rights and responsibilities according to their interaction with other users within MPEG-21.

At its most basic level, MPEG-21 provides a framework in which one user interacts with other users and the object of that interaction is a DI. Some such interactions are creating content, providing content, archiving content, rating content, enhancing and delivering content, aggregating content, syndicating content, retail selling content, consuming content, facilitating transactions that occur from any of the above, and regulating transactions that occur from any of the above. Any of these are "uses" of MPEG-21, and the parties involved are the users.

3.1.1.2 Digital Item.

The DI is the "what" of MPEG-21. Obviously, there is a need for a precise description of what constitutes such an "item." This is given in the DID, which specifies a set of abstract terms and concepts to form a useful model for defining DIs.

Example: Consider a multimedia family book. It is composed of digital photos, videos, and text documents. We would like to have a way to represent the individual digital components as a single entity, to describe the content ("myself standing in front of the Pisa Leaning Tower") of the components, and to specify the relationships among the components ("this picture was taken by Hugo"). Then, the picture itself may be included or references to it may be established. Finally, technical information required by the viewing client, such as the media format of each component, sizes of the components, and so forth, needs to be included. Finally, we like to guarantee that someone who is not a relative will not view the book. The consumption rights have to be defined.

3.1.2 MPEG-21 Structure

MPEG-21 identifies seven key architectural elements that are needed to support the multimedia delivery chain and has defined the relationships between and the operations supported by them. They are

The DID, defining an uniform and flexible abstraction and interoperable schema for declaring DIs
The Digital Item Identification (DII) and description, providing a framework for identification and description of a DI
The content management and usage, providing interfaces and protocols that enable creation, manipulation, search, access, storage, delivery, and (re)use of content across its delivery and consumption chain
The IPMP, providing means to enable DIs and their rights to be persistently and reliably managed and protected
The terminals and networks, providing tools that enable the interoperable and transparent access to content across networks and terminals
The content representation, defining how the media resources are represented
The event reporting, supplying the metrics and interfaces that enable users to understand precisely the performance of all reportable events within the framework

Taking into account these seven key elements, 12 parts of MPEG-21 are currently defined (ISO/IEC 21000-1-12):

Part 1: Vision, technologies, and strategy, covering the overview of MPEG-21
Part 2: DID
Part 3: DII
Part 4: IPMP
Part 5: REL
Part 6: Rights data dictionary (RDD)
Part 7: Digital item adaptation (DIA)
Part 8: Reference software for all parts of MPEG-21
Part 9: File format for storage and retrieval of MPEG-21 DIs
Part 10: DI processing (DIP)
Part 11: Persistent association tools
Part 12: Resource delivery test bed.

Exhibit 3.1 shows the relationship among the different parts of MPEG-21.

Exhibit 3.1: Parts of MPEG-21.

click to expand

Part 1 of the MPEG-21 standard is a technical report that describes the vision of MPEG-21, which gives an overview of the remaining parts, describes strategies for further developments, and, finally, points out the relationships and collaborations with other standardization bodies; for example, the project Mediacom 2004 issued by the ITU-T Study Group 16,^[4] proposing a framework for the harmonized and coordinated development of global multimedia communication standards.

Part 2 of the MPEG-21 standard, the DID, specifies a set of abstract terms and concepts to form a useful model for defining DIs. It offers a representation of DIs in the form of a DID Schema (see Section 3.2).

Part 3 of the MPEG-21 standard, the DIIs, introduces tools to define unique identifiers that are used throughout MPEG-21 to name and address DIs and their parts. Note that MPEG-21 does not specify any new identification systems but standardizes the syntax of the reference to identifiers and the identification resolution process. Identification systems that may be used for DIIs are, for instance, the Digital Object Identifiers (DOIs),^[5] the EAN.UCC Extensible Markup Language (XML) Schemas,^[6] the identification systems used for books, and so on.

Part 4 of the MPEG-21 standard, the IPMP, enforces the rights and permissions defined on DIs. It goes hand in hand with Parts 5 and 6, the REL and the RDD. REL specifies the expression language for issuing rights for users to act on DIs. RDD forms the basis of all expressions of rights and permissions as defined by REL. Parts 4, 5, and 6 are detailed in Section 3.3.

Part 7 of the MPEG-21 standard, the DIA, refers to the terminals and networks and defines tools to support the adaptation process of a DI with respect to usage environment descriptions (see Section 3.4). It also includes descriptions for session mobility and configuration preferences with respect to the adaptation process.

Part 8 of the MPEG-21 standard is the reference software, collecting all software for descriptor generation and use as well as the implementation of the MPEG-21 tools.

Part 9 of the MPEG-21 standard defines an MPEG-21 file format that enables the DI to be efficiently stored and accessed.

Part 10 of the MPEG-21 standard, the DIP, specifies a processing architecture and a set of operations that can be used to process DIs (operations are called Digital Item Methods, or DIMs). A DIM defines an intended method for configuring, manipulating, or validating a DI.

Part 11 of the MPEG-21 standard, the persistent association tools introduce techniques that link information to identify and describe content with the content itself. Examples of these persistent tools are fingerprinting and watermarking.

Finally, Part 12 of the MPEG-21 standard, the Resource Delivery Test Bed, is a reference platform to provide a flexible and fair test environment for evaluating MPEG streaming technologies over IP (Internet Protocol) networks. The test bed has the capability of simulating different channel characteristics of various networks. As a result, new, scalable video codecs will be developed. Therefore:

Various codecs (audio, video, scene composition) are evaluated.
Various packetization methods and file formats are evaluated.
Various multimedia streaming rate control and error control mechanisms to be plugged into the test bed are evaluated.
New scalable video codecs are developed.

Parts 1 to 7 will be International Standard (ISs) in 2003. Parts 8 to 12 are currently under review and cannot yet be detailed. More information may be accessed at http://www.chiariglione.org/mpeg/.

Event reporting, as a part of the seven MPEG-21 key elements, is not yet hosted in an MPEG-21 part. Event reporting is useful, for instance, in the following situation: outgoing network congestion is detected by an intelligent router during a streaming session. The router likes to inform the streaming server of this congestion such that the server decreases on the fly the bit rate of the stream. Thus, the router generates an MPEG-21 Event Report that is delivered to the server. The server analyzes the report, and the streamed DI is adapted accordingly. In general, many different Event Reports may be triggered for different purposes on behalf of different Users when a DI is processed. Therefore, MPEG-21 suggests at the moment that it should be possible for parties responsible for the creation, production, and distribution of DIs to specify which Event Reports they require to be produced. Similar to RDD and REL, it is proposed that an event reporting dictionary and an event reporting language may need to be specified.

^[4]http://www.itu.int/ITU-T/com16/mediacom2004/index.html.

^[5]http://www.doi.org/.

^[6]http://www.ean-ucc.org/.

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