The aim of the MPEG-21 standard, the so-called Multimedia Framework, is to enable transparent and augmented use of multimedia resources across a wide range of networks, devices, user preferences, and communities, notably for trading (of bits). As such, it provides the next step in MPEG's standards evolution, i.e., the transaction of Digital Items among Users.
A Digital Item is a structured digital object with a standard representation and metadata. As such, it is the fundamental unit of transaction and distribution within the MPEG-21 multimedia framework. In order words, it aggregates multimedia resources together with metadata, licenses, identifiers, intellectual property management and protection (IPMP) information, and methods within a standardized structure.
A User (please note the upper case “U”) is defined as any entity that interacts within this framework or makes use of Digital Items. It is important to note that Users may include individuals as well as communities, organizations, or governments, and that Users are not even restricted to humans, i.e., they may also include intelligent software modules such as agents.
The MPEG-21 standard currently comprises 18 parts which can be clustered into six major categories each dealing with different aspect of the Digital Items: declaration (and identification), digital rights management, adaptation, processing, systems, and miscellaneous aspects (i.e., reference software, conformance, etc.):
- Declaration and identification: for declaring Digital Items MPEG-21 adopted XML Schema to define the Digital Item Declaration Language (DIDL). An XML document conforming to DIDL is called Digital Item Declaration (DID) which defines the structure of the Digital Item, i.e., how resources and metadata relates to each other. Furthermore, MPEG-21 provides means for uniquely identifying DIs and parts thereof. However, it is important to emphasize that MPEG-21 does not define yet another identification scheme; in fact, it facilitates existing schemes such as International Standard Book Number (ISBN) or International Standard Serial Number (ISSN) and specifies means for establishing a registration authority for Digital Items.
- Digital rights management: how to include IPMP information and protected parts of Digital Items in a DIDL document is specified in part 4 of MPEG-21. It deliberately does not include protection measures, keys, key management, trust management, encryption algorithms, certification infrastructures or other components required for a complete DRM system. Rights and permissions on digital resources in MPEG-21 can be defined as the action (or activity) or a class of actions that a principal may perform on or using the associated resource under given conditions, e.g., time, fee, etc. Rights and permissions are accompanied by a set of clear, consistent, structured, integrated, and uniquely identified terms within a well-structured, extensible dictionary.
- Adaptation of Digital Items: is a vital and comprehensive part within MPEG-21 and with regard to Universal Multimedia Access. It is referred to as Digital Item Adaptation (DIA), which specifies normative descriptions tools to assist with the adaptation of Digital Items. In particular, the DIA standard specifies means enabling the construction of device and coding format independent adaptation engines. Note that only tools used to guide the adaptation engine are specified by DIA while the adaptation engines themselves are left open to industry competition.
- Processing of Digital Items: includes methods within the statically declared Digital Items that are usually presented to the User for selection. These methods are written in ECMAScript and may utilize Digital Item Base Operations (DIBOs) which are analogous to a standard library of functions of a programming language. However, also bindings to other programming languages (currently Java and C++) have been defined.
- MPEG-21 Systems: include a file format that forms the basis of an interoperable exchange of Digital Items (as files). MPEG's binary format for metadata (BiM) has been adopted as an alternative schema-aware XML serialization format which adds streaming capabilities to XML documents, among other useful features. Finally, this category defines how to map Digital Items on various transport mechanisms (MPEG-2 TS, RTP, etc.) whereby various parts of Digital Items can be streamed over different channels.
- Part 2: Digital Item Declaration
- Part 3: Digital Item Identification
- Part 4: Intellectual Property Management and Protection Components
- Part 5: Rights Expression Language
- Part 6: Rights Data Dictionary
- Part 7: Digital Item Adaptation and Session Mobility
- Part 8: Reference Software
- Part 9: File Format
- Part 10: Digital Item Processing and C++ bindings
- Part 11: Evaluation Tools for Persistent Association
- Part 15: Event Reporting
- Part 17: Fragment Identification for MPEG Resources
- Part 18: Digital Item Streaming
Publicly available MPEG-21 standards: