126.96.36.199.2 Representation Networks
Representation Information, which is itself an Information Object, may be expressed in physical forms (e.g., a paper document) or in digital forms. When the Representation Information is in digital form, additional Representation Information is needed to understand the bits of the Representation Information as described in the previous subsection. In principle, this recursion continues until physical forms, which can be understood by the Designated Community, are encountered. For example, Representation Information expressed in ASCII needs the additional Representation Information for ASCII, which might be a physical document giving the ASCII standard. Each item of Representation Information can have multiple components, including multiple referenced Representation Information components; each with its own Representation Information.
To preserve the meaning of an Information Object, its Representation Information must also be preserved. This is most easily accomplished when the Representation Information objects are expressed in forms that are easily understandable, such as text descriptions that use widely supported standards such as ASCII characters for electronic versions. One problem with the use of only text descriptions is that such descriptions can be ambiguous. This is addressed by the use of standardized, formal description languages containing well-defined constructs with which to describe data structures. These languages may need to be augmented with text descriptions to convey fully the semantics of the Representation Information.
As the Knowledge Base of the Designated Community changes over time, the Representation Network may need to change accordingly. As noted in 2.2, an OAIS has a choice of whether to collect all the relevant Representation Information or to reference its existence in another trusted or partner OAIS Archive; this is an implementation and organization decision.
The Content Information must be defined and separated into Content Data Object and Representation Information. It is again an implementation and organization decision related to the way Data Objects are ingested and stored in the OAIS. For example, in the case of performing arts, the Content Data Object may be the score as a PDF document, and the Representation Information would include whatever information is needed to re-perform (as the way to use and understand) the piece, such as the way to display the PDF file, the audio processing software needed, placements of hardware such as loudspeakers, movement directions, and a description of how these relate to each other and to the Content Data Object, each of which may be quite complex, encoded in a separate way, and not easily described either simply as Structure or as Semantics. Alternatively, the Content Data Object may be multiple Data Objects including the score, the audio processing software needed, placements of hardware and movement directions. Each of these Data Objects will have its own Representation Information and there will need to be additional Representation Information that describes how the several Data Objects are related.
Two special types of Representation Information are Representation Rendering Software and Access Software. Representation Rendering Software is able to display the Representation Information in understandable forms. For example, the file and directory structure of many CD-ROMs conforms to ISO 9660. This standard is Representation Information describing how most CD-ROM file structures are to be implemented, and it may be obtained as a paper document. However, it may also be obtained as a digital object that needs to be understood as a PDF object. Rather than actually obtaining the documentation of PDF and writing software to understand the ISO 9660 object, an OAIS may use available PDF display software to render the ISO 9660 documentation humanly visible and readable. In this role the PDF display software is referred to as Representation Rendering Software because it is used to render the Representation Information. It also terminates the Representation Network. If the OAIS does not also obtain the associated description of PDF, it needs to record and track this fact because when PDF objects are no longer cost-effective for access and display, the ISO 9660 documentation expressed as a PDF object will need to be migrated to a new form.
Access Software presents some or all of the information content of an Information Object in forms understandable to humans or systems. It may also provide some types of access services, such as displaying, manipulating, processing, or sub-setting, to an Information Object. For some types of Digital Objects, such software may be widely available. It is not necessary for the OAIS to maintain or provide such software. The OAIS may want to maintain and provide this software for more specialized types of Digital Objects.
Since Access software will incorporate some understanding of the Representation Information, some Archives may attempt to use Access Software as a substitute for full Representation Information. Access Software source code, which embodies at least a partial understanding of the associated Representation Information, may be used as documentation expressing such Representation Information. A problem with this approach is that the desired Representation Information may not be clearly identifiable as it may be mixed with various processing and display algorithms, and may be incomplete since the code assumes an underlying operating environment. It may be difficult to tell, from the software code, what Representation Information is missing. The use of Access Software executables, without the source code, such as may occur with proprietary formats, presents a much greater risk for loss of information because it is more difficult to maintain an operating environment for software than to migrate documentation over time. The practical use of emulation techniques to preserve working software is an area of active research. This is a significant issue for those desiring to preserve a look and feel to information access. Migration and software preservation are discussed more fully in section 5.
--Please retain original text above for reference. Propose amendments or additions below this line or respond using the Discussion tab above--
These wiki pages are licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. Attribute as "Community forum for digital preservation and curation standards http://wiki.dpconline.org/". The content on this wiki represents the opinions of the author and not the Digital Preservation Coalition. This wiki is not associated with ISO, the OAIS Standard or the CCSDS.