Digital Migrations that require some changes to the Content Information or PDI are referred to as Transformations. These changes will be to some of the bits in the Content Information or PDI with corresponding changes in the associated Representation Information. In all cases the intent is to provide maximum information preservation. The resulting AIP is intended to be a full replacement for the AIP that is undergoing Transformation. The new AIP qualifies as a new AIP Version of the previous AIP. The first version of the AIP is referred to as the original AIP and may be retained for verification of information preservation.
The Representation Information plays a key role in Transformations, and the impacts of the changes on the Representation Information may be used to categorize the Transformations. A Representation Information object can be modeled as consisting of a base set of entities, a set of resulting entities, and mapping rules that define the resulting entities and their relationships in terms of the base entities. Software, as a type of Representation Information, can be modeled in the same manner. Using this model of a Representation Information object, two types of Transformation can be defined: Reversible Transformation and Non- Reversible Transformation.
A Reversible Transformation occurs when the new representation defines a set (or a subset) of resulting entities that are equivalent to the resulting entities defined by the original representation. This means that there is a one-to-one mapping back to the original representation and its set of base entities. An example is replacing a representation that uses the ASCII codes ‘A through Z’ with a representation that uses the UNICODE UTF-16 codes for ‘A through Z’. The Transformation will result in the replacement of 7-bit codes with 16- bit codes in the AIP object undergoing change. The reverse Transformation can subsequently be performed by replacing the UNICODE UTF-16 codes for ‘A through Z’ with the ASCII codes for ‘A through Z’ and the original AIP is recovered.
A Non-Reversible Transformation occurs when a Reversible Transformation cannot be guaranteed. For example, replacing an IBM 7094 floating point value with an IEEE floating point value is a Non-Reversible Transformation because the resulting entities of these two representations are not semantically equivalent. One will have more precision than the other. However, they may be sufficiently equivalent, depending on what the values they represent are being used for, to be effectively interchangeable. If this is the case, a Non-Reversible Transformation effectively preserves the information content. For complex formats, where the meanings and relationships among groups are significant, it may be difficult to establish that a Non-Reversible Transformation has adequately preserved the Content Information. A Content Data Object for which software is playing a key role in providing much of Representation Information may be transformed into a new Content Data Object with new software. Such a Transformation is generally a Non-Reversible Transformation because the underlying data models will likely be quite complex and different.
It is useful to define a Transformational Information Property as an Information Property the preservation of the value of which is regarded as being necessary but not sufficient to verify that any Non-Reversible Transformation has adequately preserved information content. This could be important as contributing to evidence about Authenticity. Such an Information Property is dependent upon specific Representation Information, including Semantic Information, to denote how it is encoded and what it means. (The term ‘significant property’, which has various definitions in the literature, is sometimes used in a way that is consistent with its being a Transformational Information Property.) Following the example from 220.127.116.11, one can consider a simple digital book which when rendered appears as pages with margins, title, chapter headings, paragraphs, and text lines composed of words and punctuation. Information Property Descriptions for Information Properties that must be preserved (Transformational Information Properties) could be expressed as ‘paragraph identification’ and ‘characters expressing words and punctuation’. The Transformational Information
Properties would consist of all the book’s paragraph identifications, words, and punctuation as expressed by the Content Data Object and its Representation Information. This means that all formatting other than the recognition of paragraphs and readable text could be altered while still maintaining required preservation. Examples of Reversible and Non-Reversible Transformations are given in the scenarios that follow.
The following scenario identifies a Reversible Transformation that occurs when incorporating a lossless compression function on the Content Information of an AIP.
All the Content Information bits for an AIP are contained within the body of three files on a CD-ROM. The Packaging Information includes the bits used to implement the file and directory structure that provides access to these three files. The contents of the three files are transferred to a new CD-ROM and in the process they are compressed using a lossless compression algorithm. This transfer is a Transformation because the compression process has altered the Content Information, and it is a Reversible Transformation because there is a decompression algorithm that will return the original file content bits. The relevant Representation Information components of the original Content Information needs to be updated to include the decompression algorithm, and the PDI information also needs to be updated, in forming this new AIP Version.
The following scenario identifies a Non-Reversible Transformation that can occur when Content Information is migrated to a new format that can express a more varied data model than the original format.
All the Content Information bits for an AIP are contained within the body of three files on a CD-ROM. The Packaging Information includes the bits used to implement the file and directory structure that provides access to these three files. The contents of the three files are transferred to a new CD-ROM and in the process the third file is altered because there are no longer readily available tools to make effective use of the third file’s content in its current form. The new format, which is in common use, employs a different data model from that of the original format and there are many ways in which the information may be mapped into the new format. This mapping must be carefully done to ensure there is no significant information loss to the Designated Community. For example, for scientific data a Transformational Information Property could be the values of identified data elements to a specified precision; if the Content Information was a document then the page layout might be a Transformational Information Property. This mapping from the previous format to the new format must be included in the PDI, and of course the Representation Information describing the new format will replace that which was describing the previous format. The result is a new AIP Version. This is a Transformation type of Migration that is also a Non-Reversible Transformation when there is no algorithm that will reproduce the original file from the new file.
The following scenario identifies a Reversible Transformation that includes Repackaging. It occurs when the Content Information contains an embedded file name that is a pointer to one of its components, and the Content Information is moved to a new media type with new names for the files.
The Content Information for an AIP is defined to be the body of three files on a CD- ROM. The first file contains an internal name that links the third file and specifies a relationship between them. The Packaging Information includes the directory and file structure that identifies the three files. During a migration to a new media type, these three files are put into a new directory and given new names. This constitutes a Repackaging migration because there is a new implementation of the directory and file structure, which is providing the packaging function. However, the internal name must also be updated in order to maintain the link between the first and third files. This update changes the Content Information and means that the migration is also a Transformation. If the internal name had been a universal identifier, it would not have needed changing. However, the standardized framework supporting the universal identifier would contain the mapping information leading to the location of the third file and therefore would need updating. This approach would be advantageous for an OAIS because it allows updates to be centralized and more easily managed. However, the required technology is more complex and there is no universal agreement on the identification technique to use.
The final scenario identifies a Non-Reversible Transformation that includes Repackaging. It occurs when the Content Information includes file names, directory structure, and associated file attributes. The Content Information is then migrated to a new media type carrying a different implementation of the directory and file name structures that support fewer file attributes.
The Content Information and PDI bits for an AIC are defined to be an aggregation of AIUs where each AIU consists of the body of three files on a CD-ROM together with their file names, file attributes, and directory names. The Packaging Information is the bits used to implement the file and directory structure that provides access to each of the three file instances, but does not include the actual file and directory names. There may be thousands of AIU instances on a single CD-ROM. The transfer of this AIC to a new media type that employs a new representation for the file and directory structure that has fewer file attributes may result in a Non-Reversible Transformation type of Migration as well as a Repackaging migration. This is a Transformation because the Content Information that originally was stored in the file and directory structures must be re-distributed among the new file and directory structures and probably within the body of the files themselves. This is a Non-Reversible Transformation if there is no algorithmic one-to-one mapping between the resulting file and directory structures and file contents, and the original file and directory structures. It is a Repackaging because there is a new implementation of the directory and file structure, which was taken to be part of the packaging. The practice of encoding Content Information into a file or directory name increases the risk of information loss because evolution of a data management environment is facilitated by being able to update directory and file names as needed.
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