18.104.22.168.1 Content Information
The Content Information is the set of information that is the original target of preservation by the OAIS. Deciding what the Content Information is may not be obvious and may need to be negotiated with the Producer. The Content Information, which is an Information Object as shown in figure 4-12, is the Content Data Object together with its Representation Information. The Content Data Object in the Content Information may be either a Digital Object or a Physical Object (e.g., a physical sample, microfilm). Any Information Object may serve as Content Information.
The Representation Information for a digital Content Data Object (both semantic and syntactic) is needed to fully transform the bits into the Content Information. In principal, this even extends to the inclusion of definitions (e.g., dictionary and grammar) of any natural language (e.g., English) used in expressing the Content Information. Over long time periods the meaning of natural language expressions can evolve significantly in both general and in specific discipline usage.
As a practical matter, the OAIS needs to have enough Representation Information associated with the bits of the Content Data Object in the Content Information that it feels confident that the members of the Designated Community can enter the Representation Network with enough knowledge to begin accurately interpreting the Representation Information. This is a significant risk area for an OAIS, particularly for those with an expert Designated Community, because jargon and apparently widely understood terms may be short-lived. In such cases extra care needs to be exercised to ensure that the natural evolution of the Designated Community Knowledge Base does not effectively cause information loss from the Content Information.
As described above for an Information Object in general, the Representation Information can also be viewed as being augmented by Access Software that supports the presentation of the Content Information to the Consumer. Examples of this type of software include word processors supporting complex document format representations of Content Information and scientific visualization systems supporting representations of Content Information as a time series or a multidimensional array. Access Software may include rights enforcement tools that allow the access to protected content. The software uses its knowledge of the underlying Representation Information to provide these services.
Often required information will be embedded in the software packages used by the Designated Community to present and analyze the Content Information. A reason for preserving working Access Software arises from a convenience factor. Even with a complete set of Representation Information, practical access to all or part of a digital Content Data Object requires the use of Access Software. Thus a software module that provides useful access to a digital Content Data Object may be preserved in a working state as a matter of convenience.
This is not difficult to do as long as the environment, which supports the software module, is readily available. This environment consists of some underlying hardware and an operating system, various utilities that effectively augment the operating system and storage and display devices and their drivers. A change to any of these may cause the software module to no longer function, to function incorrectly, or to be unable to present results to the application or human user. The complexity of these interactions is what traditionally makes the preservation of working software such an arduous task.
In summary, the use of Access Software to replace Representation Networks is attractive from the point of view of minimizing the resources needed to ingest data and provide current users with access to data. However, the reliance on working software can provide major problems for Long Term Preservation when that software ceases to function. Indefinite Long Term information preservation requires a full and understandable description of the Representation Information. Subsection 5.2 (Preservation of Access and Use Services) discusses some techniques that can be used to preserve software over time and the risks associated with this approach.
An important function of the OAIS is deciding what parts of the Content Information are the Content Data Object and what parts are the Representation Information. This aspect is critical to a clear understanding of what is being preserved. The identification of digital Content Information with its Representation Information objects can be addressed by a series of steps, as follows:
1) Identify the bits comprising the Content Data Object of the Content Information.
2) Identify a Representation Information object that, in some way, addresses all the bits of the Content Data Object and converts them into more meaningful information.
3) For the Representation Information object identified, examine its content to identify if it requires additional Representation Information objects. If it does, obtain the required Representation Information objects. Repeat this step at least until no additional Representation Information objects are identified as required for the Designated Community. 4) Of the Representation Information objects addressed in step 3, for each that is held as a Digital Object, identify any required Representation Information object and repeat steps 3 and 4 until no new Representation Information objects are identified.
5) The Content Information consists of the Content Data Object and each of the Representation Information objects identified in steps 2 through 4.
As an example of this practice, consider an electronic file containing a sequence of values obtained from a sensor looking at the Earth’s environment. There is a second file, encoded using ASCII, which provides information on how to understand the first file. It describes how to interpret the bits of the first file to obtain meaningful numbers. It explains what these numbers mean in terms of the physics of the observation being conducted. It provides the date and time period over which the observations were made, an average value for the observed values, and who made the observations. These two files are submitted to an OAIS for preservation.
Assume that the OAIS determines that the Content Information to be preserved is the observed bits together with their values as numbers and the physical meaning of these numbers. This information is conveyed by the bit sequence within the first file together with the Representation Information from the second file that is needed to transform the first file’s bits into meaningful physical values. Neither the first file’s underlying media nor the particular file system carrying the bits is part of the Content Information in this example. Only part of the second file’s content is considered a part of the Content Information and this is the part that enables the transformation of the bits from the first file into meaningful physical values. In fact this second file does not carry all the Representation Information needed to make this transformation, because the following additional information is needed:
– information that the second file is encoded in ASCII so that it can be read as meaningful characters;
– information on how the characters are used to express the transformations from bits to numbers to meaningful physics values.
This information, typically referred to as a combination of format information and data dictionary information, may also include instrument calibration values and information on how the calibrations are to be applied. All this information may be widely understandable once the ASCII characters are visible because it has all been expressed in English (or some other natural language), or some of it may be in more structured forms that will need additional Representation Information to be understood.
Therefore, the Representation Information of the second file needs additional Representation Information, and this information may need additional Representation Information, etc., forming a linked set of Representations of Representations. This is a good example of the complex Representation Net.
In the example above, there was a determination that the Content Information consisted of the observed sensor values and their meanings. This is by no means the only choice that could have been made. It could just as easily have been decided that the Content Data Object of the desired Content Information was the bit sequences within the first file together with the all the bit sequences within the second file. The fact that some of these latter bit sequences are used to interpret the first file’s bit sequences is just an example of a set of bits that is somewhat self- describing. It is irrelevant that some of the bits in the second file are the basis for information on the date and time period over which the observations were made, the average value for the observed values, and who made the observations. Once it has been determined that all these bits constitute the Content Data Object of the Content Information, then the Representation Information is that information needed to turn them into meaningful information. How extensive this meaning is to be carried and how far the Representation Network needs to be carried are local issues for the OAIS and its related Producer and Consumer communities.
As another example, consider an electronic file containing a word processing document. This binary Data Object will have a complex format that can be seen as a document only after it has been viewed through use of associated Representation Information. In common practice, this viewing will be provided by Access Software that can use internal, or external, Representation Information. The Content Data Object is most likely to be defined as the bit sequence content of the electronic file. The Representation Information is a description of the word processing format, at a minimum, and may include information deemed needed to adequately understand the meaning of the document as viewed. If the word processing format is proprietary, and if adequate Representation Information cannot be acquired which will at the least allow simply viewing, to ensure its Long Term Preservation it may be necessary to migrate the document to another (possibly non-proprietary) format for which Representation Information is more openly available.
As a variation on the above example, it may be decided that the Content Information to be preserved is not the full word processing view of the document, but simply a sequence of text paragraphs that can be adequately represented by ASCII characters. In this case, the OAIS may decide to extract the relevant text characters and save them as a text file. The Content Data Object would be defined, most likely, as the bit stream made up of these characters. The Representation Information would be a description of how to interpret this bit stream as characters, together with any additional information deemed needed to adequately understand the meaning of the text.
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