126.96.36.199 Archival Information Unit
The AIUs can be viewed as the ‘atoms’ of information that the Archive is tasked to store. A single AIU contains exactly one Content Information object (which may consist of multiple files) and exactly one set of PDI. The Archive is free to decide how to construct the AIU and in particular an AIU does not need to be a single file. When an Information Object is ingested into the OAIS a Unit Description, which is a subtype of a Package Description, is created by extracting information from the Content Information and the PDI and adding OAIS-specific information such as a unique identifier. The AIU is illustrated in figure 4-21.
In the example of an OAIS for digital movies, the AIU for a single movie can be viewed as three objects, one containing a digital encoding of the movie in a proprietary format, one containing the Representation Information needed to understand the proprietary format (these two objects form the Content Information), and the other containing facts about the movie such as date of creation, featured actors, director, producer, sequels, movie studio, and a checksum to ensure the integrity of the digital movie (PDI). Since the OAIS reference model is implementation independent, each of these objects could be implemented as one file or multiple files. This type of implementation-dependent information is contained in the Packaging Information. When a movie is ingested into the OAIS a Unit Description for an Ordering Aid can be created by extracting information from the Content Information and the PDI and appending it to the unique ordering information. Figure 4-21: Archival Information Unit (AIU)