220.127.116.11 Unit Description
The Unit Description is a specialization of the Package Description that always contains a set of Associated Descriptions each of which describe the AIU Content Information from the point of view of a single Access Aid. Figure 4-22 is a UML diagram that illustrates the Unit Description contents.
Figure 4-22: Unit Description
All Unit Descriptions must supply an Associated Description for a Retrieval Aid that enables authorized users to retrieve the AIU described by the Unit Description from Archival Storage. This description includes the unique identifier assigned to the AIP by Archival Storage during the Ingest Process.
An important type of Access Aid is the Finding Aid, which is an application that assists the Consumer in locating information of interest. A single AIU may have a number of Associated Descriptions that describe the Content Information using different technologies. Additionally, as new description extraction and display technologies become available, an Archive may want to update the Unit Description associated with each of its AIUs, in order to add a new Associated Description that utilizes the new technology to better describe the AIUs.
In the OAIS for digital movies example, initially, there may be one Associated Description that is a free text description of a movie, another that is a five-minute clip and another that is a row in a relational database that is used by movie collectors to locate movies of interest. After the Archive has been operational for a period of time a technique for supplying compressed digital movies may be developed based on recording every tenth frame. The archivist may decide to create an additional type of Associated Description that is populated using the results of this new technique. If desired, the user can run each of the AIUs contained in the Archive though this compression technique and create a new Associated Description for each movie in the Archive or simply include this Associated Description for new AIUs as they are ingested into the OAIS.
Another important class of Associated Descriptions supplies data for Ordering Aids that allows the Consumer to discover the cost of and order AIUs of interest. The Ordering Aids also allow users to specify transformations to be applied to the AIUs prior to dissemination. These transformations can include Data Object transformations such as subsetting, subsampling or format transformations. The transformations can also involve modifying the PDI in the AIU prior to dissemination.
For example, the OAIS for digital movies could allow a user to order a digital movie as a VHS tape, a laser disc or an MPEG object delivered on-line. Each of these would involve a format transformation and, in theory, an update to the PDI information in the AIP to create accurate PDI for the DIP.