5.1.3 MIGRATION TYPES
Based on the models and concepts above, it is possible to identify four primary Digital Migration types. The primary types, ordered by increasing risk of information loss, are: Operations which do not change the bit sequences – Refreshment: A Digital Migration where a media instance, holding one or more AIPs or parts of AIPs, is replaced by a media instance of the same type by copying the bits on the medium used to hold AIPs and to manage and access the medium. As a result, the existing Archival Storage mapping infrastructure, without alteration, is able to continue to locate and access the AIP.
– Replication: A Digital Migration where there is no change to the Packaging Information, the Content Information and the PDI. The bits used to convey these information objects are preserved in the transfer to the same or new media-type instance. Refreshment is also a Replication, but Replication may require changes to the Archival Storage mapping infrastructure.
Operations which change the bit sequences
– Repackaging: A Digital Migration where there is some change in the bits of the Packaging Information.
– Transformation: A Digital Migration where there is some change in the Content Information or PDI bits while attempting to preserve the full information content.
There is the smallest risk of information loss under Refreshment because none of the bits that are used to hold AIP information or to support finding and accessing AIPs are altered. There is also little risk of information loss under Replication because none of the bits representing AIP information have changed. However, if a new media type is involved there will be some changes needed in the Archival Storage mapping infrastructure (see figure 5-1). The risk is that something may go wrong in the process and some unintended changes to bits may take place. Repackaging recognizes that some bit changes will take place, but these are mostly confined to information used to delimit the Content Information and the PDI, and so generally do not alter the information carried by the Content Information or the PDI. There is the usual risk that something will go wrong, and there are also cases where some interaction between Packaging Information and the Content Information or PDI cannot be avoided. This poses additional risk of information loss. However, it is expected that the OAIS will verify that Refreshment, Replication, or Repacking Migrations have not lost information. Finally, Transformation poses the most risk because changes to the Content Information or PDI are made.
To understand more clearly what may be involved in these migration types it is necessary to look at possible implementation approaches. It will be seen that some migrations are a mixture of Repackaging and Transformation. It is also important to recall that, for any given AIP the OAIS must first clearly identify what constitutes the Content Information, and only then can the PDI be identified. Following this the Packaging Information can also be identified. Further, there is no single ‘correct’ definition of what should be the Content Information as this must be determined by the OAIS for each AIP it constructs and stores. All these issues are discussed in more detail in the following subsections using a series of implementation and migration scenarios.