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The bits that represent digital information are stored on various types of data carriers (or storage media). Data carriers can be physically damaged. Additionally they are subject to bit rot, a naturally occurring reversal of some bits’ values that is caused by ageing of the data carrier. The obvious methodology for mitigating this risk is bit preservation, the process of copying the data carrier content bit-by-bit onto newer data carriers. This is also called storage medium refresh. In order to be able to recover from bit loss, it is additionally necessary to hold replicas remotely in order to address larger scale disasters and to create backups from which one could recover damaged digital objects locally. These replicas and backups are also created through bit copying.

The bit preservation strategies that are chosen depend on preservation requirements and vary for different digital materials. Confidentiality, bit safety, availability and cost requirements influence the preservation strategies (Zierau, Kejser, Kulovits, 2010). Data carrier properties influence data carrier choice, and failure rates influence bit preservation frequency (Rosenthal, 2010). Bits can be copied either as data carrier “image” in the form in which it was stored on the data carrier or by copying the bits of the files stored on the data carrier independent of the data carrier encoding format (Woods, Brown, 2008; Dappert, Jackson, Kimura, 2011). Bit preservation can go beyond copying (Novak, 2006). Additionally, integrity checking ensures that all files have been copied.

The LOCKSS program (LOCKSS, nd) uses bit preservation as a preservation methodology by keeping several copies decentralized and distributed at their membership organisations, while giving them local custody and control of their assets.

1.1.4.2 Migration

Migration³ (Garrett et al., 1996) is a preservation methodology in which digital objects are translated from one format to another. Migration happens proactively, when the steward of digital objects decides that all ingested objects should be “normalized”, meaning that they are transformed to a smaller set of preferable formats that are supported in the repository. It also happens reactively, if, during preservation watch, it is found that the current format of a digital object is no longer sufficiently supported. The digital object is then translated into a better supported format. This can be a translation from one file format to another; it can also be a translation of higher-level encodings, such as a port of a software program from one programming language to another.

In this form of preservation methodology the bits are not preserved, but the contents of the digital objects should be. In practice, however, different representation formats have different properties and capabilities of representing digital object content. Because of this it is often inevitable that during migration some of the characteristics of the original object are lost in

3 ISO 13008:2012 (2012) distinguishes “conversion” and “migration”, where a conversion is a shift in format, and migration is a shift from data carrier to data carrier; but this distinction is not typically observed in the digital preservation parlance.

translation. For example, when migrating a document from Microsoft Word to Adobe PDF the editing history available in Word is lost in the resulting PDF. In practice, the programmes that perform the migration are also not always correctly implemented or lose characteristics of the object in the translation (Kulovits et al., 2009). Migrations are validated by ensuring that the

“Significant Characteristics”, those characteristics that are deemed essential by the stakeholders are preserved (Thaller, M., et al., 2008; Knight, 2008; Knight, Pennock, 2009; Dappert, Farquhar, 2009b).

1.1.4.3 Emulation

Emulation (Rothenberg, 1998) is a preservation methodology in which the digital object remains (largely) unchanged, but the platform on which it is to be rendered or executed is brought up to date. Since an obsolete digital object cannot directly be used on a more modern platform, a piece of software, called an emulator has to be written, which recreates the original computer environment on the more modern platform. Emulators can be written for individual software applications, operating systems, or hardware platforms. For dynamic, digital objects, such as computer software, emulation is often the most cost-effective preservation methodology. All software that ran on the old platform can run on the newer one using the same emulator. Were the software to be migrated (ported) instead, one would have to migrate every piece of software that should be kept usable. Nonetheless, emulation is a very complex (Kuchera, 2011; Fayzullin, M., 1997-2000) and expensive task. But the most prominent obstacle to emulation is often found in potential copyright violations of preserving proprietary software or hardware (Anderson, 2011;

Charlesworth, 2012).

A Universal Virtual Computer (UVC) (Lorie, 2001) is a program which contains a set of computer instructions. At any given time one can implement this openly specified UVC on the currently available computer platforms. Computer programs that have been written in the past to run on a UVC can in this way run on the current platforms. This strategy is a combination of emulation and migration. The underlying idea is that it will be simpler in the future to create an emulation program for the UVC than it would be to emulate a complex platform. UVCs have to be considered a research product, at this point; they are not available for practical use.

Validating emulation is difficult (Guttenbrunner, Rauber, 2012), since it is often used to preserve dynamic digital objects and it is not easy to validate that every aspect of the original behaviour has been preserved in the emulated performance. Guttenbrunner, Wieners, Rauber and Thaller.

(2010) use snapshots of the emulated and original process to validate the authenticity of the

Example projects for developing emulation solutions are the KEEP project (KEEP, 2012) which has developed an emulation framework and the Dioscuri project (van der Hoeven, 2007).