Selecting Storage Media for Long-Term Access to Digital Records.

• Author: DOLLAR, CHARLES M.

The periodic replacement of digital media is a threshold solution that helps mitigate the consequences of media fragility and technological obsolescence. The replacement of digital storage media involves the use of selection criteria by information professionals to assess current and future magnetic and optical storage media. The criteria include (1) high storage capacity, (2) high data-transfer rate, (3) life expectancy of at least 20 years, (4) established and stable marketplace presence, (5) affordability, and (6) suitability. Application of these criteria leads to the conclusion that digital linear tape is the preferred storage medium to support the long-term storage of digital records.

This study explores the selection of storage media for long-term access to digital records from four different but interrelated perspectives. The first viewpoint is an overview of the current digital storage environment with an emphasis on magnetic and optical storage media. It is followed by a discussion of the so-called problem of digital storage media. The third perspective is a review of evaluation criteria for media selection. An assessment of current storage media available today comprises the fourth perspective. (Author's note: much of this discussion is indebted to the author's soon-to-be released work Authentic Electronic Records: Long-Term Access Strategies, which is being published by Cohasset Associates in 1999.)

Although media selection is a threshold issue for long-term access to digital records, it is only one component of a long-term strategy for access to digital records. Other components include maintenance of a stable storage environment, periodic media renewal, conversion to new technology platforms, and quality control procedures.

The Digital Storage Environment

The hierarchical storage model (HMS) defines digital storage media as (1) on-line, (2) near-line, and (3) off-line, based upon frequency of access. In this model, on-line storage creates higher costs, offers faster speed, but provides lower capacity than near-line storage. Near-line storage consists of optical media jukeboxes and automated tape-retrieval libraries, while off-line storage consists of shelved optical and magnetic media. Off-line storage has lower costs, slower speed, and higher capacity than near-line storage

Electronic records that have been set aside for long-term storage are no longer required for operational purposes so they fall into the off-line storage environment. Two current recording technologies -- magnetic and optical -- are used for off-line storage.

Off-line Magnetic Media

There are three different kinds of off-line magnetic media in use today: longitudinal, longitudinal serpentine, and helical. Longitudinal recording media have a fixed number of parallel tracks (e.g., 18 tracks for 3480 tape cartridges) laid across the length of the tape that are written to or read from the beginning of the tape to its end.

Longitudinal serpentine recording media also have a fixed number of tracks (128 to 204) for digital linear tape (DLT) laid down the length of the tape. Multiple write/read heads record or read simultaneously the first two or four tracks sequentially from the beginning to the end of the tape, and the next two or four tracks are read from the end of the tape, back to beginning. This stepping down technique continues until all the tracks are read or are written on.

Helical recording media involves short tracks that are recorded diagonally (at an angle of about 11 degrees) across the width of a 4mm or 8mm tape. Both helical and longitudinal serpentine recording media have a storage capacity and data transfer rate that is five to six times greater than that of longitudinal recording media.

Off-Line Optical Media

There are three different types of optical storage media: read many times (ROM), recordable (write once read many or WORM), and rewritable (RW). These three types of optical storage media share several features. Their shared physical media characteristics include

1. a rigid translucent polycarbonate substrate containing thousands of tracks

2. a reflective coating over the substrate

3. spots in the tracks, usually called pits, that represent binary 1s and "lands" (spaces without pits) that represent 1s

4. a protective clear lacquer or acrylic overcoat

   These three types of optical storage media also use a low-power laser to read the representations of binary 1s and 0s. Reading occurs when a low-power laser beam is focused on a track and reflectivity is measured. Alterations (e.g., a bubble or a pit) in the smooth recording surface causes the laser beam's reflected light to disperse. A timing mechanism tells the optical head when to expect reflectance, and the optical head interprets the absence of reflected light as a transition from one or more binary 1s to one or more binary 0s. The presence of reflected light denotes a transition from one or more binary 0s to one or more binary 1s. ROM, WORM, and RW optical storage media can be differentiated by the way a binary stream of 1s and 0s is recorded and whether the recorded binary bit stream can be changed.

   Read-Only Technology

   The manufacture of read-only optical media involves a process whereby the binary bit stream of an information object (e.g., a book, software, or music) is replicated in the on and off modulations of a high-power laser beam that etches pits and lands on a glass master disk coated with a special photoresistant material. The pits and lands on the tracks of the master disk are replicated in a metal mold into which molten polycarbonate is injected. After the molded plastic disk cools, its underside is coated with a very thin, highly reflective aluminum coating.

   Reading occurs when a low-power laser beam is focused on a track and the presence or absence of pits and lands is measured by the amount of reflected light. Hundreds of discs can be fabricated with the same mold without any error. The pits and lands in CD-ROM are irreversible, leading to the term "read only." In addition, no new digital information can be later added to the disc.

   Read-Only Systems

   There are two read-only technologies in use today. One...