Data backup systems


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Last month we looked at a few methods for backing up data. This month we will discuss what types of data backup systems are currently available. While it is possible to backup data directly to flash-type memory devices, the more practical and safer method is to back up to disk- or tape-based media. Flash memory is a better solution for the temporary backup and transport of data between machines, primarily due to its small memory capacity and high cost compared to tape or disk-based media.

Tape-based systems

Tape-based backup systems fall into one of two types of technologies: helical-scan tape or linear tape. Engineers that have had some television experience may recognize these technologies as those used for traditional videotape recording and playback systems. You may remember seeing pictures of the early 9-track tape backup systems, used primarily by the government and large corporations. These 9-track systems used about 2,400 feet of ½-inch tape on 10½-inch reels. Nine-track backup devices used all nine tracks to record data in parallel. Eight tracks were used to store the data bits and the ninth was used to store parity information that ensured data integrity. In the mid-1980s IBM incorporated this format within a single ½-inch cassette subsystem for use with its 34XX series of mainframe computers. Strangely enough these systems are still in use, but are largely being replaced by newer tape technologies.



Digital Linear Tape is a popular and reliable data storage backup format.

Helical-based tape drives use a mechanism to physically pull the tape away from its case so that the tape can be placed against a rotating drum assembly that contains multiple heads. The drum rotates at speeds as fast as 7,000 RPM. Consumer and professional VHS video machines also use this technology. Helical-scan based backup devices can store a much higher density of data because the data is written diagonally across the tape and multiple channels of data can be simultaneously written and read. The reliability of helical-scan tape devices is questionable, because the tape can break from the high tension necessary to hold the tape against the spinning drum, but in practice, these systems have proven to be reliable.

Linear tape backup devices operate similar to the all-too-familiar audiotape formats, such as reel-to-reel and cassette, in which the magnetic tape media is drawn across one or more stationary heads. Linear backup devices are considered more reliable than those of helical-based systems because the tape is simply passed across the heads without the need for the complex tensioning mechanisms and the servo-control systems required to control the speed of the drum in helical systems. In linear tape backup devices, data is written from front-to-back in a serpentine method, which means that data is written on the first track in one direction, and then moves to the next track in the opposite direction. This process continues until it reaches the end of the bottom track. The number of tracks and the amount of data per tape offered in linear drives varies by manufacturer.

A more current variation uses a technology known as Digital Linear Tape (DLT). DLT was developed by Digital Equipment Corporation (DEC) in the early 1990s as a high-capacity alternative to the IBM ½-inch tape cartridge format. DLT drives use a four-channel read and write system, where four channels can read and write data simultaneously. Quantum, the primary manufacturer of DLT devices, has introduced Super DLT (SDLT) devices touting increased data storage density using a combination of magnetic and optical (laser) technologies within a single system.

Other backup technologies using linear tape include: Linear Tape Open (LTO), a direct competitor with SDLT that integrates a memory system within the tape cartridge, along with a tracking and identification system and Sony's Advanced Intelligent Tape (AIT) Drive, which uses smart card technology within the cassette.

Disk-based systems

For single workstation or small network applications where your backup needs do not include a large amount of storage, the simplest disk based backup system is still the floppy disk. However, the low cost of optical CD-ROM writers and disk media, combined with a relatively large storage capacity, makes backing up to CD a much more practical solution.

In larger networks, magneto-optical (M/O) or DVD-RW drives may be used. The M/O drive uses a hybrid of electromagnetic and optical technologies. M/O technology uses a laser to heat specific parts of the disk to around 200 degrees centigrade. The direction of magnetic particles can be altered using a magnetic field generated by the read and write head. Because of this multistep process, M/O drives tend to be slower than other backup methods. A newer technology based on M/O, called Light Intensity Modulated Direct Overwrite (LIMDOW), uses magnets that are built into the disk rather than a separate head.



Magneto-optical formats work well in larger data installations.

The durability and relatively long shelf life of M/O media make it better suited for archival storage applications.

A lack of unified standards for DVD record products has been a stumbling block, however, the DVD manufacturers have announced an alliance to move ahead with standards for DVD+RW (DVD rewritable) and DVD+RW/+R (DVD rewritable + recordable). While DVD recorders are becoming widely available for storing and distributing video and audio information, the lack of standards has hindered its application as a viable data storage medium.

Some other optical storage technologies to look for include the Optical Super Density (OSD) that provides the benefits of M/O at speeds of current hard drives. Fluorescent Disc Technology promises the potential of enough capacity to store 20 hours of high definition TV programs.

When selecting a data backup system, decide what features are necessary for your application — reliability, speed or capacity. Other factors to consider are cost, ease of operation and the ability to grow into the future. There are several good alternatives available, but always remember that the backup system is only useful if proper backup procedures are adopted and followed.


McNamara, Radio's consultant on computer technology, is president of Applied Wireless, New Market, MD.


All of the Networks articles have been approved by the SBE Certification Committee as suitable study material that may assist your preparation for the SBE Certified Broadcast Networking Technologist exam. Contact the SBE at (317) 846-9000 or go to www.sbe.org for more information on SBE Certification.




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