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
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
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.
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
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
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.