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Unformatted text preview: rage capacity of a tape - Data recording density * Length
Data recording density refers to the amount of data that can be stored on a given length of
tape. It is measured in bytes per inch (bpi), or the number of bytes (characters) that can be
stored per linear inch of tape. Tape density varies from 800 bpi in older systems to 77,000
bpi in some of the most modern systems. Hence if a 2400 feet tape has a data recording
density of 800 bpi, then its storage capacity will be 2400 * 12 inches x 800 bpi = 23 x 10 6
bytes = 23 Giga bytes. Note that this is the total storage capacity of the tape. Its actual
storage capacity (storage available for storing user data) is much less due to the use of
IBGs, file header labels, file trailer labels, BOT and EOT markers. In fact, the actual
storage capacity of a tape may be anywhere from 35% to 70% of its total storage
capacity, depending on the storage organization.
Data Transfer Rate
Data transfer rate refers to the number of characters per second that can be transmitted to
the primary storage from the tape. It is measured in bytes per second (bps). Its value
depends on the data recording density and the speed with which the tape travels under the
read/write head. Tape speed is typically of the order of 100 inches per second. Therefore,
a tape having data recording density of 77,000 bpi and its drive having a tape speed of
100 inches per second, will have a data transfer rate of 77,000 x 100 = 77,00,000 bytes or
7.7 MB per second.
A magnetic tape drive is used for storage and retrieval of data stored on magnetic tape
medium. The tape drive is different for tape reels, tape cartridges, and tape cassettes.
However, all of them work on a similar mechanism like the audio tape recorders or video
cassette recorders (VCR) found in our homes. That is, the tape drive has read/write heads
and as the tape ribbon passes under the read/write heads, the data can be either read and
transmitted to primary storage, or transmitted from primary storage and written to the
tape by giving suitable commands to the tape drive. Instead of play and record, read and
write commands are used with the tape drive. Just as in the case of an audio tape recorder
or video cassette recorder, a magnetic tape reel or cartridge or cassette has to be first
loaded to a tape drive for processing. Once loaded to a tape drive, the magnetic tape is
said to be on-line; that is, it can now be used for storage or retrieval of data by the
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- Spring '14